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LLL
Asked: 2025-03-08 22:33:22 +0800 CST

_C 中的泛型需要类型转换吗？

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在学习宏的工作原理时，我尝试定义一个适用于不同数据类型的宏，而不必定义多种类型。我曾经_Generic让这项任务变得更容易。然而，在多次更改后，它仍然不起作用，于是我让 deepseek 来解决这个问题，最终我得到了这个：

#define FOO(var) _Generic((var), \
    int:    printf(#var " = %d\n", (int)(var)), \
    double: printf(#var " = %lf\n", (double)(var)), \
    long:   printf(#var " = %ld\n", (long)(var)), \
    char:   printf(#var " = '%c'\n", (char)(var)), \
    float:  printf(#var " = %f\n", (float)(var)), \
    default:printf(#var " = unknown type\n"))

int main(void) {
    int a = 3;
    FOO(a); // correctly prints "a = 3"
}

在 cpp-reference 和 SO 中的其他代码示例中，我从未见过有人必须进行类型转换才能使其工作。删除类型转换 (... int: printf(#var " = %d\n",(var)), \...) 会显示警告“格式指定类型 'long' 但参数类型为 'int'”，并给出编译警告（-Wall已启用）：

ue_05.c: In function ‘main’:
ue_05.c:47:14: warning: format ‘%lf’ expects argument of type ‘double’, but argument 2 has type ‘int’ [-Wformat=]
   47 |         FOO(a);
      |              ^
ue_05.c:27:25: note: in definition of macro ‘FOO’
   27 |         double: printf(#var " = %lf\n", (var)), \
      |                         ^~~
ue_05.c:47:14: warning: format ‘%ld’ expects argument of type ‘long int’, but argument 2 has type ‘int’ [-Wformat=]
   47 |         FOO(a);
      |              ^
ue_05.c:28:25: note: in definition of macro ‘FOO’
   28 |         long:   printf(#var " = %ld\n", (var)), \
      |                         ^~~
ue_05.c:47:14: warning: format ‘%f’ expects argument of type ‘double’, but argument 2 has type ‘int’ [-Wformat=]
   47 |         FOO(a);
      |              ^
ue_05.c:30:25: note: in definition of macro ‘FOO’
   30 |         float:  printf(#var " = %f\n", (var)), \

我一直没能找出为什么这里需要类型转换并且 deepseek 也没能解释清楚。

为什么这里需要类型转换以及它在哪里被记录下来？

c

1 个回答

  1. Best Answer

    只需#define编辑代码即可。

    当你写的时候

    #define FOO(var) _Generic((var), \
    int:    printf(#var " = %d\n", (var)), \
    double: printf(#var " = %lf\n",(var)), \
    long:   printf(#var " = %ld\n",(var)), \
    char:   printf(#var " = '%c'\n",(var)), \
    float:  printf(#var " = %f\n", (var)), \
    default:printf(#var " = unknown type\n"))

int main(void) {
    int a = 3;
    FOO(a);
}

    预处理完成后，它就变成了幕后工作

    int main(void) {
    int a = 3;
    _Generic((a),
    int:    printf("a" " = %d\n", (a)),
    double: printf("a" " = %lf\n", (a)),
    long:   printf("a" " = %ld\n", (a)), 
    char:   printf("a" " = '%c'\n",(a)),
    float:  printf("a" " = %f\n",(a)),
    default:printf("a" " = unknown type\n"));
}

    什么时候a编译int器仍然会看到

     printf("a" " = %lf\n", (a))

    以及其他语句。因此，即使该语句不会被执行，也会向您发出警告，抱怨变量a正在期望int中。printffloat

    类型转换只是让编译器将其视为a（float在float:的部分中_Generic，在其余部分中依此类推），因此不会对此提出抱怨。

    顺便说一下，你可能已经开启-Wall了。

    编辑：
    要消除警告，请执行以下操作

    #include <stdio.h>

#define FOO(var) printf( _Generic((var),\
                            int:    #var " = %d\n",\
                            double: #var " = %lf\n",\
                            long:   #var " = %ld\n",\
                            char:   #var " = '%c'\n",\
                            float:  #var " = %f\n",\
                            default: #var " = unknown type\n")\
                            , var )

int main() {
    float a = 2.4f;
    FOO(a);
}
    • 5

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