What is #pragma region in c++ and VScode?

The #pragma region is specific to Visual Studio only.

Using #pragma region you can specify a block of code where you can expand it and collapse it.

It has no affect on compilation.

Here is an example:

// pragma_directives_region.cpp
#pragma region Region_1
void Test() {}
void Test2() {}
void Test3() {}
#pragma endregion Region_1

int main() {}

You can read more about it here.

Like others have mentioned your compiler is allowed to silently ignore a pragma, Some will even give a warning depending on the specific compiler you are using. You need to read about your compiler's docs on pragma.

gcc -Wall also check for unknown pragmas.

First, #pragma is a C preprocessor directive. You're undoubtedly familiar with the #include and #define directives. Directives are used by the C preprocessor to perform certain actions before the code reaches the compiler.

The #pragma directive in particular is meant to pass implementation-specific information to whatever compiler you're using.

Here's what GNU has to say:

The #pragma directive is the method specified by the C standard for providing additional information to the compiler, beyond what is conveyed in the language itself.

Here's what Microsoft has to say:

Pragma directives specify machine-specific or operating system-specific compiler features.

Second, for Microsoft Visual Studio and Visual Studio Code, the #pragma region directive defines a region of code that the compiler's UI can use for code-folding (that is, hiding and showing blocks of code for readability). If a compiler cannot understand the #pragma region directive, then the directive is ignored.

Here's an example below:

1. Here's a chunk of C++ code in Visual Studio Code v1.60.0 (2021) on Mac. It would be great if I could hide and unhide the private: chunk of code. Note that when I mouse-over the UI, there is a down arrow chevron (⌄) to indicate foldable code for the comment block but not for the private: block.

2. Putting in a starting #pragma region foo and ending #pragma endregion will identify a block of code for code-folding. Notice that after the two #pragma directives are added, the code-foldable chevron symbol appears in the UI.

3. When you click on the chevron, the demarcated block will fold (hide). Clicking on the chevron again will unfold the block.