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std::forward_as_tuple

STD：向前[医]如[医]元组

Defined in header ​​
template< class... Types > tuple<Types&&...> forward_as_tuple( Types&&... args ​(since C++11) (until C++14)
template< class... Types > constexpr tuple<Types&&...> forward_as_tuple( Types&&... args ​(since C++14)

中的参数的引用的元组。args适合于作为参数转发给函数。当rvalue用作参数时，元组具有rvalue引用数据成员，否则具有lvalue引用数据成员。

参数

args-zero or more arguments to construct the tuple from

返回值

阿std::tuple对象创建的std::tuple<Types&&...>(std::forward<Types>(args)...)...

例外

noexcept规格：

noexcept

注记

如果争论是暂时的，forward_as_tuple不能延长它们的生命周期；它们必须在完整表达式结束前使用。

例

二次

#include <iostream>
#include <map>
#include <tuple>
#include <string>
 
int main()
{
    std::map<int, std::string> m;
 
    m.emplace(std::piecewise_construct,
              std::forward_as_tuple(10),
              std::forward_as_tuple(20, 'a')
    std::cout << "m[10] = " << m[10] << '\n';
 
    // The following is an error: it produces a
    // std::tuple<int&&, char&&> holding two dangling references.
    //
    // auto t = std::forward_as_tuple(20, 'a'
    // m.emplace(std::piecewise_construct, std::forward_as_tuple(10), t
}

二次

产出：

二次

m[10] = aaaaaaaaaaaaaaaaaaaa

二次

另见

make_tuplecreates a tuple object of the type defined by the argument types (function template)
tiecreates a tuple of lvalue references or unpacks a tuple into individual objects (function template)
tuple_catcreates a tuple by concatenating any number of tuples (function template)
apply (C++17)calls a function with a tuple of arguments (function template)

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