#pragma

There are two useful

#pragma

directives I like to use in my code: one let’s the preprocessor know that you want to include a header fine only once, and another deals with structure packing.

Instead of using the header include guards, which are ugly as sin, use

#pragma once

at the beginning of your header files, like this:

// This is a header file...
#pragma once // now it will be included only once
...

For structure packing, use

#pragma pack

directive. It tells the compiler about the default field alignment. On Clang 8.0.0 the

sizeof

of this structure is 32 bytes:

struct S1
{
	char c;
	short s;
	int i;
	long l;
	float f;
	double d;
};

We can pack it down to 27 bytes by using this directive (it tells the compiler to align all member fields on one byte boundary; this is useful when designing efficient network protocols or data serialization):

#pragma pack(1)

struct S2
{
	char c;
	short s;
	int i;
	long l;
	float f;
	double d;
};

You can also show, at compile time, what the current packing alignment is with

#pragma pack(show)

. Current alignment can be pushed onto a stack, then reverted back, with

#pragma pack(push, 1)

followed by

#pragma pack(pop)

.

Complete listing (pragma.cpp):

#include 

using namespace std;

struct S1
{
	char c;
	short s;
	int i;
	long l;
	float f;
	double d;
};

#pragma pack(show)
#pragma pack(push, 1)
#pragma pack(show)

struct S2
{
	char c;
	short s;
	int i;
	long l;
	float f;
	double d;
};

#pragma pack(pop)

int main()
{
	cout << sizeof(S1) << ", " << sizeof(S2) << endl;
}

32, 27

Program output.