取临时对象的地址 [英] taking the address of a temporary object

问题描述

§5.3.1一元运算符，第3节

§5.3.1 Unary operators, Section 3

operator是指向其操作数的指针。 操作数应为小值或限定ID。

应为在这种情况下？这是否意味着采取临时地址的地址是错误的？我只是想知道，因为g ++只给我一个警告，而gotau拒绝编译以下程序：

What exactly does "shall be" mean in this context? Does it mean it's an error to take the address of a temporary? I was just wondering, because g++ only gives me a warning, whereas comeau refuses to compile the following program:

#include <string>

int main()
{
    &std::string("test");
}

g ++ 警告：采取临时

comeau 错误地址：表达式必须是左值或函数指示符

comeau error: expression must be an lvalue or a function designator

有人有Microsoft编译器或其他编译器，可以测试这个程序吗？提前感谢。

Does anyone have a Microsoft compiler or other compilers and can test this program, please? Thanks in advance.

推荐答案

标准语言中的shall一词意为严格的要求。所以，是的，你的代码是错误的（它是一个错误），因为它试图将运算符应用到非值的对象。

The word "shall" in the standard language means a strict requirement. So, yes, your code is ill-formed (it is an error) because it attempts to apply address-of operator to a non-lvalue object.

但是，这里的问题不是尝试采取临时的地址。问题是，再次采取非空值的地址。临时对象可以是左值或非右值，这取决于生成该临时值的表达式或提供对该临时值的访问。在你的情况下，你有 std :: string（test） - 一个函数式样转换为非引用类型，根据定义产生非左值。因此错误。

However, the problem here is not an attempt of taking address of a temporary. The problem is, again, taking address of a non-lvalue. Temporary object can be lvalue or non-lvalue depending on the expression that produces that temporary or provides access to that temporary. In your case you have std::string("test") - a functional style cast to a non-reference type, which by definition produces a non-lvalue. Hence the error.

如果你想取一个临时对象的地址，你可以这样做，例如

If you wished to take address of a temporary object, you could have worked around the restriction by doing this, for example

const std::string &r = std::string("test");
&r; // this expression produces address of a temporary

，只要临时存在，结果指针保持有效。还有其他方法合法获取临时对象的地址。这只是你的具体方法恰巧是非法的。

whith the resultant pointer remaining valid as long as the temporary exists. There are other ways to legally obtain address of a temporary object. It is just that your specific method happens to be illegal.

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