C++ implements header file protection mechanism

In C and C++ programming, the header file protection mechanism is a technology to prevent header files from being repeatedly included, and it mainly uses the help of#ifndef、#defineand#endifThese preprocessing instructions can also be achieved.#pragma onceThis compiler-specific instruction. This mechanism is explained in detail below:

1. Problems containing duplicate header files

In large projects, a source file may contain the same header file multiple times, perhaps because of nested inclusions between header files. Repeated inclusion of header files can cause many problems, such as:

Compilation time becomes longer: The compiler will process the same code multiple times, which will slow the compilation process.
Repeated definition error: If a global variable, function or type is defined in the header file, repeated inclusion will cause a repeated definition error.

2. Use #ifndef, #define and #endif to achieve header file protection

This is a traditional and general header file protection method, and its basic syntax is as follows:

#ifndef HEADER_FILE_NAME_H
#define HEADER_FILE_NAME_H

// Header file content
#endif

Here is a specific example:

// 
#ifndef EXAMPLE_H
#define EXAMPLE_H

// Define a structuretypedef struct {
    int x;
    int y;
} Point;

// Declare a functionvoid printPoint(Point p);

#endif

In this example:

#ifndef EXAMPLE_H:examineEXAMPLE_HWhether this macro is not defined. If it is not defined, execute the subsequent code; otherwise, skip.
#define EXAMPLE_H:likeEXAMPLE_HIf not defined, define this macro.
#endif: Mark the end of the conditional compilation block.

When the first time containshour,EXAMPLE_HUndefined, so#ifndefThe conditions are true, then defineEXAMPLE_HAnd process the content of the header file. When the header file is included again,EXAMPLE_HHas been defined,#ifndefIf the conditions are not true, the content of the header file will be skipped.

3. Use #pragma once to achieve header file protection

#pragma onceis a compiler-specific directive that ensures that the header file is included only once. Its usage is very simple:

// 
#pragma once

// Define a structuretypedef struct {
    int x;
    int y;
} Point;

// Declare a functionvoid printPoint(Point p);

Just add it at the beginning of the header file#pragma once, the compiler will ensure that the header file is processed only once.

4. Comparison of the two methods

compatibility：#ifndef、#defineand#endifis a standard C and C++ preprocessing directive, supported by all compilers; and#pragma onceis a compiler-specific directive, not all compilers support it.
performance：#pragma onceIt is usually faster than traditional macro protection mechanisms, because the compiler can directly identify and process the instruction without the need for macro comparison and definition operations.
portability:because#ifndefThe method is standard, so it is more portable and is suitable for various compilers and platforms.

5. Things to note

Uniqueness of macro names: In use#ifndefWhen using the method, the macro name must be unique to avoid using the same macro name in different header files. Generally, the name of the header file can be used to add.hExtension and capitalize all letters to name the macro.
The impact of comments: In use#pragma onceWhen , be careful that comments may affect the compiler's recognition of the directive. Some compiler requirements#pragma onceIt must be located in the first line of the file, or there can only be comments before it.

To sum up, the header file protection mechanism is a very important part of C and C++ programming. It can effectively avoid the problems caused by duplicate inclusion of header files and improve the maintainability and compilation efficiency of the code.

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