C++ stringstream function does not return correctly

Question

    #include <iostream>
    #include <stack>
    #include <string>
    #include <sstream>

    using namespace std;

    stack<char> aStack;
    stringstream result;
    stack<char> operand1;
    stack<char> operand2;


    stringstream &postfixExp(string ch){
      for(int i =0; i< ch.length(); i++)
      {
        if(ch[i]== '1' || ch[i]== '2' || ch[i]== '3' || ch[i]== '4' || ch[i]== '5' || ch[i]== '6' || ch[i]== '7' || ch[i]== '8' || ch[i]== '9' || ch[i]== '0' )
        {
          aStack.push(ch[i]);
        }

        else if( ch[i]== '+')
        {
          operand2.push(aStack.top());
          aStack.pop();

          operand1.push(aStack.top());
          aStack.pop();

      result << ( operand1.top() + operand1.top());
    }

  }

  return result;
}

int main()
{
    string postfix = " 2+3";

    stringstream* answer = &postfixExp(postfix);
    cout << "Result = " << answer->str() << endl;;


  return 0;
}

Hi everyone, does anyone know what is wrong with my code ? I do not see any error message from the compiler. However it crashes when I run it.

I'm having a difficult time to display the result which I got from my function. I initially wanted to use a stack function, but I couldn't think of how to pass the value to the main function and display it.

Then I'm thinking to use stringstream function instead. Unfortunately, I still don't know how to display the corresponding result

I wonder if anyone could tell me which part is wrong in my code, or if there is a better way to display the result from the function other than using stringstream or stack

Thanks a lot!

Answer 1

As it was pointed out already, error is caused by calling aStack.pop(); while the aStack container is empty. This probably yields undefined behavior that (in this case) you are able to observe as a crash of your application.

Explanation is simple, you process string "2+3" character by character:

for each character:
    if it is digit:
        push it to stack
    if it is '+':
        pop 2 elements

... so what do you think will happen in "2+3" string once the '+' sign is reached?

---

Also consider redesigning this:

stringstream result;

stringstream& postfixExp(string ch) {
    ...
    return result;
}

... you're returning a reference to global variable, which means you either shouldn't return anything or the variable shouldn't be global. Yet even better, consider passing only std::string objects instead (use stringstream locally within the function):

std::string postfixExp(const std::string& expression) {
    std::ostringstream resultStream;
    ...
    return result.str();
}

Answer 2

Test input for a postfix arithmetic operation should be in the form of "23+", not "2+3".

Some checks should probably be added to ensure you're not popping from an empty stack, as others have mentioned.