operator<< stackoverflow

Consider the following code :

class TextMessage{
public :
    TextMessage(){};
    TextMessage(std::string _text):text(_text){}
    std::string text;
    friend std::ostream & operator<<( std::ostream & os, const TextMessage & m);
};
std::ostream & operator<<( std::ostream & os, const TextMessage & m){
    return os << "text message : " << m.text;
}

Why on earth :

• does Visual 2010 issue a C4717 warning in operator <<
• does std::cout << textMsgInstance;开发者_如何学JAVA crashes by stackoverflow as predicted by Visual ?

Btw, replacing m.text by m.text.c_str() works.

I'm guessing that you failed to #include <string>. Thus, when the compiler comes to output a std::string, it can't, and starts looking for implicit conversions- and your implicit constructor to a TextMessage looks like just the bill. But wait- now we're outputting a TextMessage in the TextMessage's output function, and bam.

Only thing I can think of is that it doesn't have an operator<< for std::string so it looks for a conversion and finds the one argument constructor TextMessage(std::string).

It is often advisable to prevent unexpected calls to one argument constructors by making them explicit.

explicit TextMessage(std::string _text):text(_text){}

Then it will not consider the constructor for implicit conversions.

Its because m.text is std::string and it gets converted inside the operator to TextMessage and the operator is called again.