我如何 return C 中的两种类型之一？

Question

假设这段代码

typedef struct A {
    ...
} A;

typedef struct B {
    ...
} B;

// If it was TypeScript I would say `type uknown = A | B;`
uknown getAorB(int k) {
    if (k > 0) return (A){...};
    return (B){...};
}

根据参数 k，函数 getAorB 应该 return A 或 B。好的，但是什么是 return 类型，是否可以在 C 中实现它？

Answer 1

一种方法是使用另一个包含 'type' 的结构返回的结构。这可能是这样的：

#define STRUCTA 1
#define STRUCTB 2

typedef struct SUPER {
   int type;
} SUPER;    

typedef struct A {
   int type;
   ...
} A;

typedef struct B {
   int type;
   ...
} B;


SUPER* getAorB(int k) {

    if (k > 0) {
        A *a;
        a = malloc(sizeof(*a));
        a->type = STRUCTA;
        return (SUPER*)a;
    } 
    
    B *b;
    b = malloc(sizeof(*b));
    b->type = STRUCTB;
    return (SUPER*)b;
}

然后在调用函数中检查 SUPER 的类型并将其转换为适当的函数。

A *a;
B *b;

if (returnedSuper->type == STRUCTA) {
    a = (A*)returnedSuper;
}
else if (returnedSuper->type == STRUCTB) {
    b = (B*)returnedSuper;
}

Answer 2

使用联合。

typedef struct A {
    ...
} A;

typedef struct B {
    ...
} B;

typedef union
{
    struct A a;
    struct B b;
}A_OR_B;

// If it was TypeScript I would say `type uknown = A | B;`
A_OR_B getAorB(int k) {
    A_OR_B c;
    if (k > 0) c.a.member = something;
        else c.b.member = somethingelse;
    return c;
}

我如何 return C 中的两种类型之一？

How do I return one of two types in C?

c

struct