C++ compilation error: A header file is referenced multiple times, how to solve it?

During the C compilation process, we often encounter errors in which header files are referenced multiple times. This error occurs because when a header file is referenced in multiple places, its contents are copied in each place. When the compiler links these files, a duplicate definition error occurs.

When facing this error, you can take the following solutions.

1. Use precompiled header files

First of all, we can use precompiled header files (PCH) to avoid this error. PCH is a technology that can compile commonly used header files in advance, which can avoid the problem of the same header file being referenced multiple times. Using PCH requires setting some compiler options. Please refer to the compiler documentation for details.

2. Use header file protection

Another solution is to use header file protection (header file guard). Header file protection refers to adding a preprocessor directive to the header file to avoid the problem of the same header file being referenced multiple times. The format of header file protection is as follows:

#ifndef HEADER_FILE_NAME
#define HEADER_FILE_NAME

// 头文件内容

#endif

Among them, HEADER_FILE_NAME can be replaced with any identifier to uniquely identify this header file. When the compiler encounters this header file for the first time, it will define HEADER_FILE_NAME, and then compile the header file; when the compiler encounters this header file for the second time, because HEADER_FILE_NAME has been is defined, the compiler will skip this file directly. This can avoid the problem of the same header file being defined multiple times.

3. Use #pragma once

In addition to header file protection, C also supports the use of #pragma once to avoid the problem of header files being referenced multiple times. #pragma once is a special preprocessor directive used to mark that a header file can only be compiled once. The format of using #pragma once is very simple. You only need to add this instruction at the top of the header file:

#pragma once

// 头文件内容

When the compiler encounters this header file for the first time, The path and file name of this file will be recorded and marked as a compiled file; when the compiler encounters this header file for the second time, it will first check whether it has been compiled before, and if it has been compiled, it will jump directly passed, otherwise continue compilation. Using #pragma once can simplify the syntax of header file protection and improve compilation speed.

In short, whether you use PCH, header file protection or #pragma once, you can effectively avoid the error of the same header file being referenced multiple times. In actual programming, we should try to avoid repeated references to header files and choose appropriate solutions to avoid this error.

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