How do I return one of two types in C?-CodePudding

Suppose this code

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){...};
}

That function getAorB should return either A or B depending on the parameter k. OK, but what is the return type and is it possible to achieve that in C?

CodePudding user response：

One way to do this would be to have another struct which contains the 'type' of the returned struct. Here is what this may look like:

#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;
}

Then in the calling function you check the type of the SUPER and cast it to the appropriate function.

A *a;
B *b;

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

CodePudding user response：

Use unions.

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;
}