The preprocessor directive is a command used in C for precompilation processing, and its main functions include header file inclusion, macro definition and conditional compilation. 1. #include is used to introduce header files, standard library use , and customize it with ""; 2. #define defines macros to implement text replacement, and it is recommended to use const or constexpr instead; 3. Conditional compilation controls whether the code segment is compiled through #ifdef, #ifndef, etc.; 4. Others such as #undef cancellation of macros, #pragma sets compilation options, #error triggers errors, etc. Rational use can improve cross-platform compatibility and debugging efficiency, but macro usage should be reduced to enhance code maintainability.

In C, preprocessor directives are part of precompilation processing. They are not real C statements, but commands processed by the preprocessor before compilation. These instructions are usually used in scenarios such as conditional compilation, macro definition, header file inclusion, etc.

#include: Include header file

This is one of the most common preprocessor instructions to insert the contents of a header file into the current source file.

 #include <iostream> // Standard library header file#include "myheader.h" // Custom header file

• Use <...> to find the header file under the standard library path.
• Using "..." is to first look up the local directory, and then look up the standard library path.
• If you write a custom tool function set, such as utils.h and utils.cpp , you can use #include "utils.h" to introduce it.

A small suggestion: Avoid repeated including the same header file, you can use Header Guards or #pragma once .

#define and macro definitions

#define can be used to define macros, that is, simple text replacement.

 #define PI 3.14159
#define SQUARE(x) ((x) * (x))

• Macros are not variables and will not participate in type checking.
• Be careful when using macro functions, so it is recommended to bracket the parameters and the entire expression.
• For example, SQUARE(ab) actually expands to (ab) * (ab) , and an error will occur if there are no brackets.

Although macros are convenient, modern C recommends using const or constexpr constants and inline functions instead of macros, which is safer and easier to debug.

Conditional compilation: #ifdef, #ifndef, #else, #endif

These instructions can determine whether to compile a certain piece of code based on certain conditions.

 #ifdef DEBUG
    std::cout << "Debug mode is on." << std::endl;
#endif

Common uses:

• Print debugging information in the development stage and will not be enabled after it is launched.
• Cross-platform compatible with code under different systems, such as the APIs of Windows and Linux are different.

For example:

 #if defined(_WIN32)
    // Windows-specific code #elif defined(__linux__)
    // Linux-specific code #endif

This structure is common in large projects, especially when it is necessary to support multiple operating systems or multiple configurations.

Other common commands

In addition to the above, there are some preprocessor instructions that are not very commonly used but occasionally encountered:

• #undef : Undefined a macro.
• #pragma : Issue some special instructions, such as #pragma once to avoid repeatedly including header files.
• #error : Force the compiler to report an error, which is often used for version checking.
• #line : Modify the current line number, mainly used for debugging or generating code tools.

These are not used much, but are useful in specific scenarios.

Basically that's it. Although the preprocessor instructions are simple, they are very practical in actual development, especially in controlling the compilation process, cross-platform adaptation and debugging. However, it should be noted that you should try to use less macros and use more features of the language itself, so that the code is easier to maintain and understand.

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