Why isn't "0" == "0"?

Question

I do not have UNICODE in this project. This is a WinAPI project and the variable "input" is a stringstream with the default value "0". Why does the first id statement run and not 2nd one even though the string itself IS "0"?

void calc::AddToInput(int number)
{
    MessageBox(NULL, input.str().c_str(), "Input", NULL); //Shows "0"

    if(input.str().c_str() != "0") //Runs even though it shouldn't
    {
        input << number;
    }
    else if(input.str().c_str() == "0") //Doesn't run though it should
    {
        input.str("");
        input << number;
    }
}

Answer 1

Comparing C-style strings with == means "Do the first elements of these strings have the same address?". It doesn't actually compare the contents of the strings. For that, you need strcmp.

However, you have no reason to compare C-style strings - just use the std::string returned from str(), which can be compared using ==, like so: input.str() != "0".

Answer 2

Because you're comparing pointers, not strings. To compare strings either A) just compare the std::string instead of using c_str() (best), or B) use strcmp.