C language__ VA_ ARGS__ Usage of

0 Preface

In the course of debugging , Sometimes a custom print is used , This is what will be used ##__VA_ARGS__, Let's talk about it in detail .

1 __VA_ARGS__ Usage of

When customizing printing , Use variable parameters , use ... Can represent variable parameters , as follows ：

#include <stdio.h>

#define LOG1(...) printf(__VA_ARGS__)//... Represents a variable parameter ,__VA_ARGS__ Will be ... The value of is copied here 
int main(int argc, char** argv)
{
    
    char *str = "test __VA_ARGS__";
    int num = 10086;
    LOG1("this is test __VA_ARGS__\r\n");
    LOG1("this is test __VA_ARGS__:%s, %d\r\n", str, num);

    return 0;
}

The results are as follows ：

this is test __VA_ARGS__
this is test __VA_ARGS__:test __VA_ARGS__, 10086

2 ##__VA_ARGS__ Usage of

##__VA_ARGS__ prefix ## The role of is ： When the number of variable parameters is 0 when , there ## You can put the superfluous in front "," Get rid of , Otherwise, there will be compilation errors .
When you want to add time to the custom debug information 、 The number of rows and other information , What should be done ？ Write the correct usage first ：

#include <stdio.h>

#define LOG3(fmt, ...) printf("<%s:%s>:"fmt"\r\n", __FILE__, __FUNCTION__, ##__VA_ARGS__)
int main(int argc, char** argv)
{
    
    char *str = "test __VA_ARGS__";
    int num = 10086;
    
    LOG3("this is test __VA_ARGS__");
    LOG3("this is test __VA_ARGS__:%s, %d", str, num);
    LOG3();

    return 0;
}

Print as follows ：

<main.c:main>:this is test __VA_ARGS__
<main.c:main>:this is test __VA_ARGS__:test __VA_ARGS__, 10086
<main.c:main>:

Then make a comparison ： If you use __VA_ARGS__, When used , The parameter is empty. , Compiler error . as follows ：

#include <stdio.h>

#define LOG2(fmt, ...) printf("<%s:%s>:"fmt"\r\n", __FILE__, __FUNCTION__, __VA_ARGS__)
int main(int argc, char** argv)
{
    
    LOG2();// Don't pass parameters , Compiler error 
    LOG2("this is test __VA_ARGS__");// Compiler error 
    LOG2("this is test __VA_ARGS__:%s, %d", str, num);// Normal compilation 
    return 0;
}

Error report after compilation ：

main.c: In function ‘main’:
main.c:3:96: error: expected expression before ‘)’ token
   3  | #define LOG2(fmt, ...)          printf("<%s:%s>:"fmt"\r\n", __FILE__, __FUNCTION__, __VA_ARGS__)
      |                                                                                                ^
main.c:6:5: note: in expansion of macro ‘LOG2’
   6  |     LOG2();// Don't pass parameters , Compiler error 
      |     ^~~~
main.c:3:96: error: expected expression before ‘)’ token
   3  | #define LOG2(fmt, ...)          printf("<%s:%s>:"fmt"\r\n", __FILE__, __FUNCTION__, __VA_ARGS__)
      |                                                                                                ^
main.c:7:5: note: in expansion of macro ‘LOG2’
   7  |     LOG2("this is test __VA_ARGS__");
      |     ^~~~

analysis ：

• The first 6 Line has no parameters , Macro definition LOG2(fmt,...) After deployment ,__VA_ARGS__ It's empty. , At this time printf There is one left ,, Inevitably, compilation fails ;
• The first 7 Although the line passed in a string , But the string is assigned to LOG2(fmt,...) The first parameter of fmt, After the grand exhibition ,printf There is one left behind ,, So compile error .

summary

##__VA_ARGS__ in ## The function of is to get rid of the superfluous ,, When using custom printing , Recommended use of 2 The way of small measure .

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