What are Inline Namespaces?

24 Apr 2021

Inline namespaces are transparent for those who are using the outer namespace. This is useful if you want to allow to narrow down the functions/classes/constants used, but do not want to add some more clutter if somebody wants to have them all.

A simple example:

Simple Example

#include <iostream>
namespace m42e {
namespace cpp {
namespace test {
const int32_t my42 = 42;
inline namespace nested {
const int32_t my23 = 23;
}  // namespace nested
}  // namespace test
}  // namespace cpp
}  // namespace m42e

int main() { 
  // this works for sure
  std::cout << m42e::cpp::test::my42 << std::endl;
  std::cout << m42e::cpp::test::nested::my23 << std::endl;
  {
    using namespace m42e::cpp::test;
    // of course we can use my42 here
    std::cout << my42 << std::endl;
    // as nested is inline it is "transparent"
    std::cout << my23 << std::endl;
  }
  
  return 0; 
}