c++ 대소문자 변환 관련해서 질문 드립니다 ㅠㅠ

#include <iostream>
#include <algorithm>
#include <string>
#include <cctype>
using namespace std;
 
void toupper_selectively(string &target, const string &search){
	string::size_type pos = 0, search_len = search.size();
	while((pos = target.find(search, pos)) != string::npos){
		auto begin = target.begin() + pos, end = begin + search_len;
		for_each(begin, end, [](char &c){c = toupper(c);});
		pos += search_len;
	}
}
 
int main() {
	string target, search;
	getline(cin, target);
	getline(cin, search);
 
	toupper_selectively(target, search);
 
	cout << target << endl;
	return 0;
}