如果要用单个空格替换任意数量的空格，则可以使用标准的unique(_copy)算法来做到这一点:

If the idea is to replace any number of spaces with a single space, then the standard unique(_copy) algorithm can be tricked to do that:

#include <iostream>
#include <string>
#include <algorithm>
#include <iterator>
#include <boost/lambda/lambda.hpp>

std::string remove_excessive_spaces(const std::string& s)
{
     using namespace boost::lambda;
     std::string result;
     //consider consequtive chars equal only if they are both spaces.
     unique_copy(s.begin(), s.end(), back_inserter(result), _1 == ' ' && _2 == ' ');
     return result;
}

int main(void)
{
    std::string s4="Hai ad                     asdasd                       asdasd";
    std::cout << remove_excessive_spaces(s4);
}