有没有一种方法可以在C语言中指定函数的默认参数?
当前回答
不，那是c++语言的特性。
其他回答
我们可以为默认值创建(仅)使用命名形参的函数。这是bk.答案的延续。
#include <stdio.h>
struct range { int from; int to; int step; };
#define range(...) range((struct range){.from=1,.to=10,.step=1, __VA_ARGS__})
/* use parentheses to avoid macro subst */
void (range)(struct range r) {
for (int i = r.from; i <= r.to; i += r.step)
printf("%d ", i);
puts("");
}
int main() {
range();
range(.from=2, .to=4);
range(.step=2);
}
C99标准定义了初始化中后面的名称覆盖前面的项。我们也可以有一些标准的位置参数，只是相应地改变宏和函数的签名。默认值参数只能在命名参数样式中使用。
项目输出:
1 2 3 4 5 6 7 8 9 10
2 3 4
1 3 5 7 9
不，但你可以考虑使用一组函数(或宏)来近似使用默认参数:
// No default args
int foo3(int a, int b, int c)
{
return ...;
}
// Default 3rd arg
int foo2(int a, int b)
{
return foo3(a, b, 0); // default c
}
// Default 2nd and 3rd args
int foo1(int a)
{
return foo3(a, 1, 0); // default b and c
}
我改进了Jens Gustedt的回答:
不使用内联函数 默认值在预处理期间计算 模块化可重用宏 可以设置编译器错误，使其与允许的默认值参数不足的情况有意义地匹配 如果实参类型保持无二义性，则不需要默认值来形成形参列表的尾部 与C11 _Generic互操作 根据参数的数量来改变函数名!
variadic.h:
#ifndef VARIADIC
#define _NARG2(_0, _1, _2, ...) _2
#define NUMARG2(...) _NARG2(__VA_ARGS__, 2, 1, 0)
#define _NARG3(_0, _1, _2, _3, ...) _3
#define NUMARG3(...) _NARG3(__VA_ARGS__, 3, 2, 1, 0)
#define _NARG4(_0, _1, _2, _3, _4, ...) _4
#define NUMARG4(...) _NARG4(__VA_ARGS__, 4, 3, 2, 1, 0)
#define _NARG5(_0, _1, _2, _3, _4, _5, ...) _5
#define NUMARG5(...) _NARG5(__VA_ARGS__, 5, 4, 3, 2, 1, 0)
#define _NARG6(_0, _1, _2, _3, _4, _5, _6, ...) _6
#define NUMARG6(...) _NARG6(__VA_ARGS__, 6, 5, 4, 3, 2, 1, 0)
#define _NARG7(_0, _1, _2, _3, _4, _5, _6, _7, ...) _7
#define NUMARG7(...) _NARG7(__VA_ARGS__, 7, 6, 5, 4, 3, 2, 1, 0)
#define _NARG8(_0, _1, _2, _3, _4, _5, _6, _7, _8, ...) _8
#define NUMARG8(...) _NARG8(__VA_ARGS__, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#define _NARG9(_0, _1, _2, _3, _4, _5, _6, _7, _8, _9, ...) _9
#define NUMARG9(...) _NARG9(__VA_ARGS__, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0)
#define __VARIADIC(name, num_args, ...) name ## _ ## num_args (__VA_ARGS__)
#define _VARIADIC(name, num_args, ...) name (__VARIADIC(name, num_args, __VA_ARGS__))
#define VARIADIC(name, num_args, ...) _VARIADIC(name, num_args, __VA_ARGS__)
#define VARIADIC2(name, num_args, ...) __VARIADIC(name, num_args, __VA_ARGS__)
// Vary function name by number of arguments supplied
#define VARIADIC_NAME(name, num_args) name ## _ ## num_args ## _name ()
#define NVARIADIC(name, num_args, ...) _VARIADIC(VARIADIC_NAME(name, num_args), num_args, __VA_ARGS__)
#endif
简化使用场景:
const uint32*
uint32_frombytes(uint32* out, const uint8* in, size_t bytes);
/*
The output buffer defaults to NULL if not provided.
*/
#include "variadic.h"
#define uint32_frombytes_2( b, c) NULL, b, c
#define uint32_frombytes_3(a, b, c) a, b, c
#define uint32_frombytes(...) VARIADIC(uint32_frombytes, NUMARG3(__VA_ARGS__), __VA_ARGS__)
使用_Generic:
const uint8*
uint16_tobytes(const uint16* in, uint8* out, size_t bytes);
const uint16*
uint16_frombytes(uint16* out, const uint8* in, size_t bytes);
const uint8*
uint32_tobytes(const uint32* in, uint8* out, size_t bytes);
const uint32*
uint32_frombytes(uint32* out, const uint8* in, size_t bytes);
/*
The output buffer defaults to NULL if not provided.
Generic function name supported on the non-uint8 type, except where said type
is unavailable because the argument for output buffer was not provided.
*/
#include "variadic.h"
#define uint16_tobytes_2(a, c) a, NULL, c
#define uint16_tobytes_3(a, b, c) a, b, c
#define uint16_tobytes(...) VARIADIC( uint16_tobytes, NUMARG3(__VA_ARGS__), __VA_ARGS__)
#define uint16_frombytes_2( b, c) NULL, b, c
#define uint16_frombytes_3(a, b, c) a, b, c
#define uint16_frombytes(...) VARIADIC(uint16_frombytes, NUMARG3(__VA_ARGS__), __VA_ARGS__)
#define uint32_tobytes_2(a, c) a, NULL, c
#define uint32_tobytes_3(a, b, c) a, b, c
#define uint32_tobytes(...) VARIADIC( uint32_tobytes, NUMARG3(__VA_ARGS__), __VA_ARGS__)
#define uint32_frombytes_2( b, c) NULL, b, c
#define uint32_frombytes_3(a, b, c) a, b, c
#define uint32_frombytes(...) VARIADIC(uint32_frombytes, NUMARG3(__VA_ARGS__), __VA_ARGS__)
#define tobytes(a, ...) _Generic((a), \
const uint16*: uint16_tobytes, \
const uint32*: uint32_tobytes) (VARIADIC2( uint32_tobytes, NUMARG3(a, __VA_ARGS__), a, __VA_ARGS__))
#define frombytes(a, ...) _Generic((a), \
uint16*: uint16_frombytes, \
uint32*: uint32_frombytes)(VARIADIC2(uint32_frombytes, NUMARG3(a, __VA_ARGS__), a, __VA_ARGS__))
和可变函数名选择，不能与_Generic组合:
// winternitz() with 5 arguments is replaced with merkle_lamport() on those 5 arguments.
#define merkle_lamport_5(a, b, c, d, e) a, b, c, d, e
#define winternitz_7(a, b, c, d, e, f, g) a, b, c, d, e, f, g
#define winternitz_5_name() merkle_lamport
#define winternitz_7_name() winternitz
#define winternitz(...) NVARIADIC(winternitz, NUMARG7(__VA_ARGS__), __VA_ARGS__)
使用宏的另一个技巧:
#include <stdio.h>
#define func(...) FUNC(__VA_ARGS__, 15, 0)
#define FUNC(a, b, ...) func(a, b)
int (func)(int a, int b)
{
return a + b;
}
int main(void)
{
printf("%d\n", func(1));
printf("%d\n", func(1, 2));
return 0;
}
如果只传递一个参数，则b接收默认值(在本例中为15)
不是真的。唯一的方法是编写一个varargs函数，并手动填充调用者没有传递的参数的默认值。
