Accessing private class in operator<< in nam

问题:

I have a class CFoo with a private inner class CBar. I want to implement a stream ouput operator for CFoo, which in turn uses a stream output for CBar in it's implementation. I can get this working when CFoo is in the common namespace, but when i place it in a new namespace (namespace foobar), the operator can no longer access the private inner class. I suspect this has something to do with the full signature of the operator, but I can't figure out the correct way to specify the friend declaration and the actual operator declaration so the implementation compiles. Can anyone suggest what I might be missing? Note that it will compile if the stream implementation is done inline in the header, but I hate to expose implementation like this unnecessarily!

in foobar.h (just comment out the usefoobarnamespace to test the non-namespaced version):

#define usefoobarnamespace
#ifdef usefoobarnamespace
namespace foobar
{
#endif // usefoobarnamespace
    class CFoo
    {
    public:
        CFoo() {}
        ~CFoo();
        void AddBar();
    private:
        class CBar
        {
        public:
            CBar() {m_iVal = ++s_iVal;}
            int m_iVal;
            static int s_iVal;
        };

        std::vector<CBar*> m_aBars;

        friend std::ostream& operator<<(std::ostream& rcStream, CFoo& rcFoo);
        friend std::ostream& operator<<(std::ostream& rcStream, CFoo::CBar& rcBar);
    };
    std::ostream& operator<<(std::ostream& rcStream, CFoo& rcFoo);
    std::ostream& operator<<(std::ostream& rcStream, CFoo::CBar& rcBar);
#ifdef usefoobarnamespace
}
#endif // usefoobarnamespace

and in foobar.cpp:

#ifdef usefoobarnamespace
using namespace foobar;
#endif // usefoobarnamespace

int CFoo::CBar::s_iVal = 0;


CFoo::~CFoo()
{
    std::vector<CBar*>::iterator barIter;
    for (barIter = m_aBars.begin(); barIter != m_aBars.end(); ++barIter)
    {
        delete (*barIter);
    }
}

void CFoo::AddBar()
{
    m_aBars.push_back(new CBar());
}


std::ostream& operator<<( std::ostream& rcStream, CFoo& rcFoo )
{
    rcStream<<"CFoo(";
    std::vector<CFoo::CBar*>::iterator barIter;
    for (barIter = rcFoo.m_aBars.begin(); barIter != rcFoo.m_aBars.end(); ++barIter)
    {
        rcStream<<(*barIter);   
    }
    return rcStream<<")";
}

std::ostream& operator<<( std::ostream& rcStream, CFoo::CBar& rcBar )
{
    return rcStream<<"CBar("<<rcBar.m_iVal<<")";
}

回答1:

Simply put the code in the .cpp file into the namespace:

namespace foobar {

// your existing code

}

回答2:

You need to put the operator definitions explicitly in a namespace. (Or fully qualify them with the namespace). The way you are doing it you declare some << operators (that are in namespace foobar), then you define some completely new << operators in the global namespace.

namespace foobar
{
    std::ostream& operator<<( std::ostream& rcStream, CFoo& rcFoo )
    {
        rcStream<<"CFoo(";
        std::vector<CFoo::CBar*>::iterator barIter;
        for (barIter = rcFoo.m_aBars.begin(); barIter != rcFoo.m_aBars.end(); ++barIter)
        {
            rcStream<<(*barIter);   
        }
        return rcStream<<")";
    }

    std::ostream& operator<<( std::ostream& rcStream, CFoo::CBar& rcBar )
    {
        return rcStream<<"CBar("<<rcBar.m_iVal<<")";
    }
}

回答3:

Your operator<< functions are now in the foobar namespace, so you should define them as foobar::operator<<.

回答4:

The issue can be resolved by specializing the stream operator overloading for the namespace:

std::ostream& foobar::operator<<( std::ostream& rcStream, CFoo& rcFoo )
{
    rcStream<<"CFoo(";
    std::vector<CFoo::CBar*>::iterator barIter;
    for (barIter = rcFoo.m_aBars.begin(); barIter != rcFoo.m_aBars.end(); ++barIter)
    {
        rcStream<<(*barIter);   
    }
    return rcStream<<")";
}

std::ostream& foobar::operator<<( std::ostream& rcStream, CFoo::CBar& rcBar )
{
    return rcStream<<"CBar("<<rcBar.m_iVal<<")";
} 

By default, the global definitions of these functions are getting overloaded. They are not friends of class CFoo and cannot access its private members.