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I need to overload the << operator for streams to work with built-in types. For strings it's not a problem, since I simply overload the function like this:

ostream& operator<<(ostream& os, const char* str) { /*...*/ }

This works because this function is global, not a member. The problem is that I need to overload the << operator for other primitive types (ints, floats, etc) but those are member functions. Is there a way I can do this? I need it to work with not only cout but other streams as well. Thanks in advance.

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    Not sure what you mean … operator << already exists for ostream& and built-in types. No need to overload it. Same for const char*. Commented Nov 6, 2010 at 13:49
  • you mean, you need to have 2 overloaded functions for <<. One global and one member. However, the parameters differ [ global for string types and member function for other datatypes ]. Right ??? Commented Nov 6, 2010 at 13:52
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    I know there is already an overload for ostream&. But I have to overload it because I need it to behave differently (not just simply printing out the characters). Commented Nov 6, 2010 at 14:30
  • How do you expect the compiler to differentiate between your new overload and an existing one if the signature is the same or compatible - meaning the call is ambiguous.? Commented Nov 6, 2010 at 14:56

2 Answers 2

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You shouldn't try to change what the operator in std::cout << 3; does. It's part of a standard API. If you need to output in some format which stream manipulators can't support, then for example you could write a little wrapper:

struct MyFormatter {
    MyFormatter (ostream &o) : o(o) {}
    ostream &o;
};

MyFormatter &operator<<(MyFormatter &mf, int i) {
    mf.o << "int(" << i << ")"; // or whatever
    return mf;
}

Then use it like this:

MyFormatter mf(std::cout);
mf << 1 << "," << 2 << "," << 3;
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7 Comments

Thx for the answer Steve. You mentioned I shouldn't change the << operator for cout. The problem is I should exactly do that. It's a homework. The task is to write a program based on manipulators that reverses the content between specific tags. Example: cout<<start<<"abc"<<end . Now the "abc" string should be printed out "cba". "start" and "end" will be manipulators but I still have to overload the << operator to get this working. Now I have succesfully done it with strings but have problem with other primtive types.
@Mirko: I don't think you can do that strictly with manipulators. A manipulator is a function (or function-like object) which, when written to a stream, is called with that stream as a parameter. All they can do is set state on the stream, and cout only has certain format flags and so on available to be manipulated. I suspect that what you want is to have cout << start return, not cout, but a wrapper. But without having seen the assignment I'm not sure, I may have missed something.
Anyway, I'm pretty sure you simply cannot provide an overload for operator<<(ostream&, int), because it would be ambiguous with the existing overload.
cout<<start returning a wrapper looks like a good idea. I'll give it a try. Thx for the hint.
I've tried the wrapper but it doesn't work. I've defined the start manipulator to take an ostream& argument and return MyFormatter&. But the manipulator function is simply not called. When I try to use it like this: cout<<start<<"abc"<<end , it prints out "1abc1". I guess it prints out the ones because of some kind of error.
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In C++, operator overloads require at least one operand of a "class type" or enumeration type.
The point is you are not allowed to overload operator for primitive types.
http://www.parashift.com/c++-faq-lite/intrinsic-types.html#faq-26.10

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