defineHow to define conditional compilation

define definition conditional compilation can be achieved using the `#ifdef`, `#ifndef`, `#if`, `#elif`, `#else` and `#endif` preprocessing directives.

Conditional compilation is a technique that selectively compiles code based on conditions, using `#ifdef`, `#ifndef`, `#if`, ` #elif`, `#else` and `#endif` preprocessing directives.

The basic syntax of conditional compilation is as follows:

#ifdef 宏名
    // 如果宏已经定义，则编译这部分代码
#else
    // 如果宏未定义，则编译这部分代码
#endif

In the above code, the `#ifdef` directive is used to check whether the macro has been defined. If the macro is defined, the code between `#ifdef` and `#else` is compiled; if the macro is not defined, the code between `#else` and `#endif` is compiled.

In addition to `#ifdef`, there is also the `#ifndef` directive, which has the opposite effect of `#ifdef`. If the macro is not defined, the code between `#ifndef` and `#else` is compiled; if the macro is defined, the code between `#else` and `#endif` is compiled.

In addition, you can also use `#if`, `#elif` and `#else` to perform more complex conditional judgments. The `#if` directive can be followed by an expression. If the value of the expression is true (non-zero), the code between `#if` and `#elif` is compiled; if the value of the expression is false (zero) ), the code between `#elif` and `#endif` is compiled.

The following is an example that demonstrates how to use conditional compilation:

#include <stdio.h>
#define DEBUG
int main() {
    #ifdef DEBUG
        printf("Debug mode\n");
    #else
        printf("Release mode\n");
    #endif
    return 0;
}

In the above code, the `DEBUG` macro is defined before the `#ifdef` directive, so `printf ("Debug mode\n")` This part of the code is included. If you change `#ifdef DEBUG` to `#ifndef DEBUG`, the `printf("Release mode\n")` part of the code will be included during compilation.

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