What is the cost of sizeof?

Question

What is the cost of sizeof?

I would expect:

• sizeof(someclass) can be known at compile time
• sizeof(someStaticArray) can be known at compile time
• sizeof(someDynamicArray) can not be known at compile time

So how does that last case work?

Answer 1

The sizeof construct in C is a completely compile time construct. There is no runtime cost.

There is at least one exception to this rule: variable length arrays. The size of these arrays are computed at runtime and that size is reused for any sizeof operators applied to them.

Please note there is a difference between a variable length array and a dynamic one. Variable length arrays were added in C99 and they do support the sizeof operator

• http://en.wikipedia.org/wiki/Sizeof

Answer 2

sizeof(dynamicArray) will just return sizeof(pointer) because in c/c++ dynamic arrays are just pointers.