Differences in const in function declarations

[SMuzzlim4]

If a function is declared as

virtual const string toString() const;

what do each of the const declarations do?

[helios]

The first one means "a constant string". The second one means "the function may used with constant this arguments (and therefore can't modify its members)".

[SMuzzlim4]

When does using this become constant? Also, should const be used in the return value only when the value return is a literal?

[helios]

const T object;
T.function(); //if function() isn't const, this gives a compiler error 

[SMuzzlim4]

I was wondering if this function's return value should be declared const.

string Object::toString() const {
	stringstream string_stream;
	string_stream << this;
	return string_stream.str();
}

I'm trying to experiment with returning a const value, and trying to throw a run-time error, but I can't seem to do so. I've tried returning "test", and then doing object.toString[1] = '1', but it ran just fine.

[jsmith]

Yes, it should be const as long as operator<< does not modify Object (which it shouldn't; I say that on the general principle that output of some data shouldn't modify the data).