Every one of the Anker 's good ideas comes mired in caveats, and all the user tweaking in the world can't solve its fundamental design problems. The software deserves praise for making macros so easy to record and use, but otherwise, the feature set is pretty standard. Whereas, the range of 16 million colors empowers you to set your desired lighting color as profile indicator, that further embellishes the look of the device. Latest: smalltech 10 minutes ago. Question Uninitialized until download 2k16 for pc Post thread.
Full content visible, double tap to read brief content. En lire plus En lire moins. Commentaires client. En savoir plus sur le fonctionnement des avis clients sur Amazon. I still refer to it from time to time. Contrary to the previous book, some items of this one are more controversial. For example, the ideas behind the item about noexcept is not so widely spread.
Another example is the use of std::async, which has some flaws. Utiliser un outil en toute connaissance de cause, c'est toujours mieux A fantastic reference.
You can hear Scott explaining the material, reading is as informative as it is enjoyable. Autant dire qu'il n'est pas hyper accessible et vous relirez certains chapitres plusieurs fois.
Afficher tous les commentaires. Just don't expect everyone else to be able to keep up. Concepts are really well explained and good examples for every point. It's an indictment on the state of the language that we need books like this to use it. Or perhaps I'm just too old and stupid now. Either way, this is a great book, if a little verbose for my tastes at times. The style of the author is witty, and light, while at the same time it conveys the educational material in an easy to follow manner.
I now ask all my PhD students to read it. Afficher ou modifier votre historique de navigation. Retour en haut. Software Images icon An illustration of two photographs. Images Donate icon An illustration of a heart shape Donate Ellipses icon An illustration of text ellipses. Metropolitan Museum Cleveland Museum of Art. Internet Arcade Console Living Room.
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In a noexcept function, optimizers need not keep the runtime stack in an unwindable state if an exception would propagate out of the function, nor must they ensure that objects in a noexcept function are destroyed in the inverse order of construction should an exception leave the function. For some functions, the case is even stronger.
You therefore ensure that Widget has move operations, either by writing them yourself or by seeing to it that the conditions for their automatic generation are fulfilled see Item But the original std::vector has been modified: n of its elements have been moved from. Restoring their original state may not be possible, because attempting to move each object back into the original memory may itself yield an exception.
In that case, having moves replace copies would be safe, and the only side effect would be improved performance. The answer is obvious: it checks to see if the operation is declared noexcept. Interestingly, whether swaps in the Standard Library are noexcept is sometimes dependent on whether user- 3 The checking is typically rather roundabout.
That, in turn, determines whether other swaps, such as the one for arrays of arrays of Widget, are noexcept. The fact that swapping higher-level data structures can generally be noexcept only if swapping their lower-level constituents is noexcept should motivate you to offer noexcept swap functions whenever you can. Alas, I must temper your enthusiasm. If you declare a function noexcept and later regret that decision, your options are bleak.
You can change the implementation such that an exception could escape, yet keep the original now incorrect exception specification.
If you do that, your program will be terminated if an exception tries to leave the function. None of these options is appealing.
The fact of the matter is that most functions are exception-neutral. Such functions throw no exceptions themselves, but functions they call might emit one. Most functions, therefore, quite properly lack the noexcept designation. Please note that I said some functions have natural noexcept implementations.
Is putting the cart before the horse. Is not seeing the forest for the trees. Is…choose your favorite metaphor. For example, callers may have to check for status codes or special return values. By default, all memory deallocation functions and all destructors—both user-defined and compiler- generated—are implicitly noexcept.
The only time a destructor is not implicitly noexcept is when a data member of the class including inherited members and those contained inside other data members is of a type that expressly states that its destructor may emit exceptions e. Such destructors are uncommon. There are none in the Standard Library, and if the 4 The interface specifications for move operations on containers in the Standard Library lack noexcept.
A function with a wide contract has no preconditions. Functions without wide contracts have narrow contracts. That suggests that f should be declared noex cept. If f were to be called with a std::string whose length is greater than 32, behavior would be undefined, because a precondition violation by definition results in undefined behavior.
The result of the cast is undefined, so there are no behavioral guarantees for the program containing the cast. As a final point, let me elaborate on my earlier observation that compilers typically offer no help in identifying inconsistencies between function implementations and their exception specifications.
There could be good reasons for their non-noexcept declarations. For example, they might be part of a library written in C. Even functions from the C Standard Library that have been moved into the std namespace lack exception specifications, e. Cutting through the confusion is worth the trouble, because when constexpr corresponds to what you want to express, you definitely want to use it.
The concept is only part of the story, though, because when con stexpr is applied to functions, things are more nuanced than this suggests. Perhaps most intriguingly, these things are features. Such objects are, in fact, const, and they do, in fact, have values that are known at compile time. Technically, their values are determined during translation, and translation consists not just of compilation but also of linking. Values known during compilation are privileged.
They may be placed in read-only memory, for example, and, especially for developers of embedded systems, this can be a feature of considerable importance. Such contexts include specification of array sizes, integral template arguments including lengths of std::array objects , enumerator values, alignment specifiers, and more. If you want compilers to guarantee that a variable has a value that can be used in contexts requiring compile-time constants, the tool to reach for is con stexpr, not const.
Usage scenarios for constexpr objects become more interesting when constexpr functions are involved. Such functions produce compile-time constants when they are called with compile-time constants. If the values of the arguments you pass to a constexpr function in such a context are known during compilation, the result will be computed during compilation. The constexpr function does it all. Suppose we need a data structure to hold the results of an experiment that can be run in a variety of ways.
For example, the lighting level can be high, low, or off during the course of the experiment, as can the fan speed and the temperature, etc. Assuming each result is an int and that n is known or can be computed during compilation, a std::array could be a reasonable data structure choice.
First, std::pow works on floating-point types, and we need an integral result. Fortunately, we can write the pow we need. Points so initialized could thus be constexpr: constexpr Point p1 9. It means that the object mid, though its initialization involves calls to constructors, getters, and a non-member function, can be created in read- only memory!
It means you could use an expression like mid. The more code taking part in the migration, the faster your software will run. Compilation may take longer, however. By using con stexpr whenever possible, you maximize the range of situations in which your objects and functions may be used. If you later decide that your use of constexpr was a mistake and you remove it, you may cause arbitrarily large amounts of client code to stop compiling. Item Make const member functions thread safe.
Within this class, it would probably be useful to have a function to compute the root s of a polynomial, i. This code has undefined behavior. It can still be moved, however. In some situations, a mutex is overkill. Because operations on std::atomic variables are often less expensive than mutex acquisition and release, you may be tempted to lean on std::atomics more heavily than you should.
Such behavior is contrary to the goal of caching. The returned value is therefore incorrect. In such cases, you can avoid the costs associated with mutexes and std::atomics, as well as the side effect of their rendering the classes containing them move-only. Item Understand special member function generation. In that case, the compiler-generated destructor for the derived class is also virtual. But you already know these things. That means that the move constructor move-constructs each non- static data member of the class from the corresponding member of its parameter rhs, and the move assignment operator move-assigns each non-static data member from its parameter.
The move constructor also move-constructs its base class parts if there are any , and the move assignment operator move-assigns its base class parts. Now, when I refer to a move operation move-constructing or move-assigning a data member or base class, there is no guarantee that a move will actually take place. Item 23 covers this process in detail. The two move operations are not independent. If you declare either, that prevents compilers from generating the other. So declaring a move constructor prevents a move assignment operator from being generated, and declaring a move assignment operator prevents compilers from generating a move constructor.
This goes in the other direction, too. Declaring a move operation construction or assignment in a class causes compilers to disable the copy operations. The copy operations are disabled by deleting them—see Item The Rule of Three states that if you declare any of a copy constructor, copy assignment operator, or destructor, you should declare all three.
It grew out of the observation that the need to take over the meaning of a copy operation almost always stemmed from the class performing some kind of resource management, and that almost always implied that 1 whatever resource management was being done in one copy operation probably needed to be done in the other copy operation and 2 the class destructor would also be participating in management of the resource usually releasing it. The classic resource to be managed was memory, and this is why all Standard Library classes that manage memory e.
A consequence of the Rule of Three is that the presence of a user-declared destructor indicates that simple memberwise copy is unlikely to be appropriate for the copying operations in the class.
This means that if you have code that depends on the generation of copy operations in classes declaring a destructor or one of the copy operations, you should consider upgrading these classes to eliminate the dependence.
Provided the behavior of the compiler-generated functions is correct i. The code is therefore likely to compile, run, and pass its functional testing. That includes testing its move functionality, because even though this class is no longer move-enabled, requests to move it will compile and run.
Such requests will, as noted earlier in this Item, cause copies to be made. The simple act of adding a destructor to the class could thereby have introduced a significant performance problem!
Generated only if the class contains no user-declared constructors. Generated only if the class lacks a user- declared copy constructor. Deleted if the class declares a move operation.
Generated only if the class lacks a user-declared copy assignment operator. Generated only if the class contains no user- declared copy operations, move operations, or destructor.
That would be the case when T is Widget. Generation of the copy operations in classes with an explicitly declared destructor is deprecated.
And sometimes about counting. Let me count the ways. If you use the wrong form, results are undefined. Missing a path leads to resource leaks, and doing the destruction more than once leads to undefined behavior. Dangling pointers arise when objects are destroyed while pointers still point to them.
Smart pointers are one way to address these issues. Smart pointers are wrappers around raw pointers that act much like the raw pointers they wrap, but that avoid many of their pitfalls.
You should therefore prefer smart pointers to raw pointers. Smart pointers can do virtually everything raw pointers can, but with far fewer opportunities for error.
The smart pointer APIs are remarkably varied. About the only functionality common to all is default construction. Mastering such information can be the difference between merely using these smart pointers and using them effectively. The source pointer is set to null. Suppose we have a hierarchy for types of investments e. Most stem from abnormal program termination. All custom deletion functions accept a raw pointer to the object to be destroyed, then do what is necessary to destroy that object.
In this case, the action is to call makeLogEntry and then apply delete. For information about decltype, see Item 3. Regardless of the actual type of object created inside makeInvestment i. When custom deleters enter the picture, this may no longer be the case. For deleters that are function objects, the change in size depends on how much state is stored in the function object.
Stateless function objects e. Machines should manage resource lifetimes, not humans. We prefer the generality and predictability of destructors, thank you. Garbage collection really is convenient, and manual lifetime management really can seem akin to constructing a mnemonic memory circuit using stone knives and bear skins.
The net effect of the assignment is that the reference count for the object originally pointed to by sp1 is decremented, while that for the object pointed to by sp2 is incremented.
Conceptually, the reference count is associated with the object being pointed to, but pointed-to objects know nothing about this. They thus have no place to store a reference count. Either way, the reference count is stored as dynamically allocated data.
As a result, no reference count manipulation is required. This is as true for assignment as for construction, so move construction is faster than copy construction, and move assignment is faster than copy assignment. Custom deleters can be function objects, and function objects can contain arbitrary amounts of data. That means they can be arbitrarily large.
It may have to use more memory. The control block contains, in addition to the reference count, a copy of the custom deleter, if one has been specified. If a custom allocator was specified, the control block contains a copy of that, too. The creation of the raw pointer pw to a dynamically allocated object is bad, because it runs contrary to the advice behind this entire chapter: to prefer smart pointers to raw pointers.
But set that aside. The second destruction is responsible for the undefined behavior. Instead, the author of the code creating spw2 would naturally use spw1 as an initialization argument i. The comment about this being wrong says it all—or at least most of it.
If the idea of a derived class inheriting from a base class templatized on the derived class makes your head hurt, try not to think about it. The design relies on the current object having an associated control block. A control block is typically only a few words in size, although custom deleters and allocators may make it larger. The usual control block implementation is more sophisticated than you might expect. Performing an operation requiring a reference count manipulation e.
In exchange for these rather modest costs, you get automatic lifetime management of dynamically allocated resources. The reverse is not true. No divorce, no annulment, no dispensations. A truly smart pointer would deal with this problem by tracking when it dangles, i. It looks anything but smart. The relationship begins at birth.
You can test for this directly, if wpw. This is easier desired than done. In that case, your dereference would yield undefined behavior. Callers should certainly receive smart pointers to cached objects, and callers should certainly determine the lifetime of those objects, but the cache needs a pointer to the objects, too.
In most implementations, each subject contains a data member holding pointers to its observers. That makes it easy for subjects to issue state change notifications. What kind of pointer should this be? With this approach, if A is destroyed, but C continues to point to B, B will contain a pointer to A that will dangle.
That would yield undefined behavior. Even if A and B are unreachable from other program data structures e. If that happens, A and B will have been leaked, for all practical purposes: it will be impossible for the program to access them, yet their resources will never be reclaimed. This avoids both problems above. In strictly hierarchal data structures such as trees, child nodes are typically owned only by their parents. When a parent node is destroyed, its child nodes should be destroyed, too.
Back-links from children to parents can be safely implemented as raw pointers, because a child node should never have a lifetime longer than its parent. For details, continue on to Item The document you want is N by Stephan T. Lavavej, dated Repeating types runs afoul of a key tenet of software engineering: code duplication should be avoided. It often evolves into inconsistent code, and inconsistency in a code base often leads to bugs.
The second reason to prefer make functions has to do with exception safety. As the comment indicates, this code could leak the Widget conjured up by new. But how? Compilers are not required to generate code that executes them in this order. That is, compilers may emit code to execute the operations in this order: 1. Execute computePriority. Direct use of new, then, requires one memory allocation for the Widget and a second allocation for the control block.
This optimization reduces the static size of the program, because the code contains only one memory allocation call, and it increases the speed of the executable code, because memory is allocated only once. The arguments for preferring make functions over direct use of new are strong ones. For some types, the answer to this question makes a big difference.
Or is the result indeterminate? That means that within the make functions, the perfect forwarding code uses parentheses, not braces. The bad news is that if you want to construct your pointed-to object using a braced initializer, you must use new directly. Both are edge cases, but some developers live on the edge, and you may be one of them. This second reference count is known as the weak count. If the reference count is zero i. And as long as a control block exists, the memory containing it must remain allocated.
The best way to do that is to make sure that when you use new directly, you immediately pass the result to a smart pointer constructor in a statement that does nothing else. This prevents compilers from generating code that could emit an exception between the use of new and invocation of the constructor for the smart pointer that will manage the newed object.
As an example, consider a minor revision to the exception-unsafe call to the process Widget function we examined earlier. And unless you have a compelling reason for doing otherwise, using a make function is what you should do. Item When using the Pimpl Idiom, define special member functions in the implementation file. Those headers increase the compilation time for Widget clients, plus they make those clients dependent on the contents of the headers.
That speeds compilation, and it also means that if something in these headers changes, Widget clients are unaffected. A type that has been declared, but not defined, is known as an incomplete type. Widget::Impl is such a type. Part 2 is the dynamic allocation and deallocation of the object that holds the data members that used to be in the original class. However, these dependencies have been moved from widget. The need to deallocate this object when a Widget is destroyed is what necessitates the Widget destructor.
Fortunately, getting the code to work is easy. At that point, its destructor is called. The type becomes complete when its definition has been seen, and Widget::Impl is defined inside widget. Arranging for that is simple.
The problem there is that compilers typically generate code to destroy pImpl in the event that an exception arises inside the move constructor, and destroying pImpl requires that Impl be complete. The original Widget class contained std::string, std::vector, and Gadget data members, and, assuming that Gadgets, like std::strings and std::vectors, can be copied, it would make sense for Widget to support the copy operations. Rather than copy the fields one by one, we take advantage of the fact that compilers will create the copy operations for Impl, and these operations will copy each field automatically.
That is, given this code in widget. A consequence of this greater efficiency is that pointed-to types must be complete when compiler-generated special functions e.
Do this even if the default function implementations are acceptable. In the same way that copy constructors and copy assignment operators give you control over what it means to copy objects, move constructors and move assignment operators offer control over the semantics of moving. Rvalue references are the glue that ties these two rather disparate features together. The world of move semantics, perfect forwarding, and rvalue references is more nuanced than it appears.
Fortunately, there is a limit to their depths. This chapter will take you to the bedrock. All those pieces will fall into place. If this surprises you, please review the overview of lvalues and rvalues that begins on page 2.
Item Understand std::move and std::forward. They generate no executable code. Not a single byte. The other is the cast that comprises the essence of the function. As you can see, std::move takes a reference to an object a universal reference, to be precise—see Item 24 and it returns a reference to the same object.
It does cast. Of course, rvalues are candidates for moving, so applying std::move to an object tells the compiler that the object is eligible to be moved from. In truth, rvalues are only usually candidates for moving. This code links. This code runs. This code sets the data member value to the content of text. Sure, text is cast to an rvalue by std::move, but text is declared to be a const std::string, so before the cast, text is an lvalue const std::string, and the result of the cast is an rvalue const std::string, but throughout it all, the constness remains.
Consider the effect that has when compilers have to determine which std::string constructor to call. The rvalue can, however, be passed to the copy constructor, because an lvalue-reference-to-const is permitted to bind to a const rvalue. The member initialization therefore invokes the copy constructor in std::string, even though text has been cast to an rvalue! Such behavior is essential to maintaining const-correctness. Moving a value out of an object generally modifies the object, so the language should not permit const objects to be passed to functions such as move constructors that could modify them.
There are two lessons to be drawn from this example. Move requests on const objects are silently transformed into copy operations. The story for std::forward is similar to that for std::move, but whereas std::move unconditionally casts its argument to an rvalue, std::forward does it only under certain conditions.
When we call logAndProcess with an lvalue, we naturally expect that lvalue to be forwarded to process as an lvalue, and when we call logAndProcess with an rvalue, we expect the rvalue overload of pro cess to be invoked.
But param, like all function parameters, is an lvalue. Every call to process inside logAndProcess will thus want to invoke the lvalue overload for process. This is precisely what std::forward does. You may wonder how std::forward can know whether its argument was initialized with an rvalue. In the code above, for example, how can std::forward tell whether param was initialized with an lvalue or an rvalue?
That parameter is passed to std::forward, which recovers the encoded information. For details on exactly how that works, consult Item From a purely technical perspective, the answer is yes: std::forward can do it all. More importantly, the use of std::move conveys an unconditional cast to an rvalue, while the use of std::forward indicates a cast to an rvalue only for references to which rvalues have been bound. Those are two very different actions. The first one typically sets up a move, while the second one just passes—forwards—an object to another function in a way that retains its original lvalueness or rvalueness.
This Item is such a lie. One is rvalue reference, of course. Their dual nature permits them to bind to rvalues like rvalue references as well as lvalues like lvalue references. Furthermore, they can bind to const or non-const objects, to volatile or non-volatile objects, even to objects that are both const and volatile.
Such unprecedentedly flexible references deserve a name of their own. I call them universal references. The initializer for a universal reference determines whether it represents an rvalue reference or an lvalue reference.
If the initializer is an rvalue, the universal reference corresponds to an rvalue reference. If the initializer is an lvalue, the universal reference corresponds to an lvalue reference.
That rules out the possibility that param is a universal reference. Okay, Args is really a parameter pack, not a type parameter, but for purposes of this discussion, we can treat it as if it were a type parameter.
The result, thanks to auto universal references, is that timeFuncInvocation can time pretty much any function execution. It will also allow you to make sense of Items 25 and 26, which rely on the distinction. So embrace the abstraction. Revel in it. Item Use std::move on rvalue references, std::forward on universal references.
Rvalue references bind only to objects that are candidates for moving. The way to do that is to cast parameters bound to such objects to rvalues. Universal references should be cast to rvalues only if they were initialized with rvalues. Even worse is the idea of using std::move with universal references, because that can have the effect of unexpectedly modifying lvalues e. You might point out that if setName had simply been overloaded for const lvalues and for rvalues, the whole problem could have been avoided.
For example, consider this use of setName: w. A call to setName would thus entail execution of one std::string constructor to create the temporary , one std::string move assignment operator to move newName into w.
Widget::setName takes only one parameter, so only two overloads are necessary, but for functions taking more parameters, each of which could be an lvalue or an rvalue, the number of overloads grows geometrically: n parameters necessitates 2n overloads.
Some functions —function templates, actually—take an unlimited number of parameters, each of which could be an lvalue or rvalue. Which is exactly what you should do. Well, usually. But not necessarily initially. Ergo the use of std::forward on only the final use of the universal reference.
For std::move, the same thinking applies i. To learn when and why, consult Item Assuming that the Matrix type supports move construction, which is more efficient than copy construction, using std::move in the return statement yields more efficient code. The situation is similar for universal references and std::forward. If the original object is an rvalue, its value should be moved into the return value thus avoiding the expense of making a copy , but if the original is an lvalue, an actual copy must be created.
My liberal use of quotation marks should tip you off that this line of reasoning is flawed. But why is it flawed? Some people draw a distinction between application of the RVO to named and unnamed i.
But the RVO is an optimization. If so, you might be willing to pay the price of a move as insurance against the cost of a copy. In that case, applying std::move to a local object would still be a bad idea. The part of the Standard blessing the RVO goes on to say that if the conditions for the RVO are met, but compilers choose not to perform copy elision, the object being returned must be treated as an rvalue. There are situations where applying std::move to a local variable can be a reasonable thing to do i.
Item Avoid overloading on universal references. Within logAndAdd, name is ultimately passed to names. Because name is an lvalue, it is copied into names. In this call, we pay for a copy, but we should be able to get by with only a move. Had that string literal been passed directly to emplace, there would have been no need to create a temporary std::string at all.
Instead, emplace would have used the string literal to create the std::string object directly inside the std::multiset. We can eliminate the inefficiencies in the second and third calls by rewriting logAndAdd to take a universal reference see Item 24 and, in accord with Item 25, std::forwarding this reference to emplace.
There are two logAndAdd overloads. The one taking a universal reference can deduce T to be short, thus yielding an exact match. The overload with an int parameter can match the short argument only with a promotion. All because the universal reference overload was a better match for a short argument than an int.
They instantiate to create exact matches for almost any type of argument. An easy way to topple into this pit is to write a perfect forwarding constructor. A small modification to the logAndAdd example demonstrates the problem. It will call the perfect- forwarding constructor. Compilers reason as follows. The copy constructor a normal function thereby trumps an instantiated template with the same signature.
The interaction among perfect-forwarding constructors and compiler-generated copy and move operations develops even more wrinkles when inheritance enters the picture. In particular, the conventional implementations of derived class copy and move operations behave quite surprisingly.
To understand why, note that the derived class functions are using arguments of type SpecialPerson to pass to their base class, then work through the template instantiation and overload-resolution consequences for the constructors in class Person. That egg can be unscrambled in a number of ways. The next Item. Item 26 explains that overloading on universal references can lead to a variety of problems, both for freestanding and for member functions especially constructors.
Yet it also gives examples where such overloading could be useful. The discussion that follows builds on the examples introduced in Item Pass by value An approach that often allows you to dial up performance without any increase in complexity is to replace pass-by-reference parameters with, counterintuitively, pass by value.
Similarly, all arguments of type std::string and things from which std::strings can be created, e. There are thus no surprises for callers. You could argue, I suppose, that some people might be surprised that using 0 or NULL to indicate a null pointer would invoke the int overload, but such people should be referred to Item 8 and required to read it repeatedly until the thought of using 0 or NULL as a null pointer makes them recoil.
Use Tag dispatch Neither pass by lvalue-reference-to-const nor pass by value offers support for perfect forwarding. The goal of this Item is to avoid that. The two functions doing the real work will be named logAndAddImpl, i. One of the functions will take a universal reference. But each function will also take a second parameter, one that indicates whether the argument being passed is integral.
Stop talking and show me the code! But, as Item 28 explains, if an lvalue argument is passed to the universal reference name, the type deduced for T will be an lvalue reference. For details, see Item 9. With that taken care of, we can shift our attention to the function being called, logAndAddImpl.
There are two overloads, and the first is applicable only to non- integral types i. That means we need a type that corresponds to true and a different type that corresponds to false. The net result is that this logAndAddImpl overload is a viable candidate for the call in logAndAdd only if T is not an integral type. The second overload covers the opposite case: when T is an integral type. Some compilers do, at least some of the time. Hence the name for this design: tag dispatch.
Constraining templates that take universal references A keystone of tag dispatch is the existence of a single unoverloaded function as the client API. Creating an unoverloaded dispatch function is usually easy, but the second problem case Item 26 considers, that of a perfect-forwarding constructor for the Person class shown on page , is an exception. Compilers may generate copy and move constructors themselves, so even if you write only one constructor and use tag dispatch within it, some constructor calls may be handled by compiler-generated functions that bypass the tag dispatch system.
Such templates are said to be disabled. If the type being passed is Person, we want to disable the perfect-forwarding constructor i. The implementation remains the same as in Item Here, I want to focus on expression of the condition that will control whether this constructor is enabled.
We want for Person and T to not be the same. This means we need a way to strip any references, consts, and volatiles from T before checking to see if that type is the same as Person. Once again, the Standard Library gives us what we need in the form of a type trait. That trait is std::decay. The condition we want to control whether our constructor is enabled, then, is! When you can use one of the other mechanisms to avoid mixing universal references and overloading and you almost always can , you should.
Given the declaration above, constructing a Person from another Person— lvalue or rvalue, const or non-const, volatile or non-volatile—will never invoke the constructor taking a universal reference. Success, right? Um, no. Belay that celebration. We need to tie it down. Pesky inheritance! You should not be surprised to hear that among the standard type traits is one that determines whether one type is derived from another. Tantalizingly close. That was, after all, our original goal; the constructor ambiguity problem was just something we got dragged into along the way.
A thing of beauty! Well, okay, the beauty is perhaps most pronounced for those with something of a template metaprogramming fetish, but the fact remains that this approach not only gets the job done, it does it with unique aplomb. Because it uses perfect forwarding, it offers maximal efficiency, and because it controls the combination of universal references and overloading rather than forbidding it, this technique can be applied in circumstances such as constructors where overloading is unavoidable.
This fundamental decision—to specify a type or not—has consequences. But perfect forwarding has drawbacks. Item 30 explores these perfect forwarding failure cases. A second issue is the comprehensibility of error messages when clients pass invalid arguments. The resulting error message is likely to be, er, impressive. In this example, the universal reference is forwarded only once from the Person constructor to the std::string constructor , but the more complex the system, the more likely that a universal reference is forwarded through several layers of function calls before finally arriving at a site that determines whether the argument type s are acceptable.
The more times the universal reference is forwarded, the more baffling the error message may be when something goes wrong. Many developers find that this issue alone is grounds to reserve universal reference parameters for interfaces where performance is a foremost concern. Item Understand reference collapsing.
Item 23 remarks that when an argument is passed to a template function, the type deduced for the template parameter encodes whether the argument is an lvalue or an rvalue.
The encoding mechanism is simple. When an lvalue is passed as an argument, T is deduced to be an lvalue reference. When an rvalue is passed, T is deduced to be a non-reference. Note the asymmetry: lvalues are encoded as lvalue references, but rvalues are encoded as non-references. And yet compilers issue no protest. Yes, you are forbidden from declaring references to references, but compilers may produce them in particular contexts, template instantiation being among them.
When compilers generate references to references, reference collapsing dictates what happens next. There are two kinds of references lvalue and rvalue , so there are four possible reference-reference combinations lvalue to lvalue, lvalue to rvalue, rvalue to lvalue, and rvalue to rvalue. Otherwise i. Reference collapsing is a key part of what makes std::forward work. Suppose that the argument passed to f is an lvalue of type Widget.
An lvalue argument passed to std::forward will thus return an lvalue reference. Now suppose that the argument passed to f is an rvalue of type Widget. The end result is that an rvalue argument passed to f will be forwarded to someFunc as an rvalue, which is precisely what is supposed to happen.
The first and most common is template instantiation. The second is type generation for auto variables. The third is the generation and use of typedefs and alias declarations see Item 9. Item Assume that move operations are not present, not cheap, and not used.
Move semantics is truly an important feature. Move semantics can really pull that off, and that grants the feature an aura worthy of legend. Legends, however, are generally the result of exaggeration. The purpose of this Item is to keep your expectations grounded. Data members or base classes of types that have disabled moving e. Objects of such container types hold as data members , conceptually, only a pointer to the heap memory storing the contents of the container.
The reality is more complex, but for purposes of this analysis, the differences are not important. Assuming that Widget is a type where moving is faster than copying, moving a std::array of Widget will be faster than copying the same std::array. So std::array certainly offers move support.
On the other hand, std::string offers constant-time moves and linear-time copies. That makes it sound like moving is faster than copying, but that may not be the case. Many string implementations employ the small string optimization SSO. The motivation for the SSO is extensive evidence that short strings are the norm for many applications.
An implication of the win, however, is that moves are no faster than copies, though one could just as well take a glass-half-full approach and say that for such strings, copying is no slower than moving.
Even for types supporting speedy move operations, some seemingly sure-fire move situations can end up making copies. The move request therefore becomes a copy request. But the title of this Item is to assume that move operations are not present, not cheap, and not used. This is typically the case in generic code, e. Perfect forwarding. This Item is devoted to familiarizing you with the epsilons. The goal is for the second function the one being forwarded to to receive the same objects that the first function the one doing the forwarding received.
We want the forwarded-to function to be able to work with the originally-passed-in objects. The fwd template, for example, accepts any type of argument, and it forwards whatever it gets. All such failure cases have the same cause.
In this case, the code fails to compile. The result is that neither 0 nor NULL can be perfect-forwarded as a null pointer. The fix is easy, however: pass nullptr instead of 0 or NULL. For details, consult Item 8. Compilers work around the missing definition as they are required to do by plopping the value 28 into all places where MinVals is mentioned.
That being the case, passing MinVals by reference is effectively the same as passing it by pointer, and as such, there has to be some memory for the pointer to point to. Passing integral static const data members by reference, then, generally requires that they be defined, and that requirement can cause code using perfect forwarding to fail where the equivalent code without perfect forwarding succeeds.
According to the Standard, passing MinVals by reference requires that it be defined. But not all implementations enforce this requirement. If you forget and provide the initializer in both places, your compilers will complain, thus reminding you to specify it only once. Overloaded function names and template names Suppose our function f the one we keep wanting to forward arguments to via fwd can have its behavior customized by passing it a function that does some of its work.
Bitfields The final failure case for perfect forwarding is when a bitfield is used as a function argument. Bitfields may consist of arbitrary parts of machine words e. The only kinds of parameters to which a bitfield can be passed are by-value parameters and, interestingly, references-to-const. You can thus make a copy yourself and call the forwarding function with the copy. You rarely have to think about it. Equally important is knowing how to work around them.
In most cases, this is straightforward. Everything a lambda can do is something you can do by hand with a bit more typing. Beyond the Standard Library, lambdas facilitate the on-the-fly specification of callback functions, interface adaption functions, and context-specific functions for one-off calls.
The vocabulary associated with lambdas can be confusing. Depending on the capture mode, closures hold copies of or references to the captured data. Each lambda causes compilers to generate a unique closure class.
The statements inside a lambda become executable instructions in the member functions of its closure class. Item Avoid default capture modes. Default by-reference capture can lead to dangling references. If the lifetime of a closure created from that lambda exceeds the lifetime of the local variable or parameter, the reference in the closure will dangle. This code is a problem waiting to happen.
The lambda refers to the local variable divisor, but that variable ceases to exist when addDivisorFilter returns. If you know that a closure will be used immediately e.
If the lambda were found to be useful in other contexts e. That is, we could add the lambda to filters as follows: filters. The lambda is dependent on divisor, but the default by-value capture mode ensures that divisor is copied into any closures arising from the lambda, right?
Completely wrong. Horribly wrong. Fatally wrong. Captures apply only to non-static local variables including parameters visible in the scope where the lambda is created. The explanation hinges on the implicit use of a raw pointer: this. From that point on, filters contains an entry with a dangling pointer.
A default capture mode is what made it possible to accidentally capture this when you thought you were capturing divisor in the first place. Such objects are defined at global or namespace scope or are declared static inside classes, functions, or files. Yet specification of a default by-value capture mode can lend the impression that they are. Rather, the code for the lambda refers to the static variable divisor.
When, at the end of each invocation of addDivisorFilter, divisor is incremented, any lambdas that have been added to filters via this function will exhibit new behavior corresponding to the new value of divisor.
If you stay away from default by- value capture clauses, you eliminate the risk of your code being misread in this way. Sometimes neither by-value capture nor by-reference capture is what you want.
If you have a move-only object e. It offers direct support for moving objects into closures. The new capability is called init capture. Using an init capture makes it possible for you to specify 1. The scope on the left is that of the closure class.
The scope on the right is the same as where the lambda is being defined. If no such configuration is necessary, i. As a result, another name for init capture is generalized lambda capture. How can you accomplish move capture in a language lacking support for move capture? I call function objects returned by std::bind bind objects. The first argument to std::bind is a callable object. Subsequent arguments represent values to be passed to that object. A bind object contains copies of all the arguments passed to std::bind.
For each lvalue argument, the corresponding object in the bind object is copy constructed. In this example, the second argument is an rvalue the result of std::move—see Item 23 , so data is move constructed into the bind object. In this example, that means that when func the bind object is called, the move- constructed copy of data inside func is passed as an argument to the lambda that was passed to std::bind.
This parameter is an lvalue reference to the copy of data in the bind object. Uses of data inside the lambda will thus operate on the move-constructed copy of data inside the bind object. By default, the operator member function inside the closure class generated from a lambda is const.
The correct way to write the lambda is to have it perfect-forward x to normalize. Doing that requires two changes to the code. If an rvalue is passed, the parameter becomes an rvalue reference. This means that in our lambda, we can determine whether the argument passed was an lvalue or an rvalue by inspecting the type of the parameter x. If an rvalue was passed, decltype x will produce an rvalue reference type. Item 28 also explains that when calling std::forward, convention dictates that the type argument be an lvalue reference to indicate an lvalue and a non-reference to indicate an rvalue.
In our lambda, if x is bound to an lvalue, decltype x will yield an lvalue reference. That conforms to convention. However, if x is bound to an rvalue, decltype x will yield an rvalue reference instead of the customary non- reference. So for both lvalues and rvalues, passing decltype x to std::forward gives us the result we want. Item Prefer lambdas to std::bind. In TR1, bind was in a different namespace, so it was std::trbind, not std::bind, and a few interface details were different.
This history means that some programmers have a decade or more of experience using std::bind. As in Item 32, I refer to the function objects returned from std::bind as bind objects.
The most important reason to prefer lambdas over std::bind is that lambdas are more readable. The alarm sound, however, remains undecided. Readers of this code simply have to know that calling setSoundB invokes setAlarm with the time and duration specified in the call to std::bind. The type of this argument is not identified in the call to std::bind, so readers have to consult the setAlarm declaration to determine what kind of argument to pass to setSoundB.
It will be evaluated when setAlarm is called. That makes sense: we want the alarm to go off an hour after invoking setAlarm. That means that the expression will be evaluated when std::bind is called, and the time resulting from that expression will be stored inside the resulting bind object. As a consequence, the alarm will be set to go off an hour after the call to std::bind, not an hour after the call to setAlarm!
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