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In computer programming, variadic templates are templates that take a variable number of arguments. Variadic templates are supported by C++ (since the C++11 standard), and the D programming language. ==C++== The variadic template feature of C++ was designed by Douglas Gregor and Jaakko Järvi 〔(【引用サイトリンク】title="Variadic Templates for C++0x", in Journal of Object Technology, vol. 7, no. 2, Special Issue OOPS Track at SAC 2007, February 2008, pp. 31-51 )〕 and was later standardized in C++11. Prior to C++11, templates (classes and functions) could only take a fixed number of arguments, which had to be specified when a template was first declared. C++11 allows template definitions to take an arbitrary number of arguments of any type. template The above template class tuple will take any number of typenames as its template parameters. Here, an instance of the above template class is instantiated with three type arguments: tuple The number of arguments can be zero, so tuple<> some_instance_name; will work as well.If one does not want to have a variadic template that takes 0 arguments, then this definition will work as well: template Variadic templates may also apply to functions, thus not only providing a type-safe add-on to variadic functions (such as printf) - but also allowing a printf-like function to process non-trivial objects. template The ellipsis (...) operator has two roles. When it occurs to the left of the name of a parameter, it declares a parameter pack. Using the parameter pack, the user can bind zero or more arguments to the variadic template parameters. Parameter packs can also be used for non-type parameters. By contrast, when the ellipsis operator occurs to the right of a template or function call argument, it unpacks the parameter packs into separate arguments, like the args... in the body of printf below. In practice, the use of an ellipsis operator in the code causes the whole expression that precedes the ellipsis to be repeated for every subsequent argument unpacked from the argument pack; and all these expressions will be separated by a comma.The use of variadic templates is often recursive. The variadic parameters themselves are not readily available to the implementation of a function or class. Therefore, the typical mechanism for defining something like a C++11 variadic printf replacement would be as follows: void printf(const char *s) } std::cout << *s++; } } template void printf(const char *s, T value, Args... args) } std::cout << *s++; } } This is a recursive template. Notice that the variadic template version of printf calls itself, or (in the event that args... is empty) calls the base case. There is no simple mechanism to iterate over the values of the variadic template. There are few ways to translate the argument pack into single argument use. Usually this will rely on function overloading, or - if the function can simply pick one argument at a time - using a dumb expansion marker: template }; pass; Instead of executing a function, a lambda expression may be specified and executed in place, which allows executing arbitrary sequences of statements in-place. pass; However, in this particular example, a lambda function is not necessary. A more ordinary expression can be used instead: pass; Another way is to use overloading with "termination versions" of functions. This is more universal, but requires a bit more code and more effort to create. One function receives one argument of some type ''and'' the argument pack, whereas the other receives neither. (If both have the same list of initial parameters, the call would be ambiguous - a variadic parameter pack alone cannot disambiguate a call.) For example: void func() }; The unpack operator will replicate the types for the base classes of ClassName, such that this class will be derived from each of the types passed in. Also, the constructor must take a reference to each base class, so as to initialize the base classes of ClassName.With regard to function templates, the variadic parameters can be forwarded. When combined with universal references (see above), this allows for perfect forwarding: template This unpacks the argument list into the constructor of TypeToConstruct. The std::forward syntax is the syntax that perfectly forwards arguments as their proper types, even with regard to rvalue-ness, to the constructor. The unpack operator will propagate the forwarding syntax to each parameter. This particular factory function automatically wraps the allocated memory in a std::shared_ptr for a degree of safety with regard to memory leaks.Additionally, the number of arguments in a template parameter pack can be determined as follows: template The expression SomeStruct will yield 2, while SomeStruct<>::size will give 0.抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Variadic template」の詳細全文を読む スポンサード リンク
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