宏定义中的##操作符和... and _ _VA_ARGS_ _

本文详细介绍了C语言中预处理宏的两个重要特性：##操作符用于连接两个符号成为一个新的标识符，以及...and__VA_ARGS__用于创建变参宏的方法。通过具体的代码示例展示了如何利用这些特性实现字符串拼接及变量参数列表的功能。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

1.Preprocessor Glue: The ## Operator

预处理连接符：##操作符

Like the # operator, the ## operator can be used in the replacement section of a function-like macro.Additionally, it can be used in the replacement section of an object-like macro. The ## operator combines two tokens into a single token.

##将两个符号连接成一个。

For example, you could do this:

#define XNAME(n) x ## n

Then the macro

XNAME(4)

would expand to the following:

Listing 1 uses this and another macro using ## to do a bit of token gluing.

// glue.c -- use the ## operator

#include

#define XNAME(n) x ## n

#define PRINT_XN(n) printf("x" #n " = %d\n", x ## n);

int main(void)

{

int XNAME(1) = 14; // becomes int x1 = 14;

int XNAME(2) = 20; // becomes int x2 = 20;

PRINT_XN(1); // becomes printf("x1 = %d\n", x1);

PRINT_XN(2); // becomes printf("x2 = %d\n", x2);

return 0;

}

Here's the output:

x1 = 14

x2 = 20

Note how the PRINT_XN() macro uses the # operator to combine strings and the ## operator to combine tokens into a new identifier.

2.Variadic Macros: ... and _ _VA_ARGS_ _

Some functions, such as printf(), accept a variable number of arguments. The stdvar.h header file,provides tools for creating user-defined functions with a variable number of arguments. And C99 does the same thing for macros.Although not used in the standard, the word variadic has come into currency to label this facility. (However, the process that has added stringizing and variadic to the C vocabulary has not yet led to labeling functions or macros with a fixed number of arguments as fixadic functions and normadic macros.)

The idea is that the final argument in an argument list for a macro definition can be ellipses (that is, three periods)（省略号）. If so, the predefined macro _ _VA_ARGS_ _ can be used in the substitution part to indicate what will be substituted for the ellipses. For example, consider this definition:

#define PR(...) printf(_ _VA_ARGS_ _)

Suppose you later invoke the macro like this:

PR("Howdy");

PR("weight = %d, shipping = $%.2f\n", wt, sp);

For the first invocation, _ _VA_ARGS_ _ expands to one argument:

"Howdy"

For the second invocation, it expands to three arguments:

"weight = %d, shipping = $%.2f\n", wt, sp

Thus, the resulting code is this:

printf("Howdy");

printf("weight = %d, shipping = $%.2f\n", wt, sp);

Listing 2 shows a slightly more ambitious example that uses string concatenation and the # operator: