C++ – Return value for a << operator function of a custom string class in C++


I am trying to create my own std::string wrapper to extend its functionality.
But I got a problem when declaring the << operator.
Here's my code so far:

my custom string class:

class MyCustomString : private std::string
  std::string data;
  MyCustomString() { data.assign(""); }
  MyCustomString(char *value) { data.assign(value); }
  void Assign(char *value) { data.assign(value); }
  // ...other useful functions
  std::string & operator << (const MyCustomString &src) { return this->data; }

the main program:

int main()
  MyCustomString mystring("Hello");
  std::cout << mystring; // error C2243: 'type cast' : conversion from 'MyCustomString *' to 'const std::basic_string<_Elem,_Traits,_Ax> &' exists, but is inaccessible

  return 0;

I wanted cout to treat the class as a std::string, so that I won't need to do something like:

std::cout << mystring.data;

Any kind of help would be appreciated!


Just fyi: my IDE is Microsoft Visual C++ 2008 Express Edition.

Best Solution

If you look at how all stream operators are declared they are of the form:

ostream& operator<<(ostream& out, const someType& val );

Essentially you want your overloaded function to actually do the output operation and then return the new updated stream operator. What I would suggest doing is the following, note that this is a global function, not a member of your class:

ostream& operator<< (ostream& out, const MyCustomString& str )
    return out << str.data;

Note that if your 'data' object was private, which basic OOP says it probably should, you can declare the above operator internally as a 'friend' function. This will allow it to access the private data variable.

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