c++ - Why isn't #pragma once automatically assumed?

Question

What's the point of telling the compiler specifically to include the file only once? Wouldn't it make sense by default? Is there even any reason to include a single file multiple times? Why not just assume it? Is it to do with specific hardware?

Answer 1

There are multiple related questions here:

• Why is #pragma once not automatically enforced?
  Because there are situations in which you want to include files more than once.

• Why would you want to include a file multiple times?
  Several reasons have been given in other answers (Boost.Preprocessor, X-Macros, including data files). I would like to add a particular example of "avoid code duplication": OpenFOAM encourages a style where #includeing bits and pieces within functions is a common concept. See for example this discussion.

• Ok, but why is it not the default with an opt-out?
  Because it is not actually specified by the standard. #pragmas are by definition implementation-specific extensions.

• Why has #pragma once not become a standardized feature yet (as it is widely supported)?
  Because pinning down what is "the same file" in a platform-agnostic way is actually surprisingly hard. See this answer for more information.