c++ - 写 "::namespace::identifier"和 "namespace::identifier"有什么区别？

Question

我在代码中看到了这两种方法。你能解释一下这两者有什么区别吗？正如我认为它与 C++ 完成命名空间查找的方式有关，您能否也提供一些相关信息，或者提供一个好的文档的链接？谢谢。

Answer 1

示例:

#include <cstdio>

namespace x {
    const int i = 1;
}

namespace y {
    namespace x {
        const int i = 2;
    }

    void func()
    {
        std::printf("x::i = %d\n", x::i);
        std::printf("::x::i = %d\n", ::x::i);
    }
}

int main()
{
    y::func();
    return 0;
}

输出:

x::i = 2
::x::i = 1

Explanation:

• When you refer to an identifier like x::i, the definition used is the "closest" x::i. Inside ::y::func, the definition ::y::x::i is closer than the definition ::x::i. By contrast, there is no such function ::y::std::printf so ::std::printf is used instead.

• When you refer to an identifier like ::x::i, there is no possible ambiguity: it looks for a top-level namespace named x, then finds an i inside.

So using :: at the beginning allows you to spell the full name of a global something. This also allows you to distinguish between local and global variables.

Example 2:

#include <cstdio>
const int x = 5;
int main()
{
    const int x = 7;
    std::printf("x = %d\n", x);
    std::printf("::x = %d\n", ::x);
    return 0;
}

输出:

x = 7
::x = 5