Why can't I overload ostream's << operator?

Question

EDIT: Passed Expression exp and string expression by const reference

I'm trying to allow a class to be display via cout in the following manner:

#include <iostream>

class Expression {
private:
    std::string expression;
public:
    Expression(const std::string& expression):
        expression(expression) { }
    friend std::ostream& operator <<(ostream& os, const Expression& exp) {
        return os << exp.expression; }
};

however, on compiling I get the errors:

main.cpp(9) : error C2061: syntax error : identifier 'ostream'
main.cpp(9) : error C2809: 'operator <<' has no formal parameters

this is especially confusing because VC++ is giving me ostream as an autocompletion suggestion when I enter std::. What's causing these errors, and how can they be resolved?

Answer 1

Surely you need std::ostream in all locations? i.e.:

friend std::ostream& operator <<(std::ostream& os, Expression& exp) ...
                                 ^^^

Answer 2

Without a using namespace std; clause (which has its own set of problems), you need to fully qualify all the iostream stuff.

You can see this with the following program:

#include <iostream>

class Expression {
private:
    std::string expression;
public:
    Expression(std::string expression):
        expression(expression) { }
    //                                  added this bit.
    //                                _/_
    //                               /   \
    friend std::ostream& operator <<(std::ostream& os, Expression& exp) {
        return os << exp.expression; }
};

int main (void) {
    Expression e ("Hi, I'm Pax.");
    std::cout << e << std::endl;
    return 0;
}

which prints out:

Hi, I'm Pax.

as expected.

---

And, as some comments have pointed out, you should pass the string as const-reference:

#include <iostream>

class Expression {
private:
    std::string expression;
public:
    Expression(const std::string& expression)
    : expression(expression) {
    }
    friend std::ostream& operator <<(std::ostream& os, const Expression& exp) {
        return os << exp.expression;
    }
};

int main (void) {
    Expression e ("Hi, I'm Pax.");
    std::cout << e << std::endl;
    return 0;
}