C++ segmentation fault

Question

I'm new to C++ and the memory nuances that are needed to write and debug the language. Can anyone tell me why the following code is giving me a segmentation fault?

string Polynomial::toString(){
    int i, exponent;
    stringstream result;

    for (i = 0; i < coeffs.size(); i++){

        // For first non-zero coefficient
        if (result.str().empty()){
            if(coeffs[i] < 0)
                result << "-";
            if(coeffs[i] != 0)
                result << coeffs[i];
        }
        else{
            if(coeffs[i] < 0)
                result << " - " << abs(coeffs[i]);
            else if(coeffs[i] > 0)
                result << " + " << coeffs[i];
        }

        exponent = (coeffs.size() - i - 1);
        if (coeffs[i] != 0){
            if (exponent > 1)
                result << coeffs[i] << "x^" << exponent;
            else if(exponent == 1)
                result << coeffs[i] << "x";
        }
    }

    result.str();
}

Answer 1

Your function is missing a return statement (probably just the return part). According to the standard, reaching the closing brace on any function besides main that has a return type is undefined behaviour, as per §6.6.2/2 of the C++11 standard, shown below, which usually results in a crash, though it might not always do so.

Other than that, there's nothing that could cause undefined behaviour or a crash. What you want to do is add the return part:

return result.str();

For what it's worth, GCC 4.7.2 gives the following warning:

warning: no return statement in function returning non-void [-Wreturn-type]

Always take advantage of the warnings compilers can and will give you.

---

Standard reference:

Flowing off the end of a function is equivalent to a return with no value; this results in undefined behavior in a value-returning function.

Answer 2

You are probably calling the function and assigning the result to something:

Polynomial p = ....;
std::string s = p.toString();

Since you have no return statement in Polynomial::toString(), which in itself is undefined behaviour, this can easily result in a segmentation violation. You can easily fix this by returning the stringstreams's `string:

return result.str();