remove trailing spaces

trailing_spaces.cpp

#include <iostream>
#include <string>
#include <cstddef>

void remove_trailing_spaces(std::string& str)
{
    bool trailing{false};
    int prev_begin{};
    int removed_spaces{};

    for(std::size_t i = 0; i < str.size(); ++i)
    {
        if(str[i] == ' ')
        {
            if(not trailing)
            {
                prev_begin = i + 1 - removed_spaces;
                trailing = true;
            }
            else
            {
                removed_spaces++;
            }
        }
        else
        {
            trailing = false;
            std::swap(str[prev_begin++], str[i]);
        }
    }

    str.erase(str.size() - removed_spaces, removed_spaces);
}

int main()
{
    std::string str{"aaa    bbbb     c       g"};
    remove_trailing_spaces(str);
    std::cout << '"' << str << '"' << '\n';
}