CODEWITHSUNDEEP
cvisual-c++visual-studio-2008struct
Calmarius
Calmarius

Reputation: 19431

Does C allow casting a struct type to itself?

So we have a struct type like this:

typedef struct
{
    U64 low;
    U64 high; 
} U128;

Then somewhere in the code as a result of macro expansion there is an assignment like this:

*ptr = (U128)value;

Where ptr is of U128*. And this causes the following error:

error C2440: 'type cast' : cannot convert from 'U128' to 'U128'

The question is: Are such self casts allowed in C? Am I just spotted a compiler bug?

Additional question:

This is a generalized macro that allows 8, 16, 32, 64, 128, etc. as arguments, and other types are typedefd as numbers and work without problems: are there any workaround? I would like to avoid memcpy for performance reasons.

Upvotes: 3

Views: 303

Answers (1)

unwind
unwind

Reputation: 399803

No.

The (draft C11) standard text that requires this behavior is in §6.5.4.2:

Unless the type name specifies a void type, the type name shall specify atomic, qualified, or unqualified scalar type, and the operand shall have scalar type

In other words, you can't cast structs at all.

One fix could of course be to remove the right-hand side cast, or do a bitwise copy:

memcpy(ptr, &value, sizeof *ptr);

This will perhaps be optimized away, since the size being copied is quite small. Note that sizeof *ptr is a safer choice, using sizeof value would risk overflowing if the incoming value has an unexpected type.

Upvotes: 5

Related Questions