c++ 演员的结果是高价值吗？

让

int a = 0;

那么(C)是标准C中的值？

不同的编译器对此代码显示不同的结果：

#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);
}

编译器具有不同的结果：

1) http://cpp.sh/2r6

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

)禁用.

我们可以看到,这应该是C标准草案5.4的显着类型转换(铸造符号)第1段的重要值(特别是prvalue)(强调我的)：

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 都会导致r值,这是预期的结果.

除此之外,我建议在web编译器上使用 rextester ,因为rextester可以让您共享程序,并且还可以实时共享.

更新

Ben Voigt指出这个 bug report ,所以似乎Visual Studio实际上产生了一个左值.所以这不仅仅是 extension which allows temporary objects to be bound to non-const lvalue references 的一个例子.

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

更新2

Mgetz提交了 bug report ,答复是这是通过使用 /Zc:rvalueCast flag 修复的,标志的描述如下：

When the /Zc:rvalueCast option is specified, the compiler correctly  identifies an rvalue reference type as the result of a cast operation  in accordance with the C++11 standard. When the option is not  specified, the compiler behavior is the same as in Visual Studio 2012.  By default, /Zc:rvalueCast is off. For conformance and to eliminate  errors in the use of casts, we recommend that you use /Zc:rvalueCast.

这个标志在未来的版本中是否默认启用是不清楚的.