Your code is fine. What you are seeing is a linker issue.
If you put your code in a single file like this:
#include <iostream>
#include <string>
using namespace std;
string trim(const string& str)
{
size_t first = str.find_first_not_of(' ');
if (string::npos == first)
{
return str;
}
size_t last = str.find_last_not_of(' ');
return str.substr(first, (last - first + 1));
}
int main() {
string s = "abc ";
cout << trim(s);
}
then do g++ test.cc and run a.out, you will see it works.
You should check if the file that contains the trim function is included in the link stage of your compilation process.