c++ 演員的結果是高價值嗎？

讓

int a = 0;

那麼(C)是標準C中的值？

不同的編譯器對此程式碼顯示不同的結果：

#include <iostream>
using namespace std;

void f(int& x)
{
cout << "l value" << endl;
}

void f(int&& x)
{
cout << "r value" << endl;
}

int main()
{
int a = 0;
f((int)a);
}

編譯器具有不同的結果：

1)ofollow,noindex" target="_blank">http://cpp.sh/2r6

2)http://webcompiler.cloudapp.net/

)禁用.

我們可以看到,這應該是C標準草案5.4的顯著型別轉換(鑄造符號)第1段的重要值(特別是prvalue)(強調我的)：

The result of the expression (T) cast-expression is of type T. The

result is an lvalue if T is an lvalue reference type or an rvalue

reference to function type and an xvalue if T is an rvalue reference

to object type;otherwise the result is a prvalue. [ Note: if T is

a non-class type that is cv-qualified, the cv-qualifiers are ignored

when determining the type of the resulting prvalue; see 3.10. —end

note ]

gcc 和clang 都會導致r值,這是預期的結果.

除此之外,我建議在web編譯器上使用rextester ,因為rextester可以讓您共享程式,並且還可以實時共享.

更新

Ben Voigt指出這個bug report ,所以似乎Visual Studio實際上產生了一個左值.所以這不僅僅是extension which allows temporary objects to be bound to non-const lvalue references 的一個例子.

因為dyp也指出了gcc used to have a cast to lvalue extension .

更新2

Mgetz提交了bug report ,答覆是這是通過使用/Zc:rvalueCast flag 修復的,標誌的描述如下：

When the /Zc:rvalueCast option is specified, the compiler correctlyidentifies an rvalue reference type as the result of a cast operationin accordance with the C++11 standard. When the option is notspecified, the compiler behavior is the same as in Visual Studio 2012.By default, /Zc:rvalueCast is off. For conformance and to eliminateerrors in the use of casts, we recommend that you use /Zc:rvalueCast.

這個標誌在未來的版本中是否預設啟用是不清楚的.