[英]Adding default arguments to variadic macro
Is it possible to add default arguments before variable argument in variadic macro? 是否有可能在可变参数宏中的变量参数之前添加默认参数? eg I have the version of macro something like
例如,我有像宏的版本
#define MACRO(arg1, ...) func(arg1, ##__VA_ARGS__)
I would like to add 2 more default arguments in the macro before variable arguments so that it should not affect previous version. 我想在变量参数之前在宏中添加另外2个默认参数,这样它就不会影响以前的版本。 Like:
喜欢:
#define MACRO(arg1, arg2 = "", arg3 = "", ...) func(arg1, arg2, arg3, ##__VA_ARGS__)
Any help would be appreciated. 任何帮助,将不胜感激。
What you can do: 你可以做什么:
struct foo {
int aaa;
char bbb;
char *ccc;
};
#define BAR(...) bar((struct foo){__VA_ARGS__})
void bar(struct foo baz)
/* set defaults */
if (!baz.aaa)
baz.aaa = 10;
if (!baz.bbb)
baz.bbb = 'z';
if (!baz.ccc)
baz.ccc = "default";
printf("%d, %c, %s\n", baz.aaa, baz.bbb, baz.ccc);
}
...
BAR(); // prints "10, z, default"
BAR(5); // prints "5, z, default"
BAR(5,'b'); // prints "5, b, default"
BAR(5, 'b', "something"); // prints "5, b, something"
Bad thing about this - zero parameter is treated like no parameter, eg BAR(0, 'c')
will produce string 10, c, default
关于这一点的坏事 - 零参数被视为没有参数,例如
BAR(0, 'c')
将产生字符串10, c, default
I do not think this is possible. 我不认为这是可能的。 How would the compiler/preprocessor knows if the second and third arguments are part of the variable arguments or override the default values ?
如果第二个和第三个参数是变量参数的一部分还是覆盖默认值,编译器/预处理器将如何知道? That's the reason why parameters with default values have to be in last position in a function.
这就是为什么具有默认值的参数必须位于函数中的最后位置的原因。
I'm afraid you'll have to have two macros or three if you to be able to specify arg2 and use arg3 default value, but this is error-prone. 如果你能够指定arg2并使用arg3默认值,我担心你必须有两个宏或三个宏,但这很容易出错。
#define MACRO(arg1, ...) func(arg1, "", "", ##__VA_ARGS__)
#define MACRO2(arg1, arg2, ...) func(arg1, arg2, "", ##__VA_ARGS__)
#define MACRO3(arg1, arg2, arg3, ...) func(arg1, arg2, arg3, ##__VA_ARGS__)
Not as an answer to your question but as a way to simply solve your problem: 不是作为您问题的答案,而是作为简单解决问题的方法:
#define MACRO(arg1, ...) \
/* pre-treatment */ \
func(arg1, ##__VA_ARGS__) \
/* post-treatment */ \
void func (type1 arg1, type2 arg2 = "", type3 arg3 = "", ...);
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