#pragma once: A Safe Cross-Platform Include Guard

Including header files multiple times can lead to compilation errors due to duplicate declarations. To prevent this, it's common practice to use include guards, such as the standard #ifndef directive.

Non-Standard but Efficient: #pragma once

pragma once is a compiler directive supported by many modern compilers, including non-Windows platforms like gcc. It acts like an include guard, but is more efficient than #ifndef because it uses the preprocessor to mark the associated file as included. This optimization can result in faster compilation times.

#pragma once Drawback: File Collision

One potential drawback of using #pragma once is that it assumes each header file exists in only one location. If you have the same header file in different locations (e.g., due to linking or build system actions), the compiler may treat them as distinct files, which can lead to unpredictable behavior.

Cross-Platform Compatibility Concerns

Since #pragma once is non-standard, its implementation can vary across compilers. However, most modern compilers support it consistently. Using #pragma once generally does not pose cross-platform compilation issues.

Recommendation

Whether or not you should be concerned with this potential issue depends on your specific project and compilation setup. If you have a controlled environment where header files are not duplicated in different locations, using #pragma once can provide performance benefits. However, if there's a possibility of file collisions, it's advisable to stick with the fallback include guard approach using #ifndef or #ifdef directives.

The above is the detailed content of #pragma once vs. #ifndef: Which Include Guard Should You Choose?. For more information, please follow other related articles on the PHP Chinese website!