Modern C++ makes this super simple.

C++20

C++20 introduces std::format, which allows you to do exactly that. It uses replacement fields similar to those in python:

#include <iostream>
#include <format>
 
int main() {
    std::cout << std::format("Hello {}!\n", "world");
}

Code from cppreference.com, CC BY-SA and GFDL

Check out the compiler support page to see if it's available in your standard library implementation. As of 2021-11-28, partial support is available in Visual Studio 2019 16.10, which was released on 2021-05-25 and Clang 14, which is tracked here. In all other cases, you can resort to the C++11 solution below, or use the {fmt} library, which has the same semantics as std::format.