Merge pull request #114 from amhndu/pragma-once-faq

resource: Add pragma-once-faq for C++

src/content/resources/cpp/pragma-once-faq.md

@@ -0,0 +1,39 @@
+---
+authors:
+  - "sudonym#8623"
+created_at: 2019/10/6
+title: "pragma once FAQ"
+---
+
+## What is `#pragma once`?
+
+The `#pragma once` directive tells a compiler to only include and parse a header file _once_ even if it is included multiple times in the same source file.
+They provide a cleaner alternative to [include-guards](https://en.wikipedia.org/wiki/Include_guard) that are conventionally used for this purpose.
+
+### Is it portable ?
+
+No. `#pragma` directives are used to control _implementation-defined_ behavior and are thus compiler-specific.
+
+### Why should I use it ?
+
+1.  Less typing if your IDE/editor doesn't do it for you ¯\\\_(ツ)\_/¯
+2.  Faster compiling, as the compiler can now completely ignore processing/loading this file, it'd have to load it all if you were using a header guard.
+3.  You find header guards icky.
+
+### Why should I not use it ?
+
+1.  You want to be standards-compliant
+2.  You want to follow CppCoreGuidelines: <https://github.com/isocpp/CppCoreGuidelines/blob/master/CppCoreGuidelines.md#Rs-guards>
+3.  You use a `compiler:version` which doesn't support `pragma once` or has a buggy implementation (see the section on compiler support)
+
+### Why not both?
+
+![Why not both](https://i.imgur.com/DBcUtoo.png)
+
+Some people/projects do this, so that you can both be standards-compliant (since unrecognized pragmas will be ignored) _and_ get that speed bonus.
+I can't tell you if you should either use pragma, header guards or both, that's just one of those opinion topics nobody agrees on.
+
+### What compilers support it?
+
+Virtually all popular and modern compilers.
+See [this list](https://en.wikipedia.org/wiki/Pragma_once#Portability) for more information.