All you need to know about #PRAGMA in C++

#	User	Rating
1	tourist	3985
2	jiangly	3814
3	jqdai0815	3682
4	Benq	3529
5	orzdevinwang	3526
6	ksun48	3517
7	Radewoosh	3410
8	hos.lyric	3399
9	ecnerwala	3392
9	Um_nik	3392

#	User	Contrib.
1	cry	169
2	maomao90	162
2	Um_nik	162
4	atcoder_official	161
5	djm03178	158
6	-is-this-fft-	157
7	adamant	155
8	awoo	154
8	Dominater069	154
10	luogu_official	150

In C++, pragmas are directives provided by the compiler to control various aspects of the compilation process. Pragmas are typically specific to a particular compiler, and their usage might not be standardized across different compilers. Here are some common C++ pragmas and their purposes:

pragma once:

This pragma is used for header file guards. It ensures that the header file is included only once during compilation, helping to prevent multiple inclusions and potential issues with redefinitions.

#pragma once

pragma comment(lib, "library_name"):

This pragma is often used in Microsoft Visual Studio to specify linking with a particular library. It's a way to include a library without explicitly adding it to the project settings.

#pragma comment(lib, "user32.lib")

pragma message("message text"):

This pragma allows you to generate a compiler message. It's often used for informational or debugging purposes. The message specified will be displayed during compilation.

#pragma message("Compiling: This is an informational message.")

pragma warning:

This pragma allows you to control warning messages issued by the compiler. You can enable or disable specific warnings or set their severity level.

#pragma warning(disable: 4996) // Disable warning 4996

pragma pack(n):

This pragma controls the alignment of structure members in memory. It specifies the alignment boundary for structure members.

#pragma pack(1) // Set the alignment to 1 byte

pragma GCC optimize:

This pragma is used in GCC (GNU Compiler Collection) to control optimization options. It allows you to specify optimization levels for specific functions or code sections.

#pragma GCC optimize("O3") // Optimize with level 3

pragma omp:

OpenMP (Open Multi-Processing) directives are often used with this pragma to enable parallel programming in C++. It allows developers to specify parallel regions and control parallel execution.

#pragma omp parallel for
for (int i = 0; i < size; ++i) {
    // Parallelized loop
}

Keep in mind that while pragmas can be powerful tools for controlling compiler behavior, excessive or inappropriate use of pragmas can lead to non-portable code. It's essential to be aware of the compiler-specific nature of pragmas and use them judiciously based on the targeted compiler and platform.