“在命名空间内的类中使用带有运算符重载的stringstream时，不匹配'运算符>>'”

Question

I'm trying to overload the >> operator in a class inside a namespace, but as soon as I try to use it with a string stream it doesn't work. Here's a distilled version of my code:

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

namespace Foo {
    class Bar {
    public:
        string str;
        friend istream& operator >>(istream& in, Bar& t);
    };
}

inline istream& operator >>(istream& in, Foo::Bar& t) {
    in >> t.str;
    return in;
}

int main() {
    Foo::Bar foo;
    stringstream("foo") >> foo;

    cout << foo.str << endl;

    return 0;
}

and here's the error:

main.cpp:22:22: error: no match for ‘operator>>’ (operand types are ‘std::stringstream {aka std::__cxx11::basic_stringstream<char>}’ and ‘Foo::Bar’)

The thing is these other ways of doing it work:

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

namespace Foo {
    class Bar {
    public:
        string str;
        friend istream& operator >>(istream& in, Foo::Bar& t) {
            in >> t.str;
            return in;
        }
    };
}



int main() {
    Foo::Bar foo;
    stringstream("foo") >> foo;

    cout << foo.str << endl;

    return 0;
}

#include <iostream>
#include <string>
#include <sstream>

using namespace std;

class Bar {
public:
    string str;
    friend istream& operator >>(istream& in, Bar& t);
};

inline istream& operator >>(istream& in, Bar& t) {
    in >> t.str;
    return in;
}

int main() {
    Bar foo;
    stringstream("foo") >> foo;

    cout << foo.str << endl;

    return 0;
}

The thing is, I have no idea why the first way of doing it should be wrong. I'm using the g++ compiler on linux if that helps. Could someone help me understand what's going on?

Answer 1

Thanks to hints from Sam Varshavchik (in the comments above), I've been able to come up with a correct version of the first version:

#include <iostream>
#include <string>
#include <sstream>

namespace Foo {
    class Bar {
    public:
        std::string str;
        friend std::istream& operator >>(std::istream& in, Bar& t);
    };

    std::istream& operator >>(std::istream& in, Bar& t);
}

std::istream& Foo::operator >>(std::istream& in, Foo::Bar& t) {
    in >> t.str;
    return in;
}

using namespace std;

int main() {
    Foo::Bar foo;
    stringstream("foo") >> foo;

    cout << foo.str << endl;

    return 0;
}

The key was making sure that the operator>> function was both declared and defined in the same scope. I still wanted to be able to define the function outside of the namespace braces, so I had to add a declaration inside the namespace so the compiler would know that there's supposed to be that function in the namespace. Keeping the function definition separate allowed me to separate my code into three files, main.cpp, foo.hpp, and foo.cpp:

// main.cpp

#include <iostream>
#include <string>
#include <sstream>

#include "foo.hpp"

using namespace std;

int main() {
    Foo::Bar foo;
    stringstream("foo") >> foo;

    cout << foo.str << endl;

    return 0;
}

// foo.hpp

#ifndef FOO_HPP
#define FOO_HPP

#include <string>
#include <iostream>

namespace Foo {
    class Bar {
    public:
        std::string str;
        friend std::istream& operator >>(std::istream& in, Bar& t);
    };

    std::istream& operator >>(std::istream& in, Bar& t);
}

#endif

// foo.cpp

#include "foo.hpp"

std::istream& Foo::operator >>(std::istream& in, Foo::Bar& t) {
    in >> t.str;
    return in;
}

Anyways, thanks so much for the help! And thanks for not hand-feeding me a solution; it's so much better to learn by figuring it out myself, even if I did get some help pointing me in the right direction.