title: C# extension methods analogue in C++
body: <p>C# has this little nice feature: <a href="https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods" rel="nofollow noreferrer">https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods</a></p>
  
  <p>This is really cool. Let me give you an example:</p>
  
  <p>I want to add <code>concat</code> method to <code>std::vector</code> but I don't want to inherit it. This feature would be very useful. Do you have any analogues feature in C++ that allows to add a function to a type without inheriting from the original type? I am asking for a language-level feature, please.</p>