Storing floats to a string?

[Geralds Mod]

Is it possible to do this?
I have tried something like this but it has not worked:

float x = 9;
float y = 33;

string sum = x;
sum += "+";
sum += y;

I would want the outcome to look like "9+33"

I know that seems dumb but I am actually trying something on a larger scale.
I know I could use:

cout << x << "+" << y

but I need to display this a lot so I would like it to be a string.

Thanks.