std::forward

 

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關係運算符 Original: Relational operators The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

rel_ops::operator!= rel_ops::operator> rel_ops::operator<= rel_ops::operator>=

雙和元組 Original: Pairs and tuples The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

pair

tuple （C++11）

piecewise_construct_t （C++11）

piecewise_construct （C++11）

掉期，遠期和移動 Original: Swap, forward and move The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

swap

forward （C++11）

move （C++11）

move_if_noexcept （C++11）

declval （C++11）

 

Defined in header <utility>
template< class T > T&& forward( typename std::remove_reference<T>::type& t );	(1)	（C++11 起）
template< class T > T&& forward( typename std::remove_reference<T>::type&& t );	(2)	（C++11 起）

轉發。 使用時可根據下面的配方在一個函數模板，轉發給另一個函數的參數，正是因為它是傳遞給調用函數.

Original:

When used according to the following recipe in a function template, forwards the argument to another function exactly as it was passed to the calling function.

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template<typename T>
wrapper(T&& arg) {
  foo(std::forward<T>(arg));
}

• 如果調用wrapper()通過一個rvaluestd::string，那麼T推導出std::string（不std::string&，const std::string&，或std::string&&），和std::forward確保一個右值引用傳遞給foo.

  Original:

  If a call to wrapper() passes an rvalue std::string, then T is deduced to std::string (not std::string&, const std::string&, or std::string&&), and std::forward ensures that an rvalue reference is passed to foo.

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• 如果調用wrapper()通過一個常量左值std::string，然後T推導出const std::string&，並std::forward確保一個常量左值的引用被傳遞給foo.

  Original:

  If a call to wrapper() passes a const lvalue std::string, then T is deduced to const std::string&, and std::forward ensures that a const lvalue reference is passed to foo.

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• 如果調用wrapper()通過一個non-const左值std::string，那麼T推導出std::string&，並std::forward確保一個non-const左值的引用被傳遞給foo.

  Original:

  If a call to wrapper() passes a non-const lvalue std::string, then T is deduced to std::string&, and std::forward ensures that a non-const lvalue reference is passed to foo.

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注釋

嘗試發送一個右值作為左值，如表2）與左值的引用類型T的實例，是一個編譯時錯誤.

Original:

Attempting to forward an rvalue as an lvalue, such as by instantiating the form 2) with lvalue reference type T, is a compile-time error.

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參數

t

-

該對象被轉發 Original: the object to be forwarded The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions.

返回值

static_cast<T&&>(t)

例外

noexcept specification:  

noexcept

  （C++11 起）

為例

這個例子演示了完美的參數的功能make_unique（）的參數的構造函數的類T

Original:

This example demonstrates perfect forwarding of the parameter of the function make_unique() to the argument of the constructor of class T

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#include <iostream>
#include <memory>
#include <utility>
 
struct A {
    A(int&& n) { std::cout << "rvalue overload, n=" << n << "\n"; }
    A(int& n)  { std::cout << "lvalue overload, n=" << n << "\n"; }
};
 
template<class T, class U>
std::unique_ptr<T> make_unique(U&& u)
{
    return std::unique_ptr<T>(new T(std::forward<U>(u)));
}
 
int main()
{
    std::unique_ptr<A> p1 = make_unique<A>(2); // rvalue
    int i = 1;
    std::unique_ptr<A> p2 = make_unique<A>(i); // lvalue
}

Output:

rvalue overload, n=2
lvalue overload, n=1

複雜性

常數

Original:

Constant

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另請參閱

move （C++11）	獲得一個右值引用 Original: obtains an rvalue reference The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. （函數模板）
move_if_noexcept （C++11）	獲得一個右值引用的舉動構造方法不拋出 Original: obtains an rvalue reference if the move constructor does not throw The text has been machine-translated via Google Translate. You can help to correct and verify the translation. Click here for instructions. （函數模板）