Replace the last word of a string with number

#include <iostream>
#include <string>

int main() {
    std::string str = "The quick brown fox";
    std::string new_word = "42";
    size_t last_space = str.find_last_of(" ");
    str.replace(last_space + 1, str.size() - last_space, new_word);
    std::cout << str << std::endl;
    return 0;
}

Output:

The quick brown 42

#include <iostream>
#include <string>

int main() {
    std::string str = "The quick brown fox";
    std::string new_word = "42";
    size_t first_space = str.find_first_of(" ");
    str.replace(0, first_space, new_word);
    std::cout << str << std::endl;
    return 0;
}

Output:

42 quick brown fox