What is the C++ equivalent of the % or .format operator in Python?

The C++20 std::format library serves this purpose:

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

For more info and guides on how to use it, see:

• https://en.cppreference.com/w/cpp/utility/format
• https://www.bfilipek.com/2020/02/extra-format-cpp20.html
• https://www.zverovich.net/2019/07/23/std-format-cpp20.html
• How can I use C++20 std::format?

However, <format> isn't provided in some standard library implementations yet — see C++20 library features. In the meantime you can use https://github.com/fmtlib/fmt, which is equivalent (and was the inspiration for <format>).

C++ has several ways to do IO, mostly for historical reasons. Whichever style your project uses should be used consistently.

1. C-style IO: printf, sprintf, etc.

#include <cstdio>

int main () {
  const char *name = "world";
  // other specifiers for int, float, formatting conventions are avialble
  printf("Hello, %s\n", name); 
}

2. C++ style IO: iostreams

#include <iostream>

int main() {
  std::string name = "world";
  std::cout << "Hello, " << name << std::endl;
}

3. Libraries/C++20 std::format:

Pre C++20 quite a few people have provided their own formatting libraries. One of the better ones is {fmt}. C++ adopted this kind of formatting as [std::format][2]

#include <format>
#include <iostream>
#include <string>

int main() {
  std::string name = "world";
  std::cout << std::format("Hello, {}", name) << std::endl;
}

Notice that format produces format-strings, so it works with both ways to do IO, and/or other custom approaches, but if you use C-style IO it would probably be mostly weird to layer std::format on top, where the printf specifier would also work.

printf("%i", 123456789);