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 m

Answer 1

No, this would significantly hinder the options available to, for example, library writers. For example, Boost.Preprocessor allows one to use pre-processor loops, and the only way to achieve those is by multiple inclusions of the same file.

And Boost.Preprocessor is a building block for many very useful libraries.