Using #pragma once in Header Files

Using #pragma once in header files offers several advantages over traditional header guards:

Simplicity and Readability

With #pragma once, you only need a single line at the beginning of the header file. It makes the code cleaner and more readable compared to header guards, which require three lines of code.

1// MyHeader.h
2#pragma once
3
4// Header content goes here

Reduced Typing and Maintenance:

When using header guards, you need to choose a unique name for each header file and ensure that the same name is used for the #ifndef, #define, and #endif directives. With #pragma once, you don't need to worry about unique names or maintaining the consistency of guard macros.

1// MyHeader.h
2#ifndef MYHEADER_H
3#define MYHEADER_H
4
5// Header content goes here
6
7#endif

Compilation Speed

The #pragma once directive is typically faster for the compiler to process compared to header guards. When the compiler encounters #pragma once, it can quickly check if the header file has already been included and skip processing it again, improving compilation speed.

1// MyHeader.h
2#pragma once
3
4// Header content goes here

Compatibility

Most modern compilers support #pragma once, including GCC, Clang, and Microsoft Visual C++. However, if you need to support older compilers or ensure maximum portability, you can use a combination of #pragma once and header guards, as shown in the example above.

1// MyHeader.h
2#if defined(__GNUC__)   \
3  || defined(__clang__) \
4  || defined(_MSC_VER)    
5  #pragma once
6#else
7  #ifndef MYHEADER_H
8  #define MYHEADER_H
9
10// Header content goes here
11
12  #endif
13#endif

While #pragma once offers several advantages, it's important to note that it's a non-standard extension. If portability across all compilers is a concern, you may still choose to use header guards. However, for most modern C++ projects, #pragma once is widely supported and provides a simpler and more efficient way to prevent multiple header inclusions.