#include <stdio.h>
#include <string>
#include <functional>

using namespace std;

string replace (const char *pt, std::function<const char*(const char)> func)
{
	string ret;
	while (*pt != '\0'){
		const char *p = func(*pt);
		if (p != nullptr){
			ret += p;
		}else{
			ret += *pt;
		}
		pt++;
	}
	return ret;
}

int main ()
{
	string ret = replace("allo",[](auto a){return a=='a' ? "AA" : nullptr;});
	printf ("ret=%s\n",ret.c_str());
	return 0;
}