c++ - 将一个命名空间数据成员访问到另一个

Question

如何从 n1 命名空间访问 y(位于命名空间 n2 内):测试代码如下:

#include<iostream>
using namespace std;

namespace n1
{
    int x =  20;
    int m = ::n2::y;
    void printx()
    {
        cout << "n1::x is " << x << endl;
        cout << "n2::y is " << m << endl;
    }
}

namespace n2
{
    int y = 10;
}

int main()
{
    cout << n1::x << endl;
    n1::printx();
    cout << n2::y << endl;

    return 0;
}

我遇到以下错误:test.cpp:7:15: 错误: ‘::n2’ 尚未声明 int m =::n2::y;

Answer 1

只需更改顺序，使 n2 在 n1 中可解析:

#include<iostream>
using namespace std;


namespace n2
{
    int y = 10;
}

namespace n1
{
    int x =  20;
    int m = n2::y;
    void printx()
    {
        cout << "n1::x is " << x << endl;
        cout << "n2::y is " << m << endl;
    }
}


int main()
{
    cout << n1::x << endl;
    n1::printx();
    cout << n2::y << endl;

    return 0;
}