c++ - 不具名命名空間不可以放在.h

Question

不具名命名空間不可以放在.h，可以舉例為什麼不行？
我想來想去沒找到例子。

Answer 1

關鍵看怎麼用，在unnamed namespace中放什麼東西，一個例子，如果放變量，就有問題了。

//one.h
#include <iostream>
#include <typeinfo>

namespace {
class TestClass {
};
int i;
}

const std::type_info& one_get_TestClass_Info();
const std::type_info& two_get_TestClass_Info();

//one.cpp
#include "one.h"
const std::type_info& one_get_TestClass_Info()
{
    i = 10;
    std::cout << "val: " << i << "   addr: " << &i << std::endl;
    return typeid(TestClass);
}

//two.cpp
#include "one.h"
#include <iostream>
#include <typeinfo>
using namespace std;

const std::type_info& two_get_TestClass_Info()
{
    std::cout << "val: " << i << "   addr: " << &i << std::endl;
    return typeid(TestClass);
}
//main.cpp

#include "one.h"
using namespace std;

int main()
{
    const std::type_info& t1 = one_get_TestClass_Info();
    const std::type_info& t2 = two_get_TestClass_Info();
    std::cout << "one has type: " << t1.name() << '\n'
              << "two has type: " << t2.name() << '\n';
    if (t1 == t2) {
        cout << "same type";
    }

    return 0;
}

輸出為：

val: 10   addr: 0x602200
val: 0   addr: 0x602208
one has type: N12_GLOBAL__N_19TestClassE
two has type: N12_GLOBAL__N_19TestClassE

稍微詳細點，看這裡 : https://zsounder.github.io/20...。