Typecasting user-define datatype in C

Question

//data type 1
typedef struct t_A{
    int mJ;
}A;

//data type 2        
typedef struct t_B{
    int mK;
}B;

//Function returning B object which is a rValue
B funcRetB(void)
{
    B test;
    test.mK = 9;
    return test;
}

void main(void)
{
    A a;
    a = (A)funcRetB(); //How to typecast this without defining a variable of B?
}

The above code gives error "error C2440: 'type cast' : cannot convert from 'B' to 'A'". Is it possible to resolve this error in C?

Could you explain why you want to do this? What do you wish to achieve? — Thomas Padron-McCarthy, Feb 19 '17 at 09:06
@VishvajeetT - Then your curiosity will be appeased with a simple "you can't do that". C doesn't support structural equivalence. Have a read http://stackoverflow.com/questions/4486401/structural-equivalence-vs-name-equivalence — StoryTeller - Unslander Monica, Feb 19 '17 at 09:33
@StoryTeller Thanks. Also, could you explain; B b = funcRetB();//Works B b = (B)funcRetB();// error C2440: 'type cast' : cannot convert from 'B' to 'B'. How typecasting same datatype (Name equivalence) gives error? — Vishvajeet T, Feb 19 '17 at 09:59
What are you compiling with? I cannot reproduce http://ideone.com/trCWXZ — StoryTeller - Unslander Monica, Feb 19 '17 at 10:05
msvc 2010 ultimate. You have C++14 as language option; try with C or save the file as .c(offline). — Vishvajeet T, Feb 19 '17 at 13:33

score 0 · Answer 1 · answered Feb 19 '17 at 09:29

I believe, but couldn't quickly find it in the standard, that typedef introduces a new type that is only compatible with itself.

Although A and B are defined the same, the compiler will treat them as different types. When the function returns a complete object of type B, the compiler now does not know how to copy that to an object of type A. Because you copy objects, the cast won't work because, as far as the compiler is concerned, all members may be differently laid out so the compiler doesn't know how to copy the objects to one another.

See the links provided in the comments for tips and tricks.

No, typedef only creates an alias for an existsing type. But the struct definition does create a new type that is only compatible with itself. — Thomas Padron-McCarthy, Feb 19 '17 at 11:48

Typecasting user-define datatype in C

1 Answers1