How does sizeof know the size of the operand array?

Question

This may be a stupid question but how does the sizeof operator know the size of an array operand when you don't pass in the amount of elements in the array. I know it doesn't return the total elements in the array but the size in bytes, but to get that it still has to know when the array ends. Just curious as to how this works.

Answer 1

sizeof is interpreted at compile time, and the compiler knows how the array was declared (and thus how much space it takes up). Calling sizeof on a dynamically-allocated array will likely not do what you want, because (as you mention) the end point of the array is not specified.

Answer 2

The problem underlying your trouble to understand this might be because you are confusing arrays and pointers, as so many do. However, arrays are not pointers. A double da[10] is an array of ten double, not a double*, and that's certainly known to the compiler when you ask it evaluate sizeof(da). You wouldn't be surprised that the compiler knows sizeof(double)?

The problem with arrays is that they decay to pointers to their first elements automatically in many contexts (like when they are passed to functions). But still, array are arrays and pointers are pointers.