C++从string中删除所有的某个特定字符

#include   <string>   
#include   <iostream>   
using   namespace   std;   
string&   replace_all(string&   str,const   string&   old_value,const   string&   new_value)   
{   
    while(true)   {   
        string::size_type   pos(0);   
        if(   (pos=str.find(old_value))!=string::npos   )   
            str.replace(pos,old_value.length(),new_value);   
        else   break;   
    }   
    return   str;   
}   
string&   replace_all_distinct(string&   str,const   string&   old_value,const   string&   new_value)   
{   
    for(string::size_type   pos(0);   pos!=string::npos;   pos+=new_value.length())   {   
        if(   (pos=str.find(old_value,pos))!=string::npos   )   
            str.replace(pos,old_value.length(),new_value);   
        else   break;   
    }   
    return   str;   
}   
int   main()   
{   
    cout   <<   replace_all(string("12212"),"12","21")   <<   endl;   
    cout   <<   replace_all_distinct(string("12212"),"12","21")   <<   endl;   
}   
/*
输出如下:   
22211   
21221
*/