命名空间中的好友功能 [英] friend function within a namespace

问题描述

当名称空间中包含一个朋友功能时，其定义需要以名称空间为前缀进行编译，这是示例代码:

When a friend function is included in the namespace, its definition needs to be prefixed with namespace to compile it, here is the sample code:

test.h:

#ifndef TEST_H
#define TEST_H
namespace TestNamespace
{
    class TestClass
    {
    public:
        void setValue(int &aI);
        int value();
    private:
        int i;
        friend void testFunc(TestClass &myObj);
    };
void testFunc(TestClass &myObj);
}
#endif

test.cpp:

#include "test.h"

using namespace TestNamespace;

void TestClass::setValue(int &aI)
{
    i=aI;
}

int TestClass::value()
{
    return i;
}

void testFunc(TestClass &myObj)
{
    int j = myObj.i;
}

编译上面的代码会给出错误:

Compiling above code give the error :

1>c:\qtprojects\namesp\test.cpp(17) : error C2248: 'TestNamespace::TestClass::i' : cannot access private member declared in class 'TestNamespace::TestClass'
1>        c:\qtprojects\namesp\test.h(11) : see declaration of 'TestNamespace::TestClass::i'
1>        c:\qtprojects\namesp\test.h(6) : see declaration of 'TestNamespace::TestClass'

但是，如果我使用

void TestNamespace::testFunc(TestClass &myObj)
{
    int j = myObj.i;
}

它会编译，为什么函数需要使用名称空间TestNamespace :: testFunc而不是类作为前缀，类TestClass和函数testFunc都包含在标头的名称空间中.

It compiles, Why the function needs to be prefixed with namespace TestNamespace::testFunc but not the class, Both the class TestClass and function testFunc are included in the namespace in header.

推荐答案

testFunc方法中正在使用TestClass.由于您已包含名称空间"using namespace TestNamespace;"，因此可以正常工作.

The TestClass is being used in the testFunc method. Since you have included the namespace "using namespace TestNamespace;" it works fine.

对于要定义实现的testFunc方法，您需要告诉编译器testfunc属于名称空间TestNamespace:

For testFunc method you are defining the implementation, you need to tell compiler that testfunc belongs to namespace TestNamespace:

您可以执行以下操作之一:

you can do it either:

.cpp

namespace TestNamespace
{
    void testFunc(TestClass &myObj)
     {
        int j = myObj.i;
     }
 }

或

void TestNamespace::testFunc(TestClass &myObj)
{
    int j = myObj.i;
}

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