Question

我喜欢voverload一个C函数。我找到了以下内容：Function overloading in C using GCC - functions with mutiple arguments。这非常好，但现在我想根据两个参数的类型选择函数。

#define dataioWriteBin(file, data, len)                                                                                                                                                 \
    (__builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint8_t )    && __builtin_types_compatible_p(typeof(len), uint64_t ), dataioWriteBin_uint8Data_uint64Len,    \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint8_t )    && __builtin_types_compatible_p(typeof(len), uint32_t ), dataioWriteBin_uint8Data_uint32Len,    \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint32_t )   && __builtin_types_compatible_p(typeof(len), uint64_t ), dataioWriteBin_uint32Data_uint64Len,   \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint32_t )   && __builtin_types_compatible_p(typeof(len), uint32_t ), dataioWriteBin_uint32Data_uint32Len,   \
     (void)0))))(file, data, len))

但是使用这段代码我得到了一个编译跟随错误：

错误：被调用对象不是函数或函数指针（__builtin_choose_expr（__ builtin_types_compatible_p（typeof运算（*数据）， uint8_t）

任何人都有一个想法，是不是可能派遣两个argumnents。构造本身也有效。它失败了我用uint32_t数据和uint32_t长度调用函数。使用uint8_t的函数调用正在运行。

感谢您的帮助。

Answer 1

您可以使用C.11的_Generic：

来实现此目的

#define uint8Data_(file, data, len)                          \
_Generic((len),                                              \
    uint64_t : dataioWriteBin_uint8Data_uint64Len,           \
    uint32_t : dataioWriteBin_uint8Data_uint32Len)

#define uint32Data_(file, data, len)                         \
_Generic((len),                                              \
    uint64_t : dataioWriteBin_uint32Data_uint64Len,          \
    uint32_t : dataioWriteBin_uint32Data_uint32Len)

#define dataioWriteBin(file, data, len)                      \
_Generic(                                                    \
    (data),                                                  \
    uint8_t * :        uint8Data_(file, data, len),          \
    const uint8_t * :  uint8Data_(file, data, len),          \
    uint32_t * :       uint32Data_(file, data, len),         \
    const uint32_t * : uint32Data_(file, data, len)          \
    )(file, data, len)

accepted answer to the question you cited in your post中描述了使用GCC __builtin_choose_expr的表达式选择语法。

Answer 2

这可能不是基于类型的重载的最佳用法（无论是基于__builtin_choose_expr还是标准化的_Generic），但这里有一个示例，表明您的代码可以使用uint32_t *data和uint32_t len。代码编译：

#include <stdint.h>
#include <stdio.h>
//prototypes
void dataioWriteBin_uint8Data_uint64Len(FILE *f, uint8_t const *data, uint64_t len);
void dataioWriteBin_uint8Data_uint32Len(FILE *f, uint8_t const *data, uint32_t len);
void dataioWriteBin_uint32Data_uint64Len(FILE *f, uint32_t const *data, uint64_t len);
void dataioWriteBin_uint32Data_uint32Len(FILE *f, uint32_t const *data, uint32_t len);

//
#define dataioWriteBin(file, data, len)                                                                                                                                                 \
    (__builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint8_t )    && __builtin_types_compatible_p(typeof(len), uint64_t ), dataioWriteBin_uint8Data_uint64Len,    \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint8_t )    && __builtin_types_compatible_p(typeof(len), uint32_t ), dataioWriteBin_uint8Data_uint32Len,    \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint32_t )   && __builtin_types_compatible_p(typeof(len), uint64_t ), dataioWriteBin_uint32Data_uint64Len,   \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint32_t )   && __builtin_types_compatible_p(typeof(len), uint32_t ), dataioWriteBin_uint32Data_uint32Len,   \
     (void)0))))(file, data, len))

//
void use_it(void)
{
    uint32_t data[4]={};
    dataioWriteBin(stdout,data,(uint32_t)sizeof(data));
}

函数重载C多个参数

2 个答案: