要用istrstream哦！

#include <iostream>
#include <string>
#include<sstream>
int main() {
    std::string str,to_replace,replacement,word;
    std::getline(std::cin,str);
    std::cin>>to_replace>>replacement;
    std::istringstream iss(str);
    while (iss >> word) {
        if (word==to_replace) std::cout << replacement << ' ';
        else std::cout<<word<<' ';
    }
    return 0;
}