You can resize a c++ string using string::resize.
// resizing string
#include <iostream>
#include <string>
int main ()
{
std::string str ("I like to code in C");
std::cout << str << '\n';
unsigned sz = str.size();
.resize (sz+2,'+');
std::cout << str << '\n'; //I like to code in C++
str.resize (14);
std::cout << str << '\n';//I like to code
return 0;
}