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

You seem to be operating under the assumption that the purpose of the "#include" feature even existing in the language is to provide support for decomposition of programs into multiple compilation units. That is incorrect.

It can perform that role, but that was not its intended purpose. C was originally developed as slightly higher-level language than PDP-11 Macro-11 Assembly for reimplementing Unix. It had a macro preprocessor because that was a feature of Macro-11. It had the ability to textually include macros from another file because that was a feature of Macro-11 that the existing Unix they were porting to their new C compiler had made use of.

Now it turns out that "#include" is useful for separating code into compilation units, as (arguably) a bit of a hack. However, the fact that this hack existed meant that it became The Way that is done in C. The fact that a way existed meant no new method ever needed to be created to provide this functionality, so nothing safer (eg: not vulnerable to multiple-inclusion) was ever created. Since it was already in C, it got copied into C++ along with most of the rest of C's syntax and idioms.

There is a proposal for giving C++ a proper module system so this 45 year old preprocessor hack can finally be dispensed with. I don't know how imminent this is though. I've been hearing about it being in the works for more than a decade.