Programming :: If Test Validation - Using Char Arrays

I'm writing a code to get the index of the last occurrence of a given substring in a string.

Code:
int StringHandler::lastIndexOf(string src, const string s) {
unsigned int lastIndex = string::npos;
if (src.find(s, 0) == string::npos) {
return -1; } else {
bool isLast = false; unsigned int i = 0; do {
lastIndex = src.find(s, i);
if (lastIndex != string::npos) { i = lastIndex + 1;
if (i == src.size()) { isLast = true;
} } else { isLast = true;
} } while (!isLast);
} return lastIndex;
}

My problem is: execution steps into an if with a false condition! Check the values of i and src.size() at the right panel. How could be possible for the program to run the highlighted statement? The if condition above is false! [URL]. I could use char arrays, for example, but this kind of false validation has happened to me more than once, I'm using G++ 4.5 with these flags: -O0 -g -Wall -c