C++ function call in cout leads to "Reference to non-static member function must be called"

Question

I wanted to print some properties directly to cout. Luckily I found a solution for static calls like this:

// somewhere:
ostream & Foo(ostream &ostr){
    ostr << "Foo";
    return ostr;
}

and another static call where it can be called directly like that:

// somoewhere else:
void doSomething(){
    cout << 123 << Foo << endl;
}

This works fine and prints simply "123Foo". But I want to use this for an object I have, so I could print its properties. In my class A I have a property b of the class B. There the function is implemented similar to the static one before:

ostream & Foo(ostream &ostr) {
    ostr << "my foo is 123";
    return ostr;
}

but it doesn't compile. I also tried to specify the namespace like

ostream & B::Foo(ostream &ostr) { ... }

and assumed I could just call the object's function like this:

cout << 123 << b->Foo << endl;

but it always tells me there is an error: "Reference to non-static member function must be called".

I just can't get this done. What am I doing wrong? Is this even possible?

NOTE: I don't want to overload the << operator or anything. I don't want a "toString()" method or anything similar but would like to understand why this instance method can not be called like anywhere else. As a Java developer I would have guessed something like "My object " + b.foo() would be possible. But it seems, it is not.

Answer 1

The problem is that your standalone foo is a function, whereas b->foo is, according to C++, not one. Namely, the latter binds its this argument, so it requires some extra data on top of just the function's behavior. You can make it a valid std::function with an explicit lambda.

cout << 123 << [&b_object](ostream& out) { return b_object->foo(out); } << endl;

and if that's your only intended use case for this function, then you can return it from B::Foo directly.

// Be careful with lifetimes! The return value cannot
// outlive `this`!
std::function<ostream&(ostream&)> B::foo() {
  return [&this](ostream& out) { out << "my foo is " << my_foo; return out; };
}

Then you can write

cout << 123 << b_object->foo() << endl;

But, as indicated in the comments, it's much more common to use a specialized datatype for this. If the property you're trying to print is cheap, then just return it or a reference to it from foo() and you're done. If not, or if you want some complicated custom printing operation to happen, I recommend writing your own custom class and encapsulating this custom print behavior.

class ViewOfObjectB {
private:
  B& owner;
public:
  ViewOfObjectB(B& owner) : owner(owner) {}
  friend ostream& operator<<(ostream&, const ViewOfObjectB&);
};

ostream& operator<<(ostream& out, ViewOfObjectB& obj) {
  // Whatever you want to print goes here ...
  return out;
}

class B {
public:
  ViewOfObjectB foo() {
    return ViewOfObjectB(&this);
  }
};

Same idea as using a lambda, but since there's a ton of hidden complexity, we make that complexity explicit with a named view class that we can document independently as our code grows and becomes more complicated. You still call it using

cout << 123 << b_object->foo() << endl;

All of these calls are static (in the sense of "no vtable involved", not in the sense of the static keyword), so a clever optimizer should be able to inline it all for you.