What does ((Struct*)0) Mean?

Question

I encountered a problem in reading a piece of C code. code show as below:

#define size_of_attribute(Struct, Attribute) sizeof(((Struct*)0)->Attribute)

The function of this macro function is gets the length of the attribute in the struct. I know what this function is for, but i can't understand the meaning of " ((Struct*)0) ".

I will appeaciate If you can give me some explanation:).

Answer 1

The constant value 0 qualifies as a null pointer constant . The expression (Struct*)0 is therefore casting that null pointer constant to a pointer of type Struct * . The expression then gets the Attribute member.

Attempting to evaluate ((Struct*)0)->Attribute would result in a null pointer defererence, however this expression is the argument to the sizeof operator. This means the expression is not actually evaluated but simply examined to determine its type.

So sizeof(((Struct*)0)->Attribute) gives you the size of the Attribute member of the struct named Struct without having to have an object of that type.

Answer 2

It's casting a null pointer to the Struct* type so it can determine the size of the attribute of that struct. Normally, reading an attribute from NULL is illegal, but for sizeof , it doesn't actually read anything, it just looks at the definition of the struct to determine the statically defined size of the attribute of any such struct.

At least for C++, this is useful because unlike a non-pointer-based:

sizeof(Struct{}.Attribute)

it doesn't require Struct to have a default constructor. A pointer can be made with no knowledge of how to construct the object, while an actual object (even if none is actually constructed) must still be constructed in a valid way, and you can't say with any reliability how an arbitrary struct can be legally constructed.

Answer 3

This is basically accessing a member variable type without actually mentioning / creating any variable of that structure type.

Here,

• the 0 is casted to the structure type pointer, and
• then that pointer is used to access the member variable
• which is used as the operand of sizeof operator.

Since sizeof is a compile time operation, the NULL dereference never actually executes at runtime.