Program crashes - probably on type conversion

[redpaperlantern]

Hey everybody!
My program crashes... and I have no real Idea why.
The basic task here is to implement a program which converts a infix-notation to a postfix-notation. I've done that, it works fine. But on solving the notation, the program somehow crashed.

Two additional notes: We have to use a self-written stack. We can't use the one from the stl. This stack contains strings. The front-function gives the top element, the pop deletes it, the push adds one. Anything else is pretty much stl.

I feel that the program crashes on a conversion.
I'm doing several, all work the same. If anyobody might tell me if this stuff here works, and if not, why not, I'd be glad:

if (income.front() == "*" )
         {
         std::stringstream bb(neues.top());
         neues.pop();
         std::stringstream aa(neues.top());
         neues.pop();
         float a, b;
         bb >> b;
         aa >> a;
         float c = a*b;
         std::string result;
         std::ostringstream neuest;
         neuest << c;
         result = neuest.str();
         neues.push(result);
         }

Thanks already!

[redpaperlantern]

The problem was not the type conversion.

Since I did not pop the upper element of the income-list, the element would always stay in the list. The loop, however, still popped the elements from the neues-stack, and boom, an empty stack and a pop-function don't match that well.

Thanks to anyone thinking about this.