sizeof(value) vs sizeof(type)?

Question

Considering :

double data; 
double array[10]; 
std::vector vec(4, 100); 
MyClass myclass;   

Is there a difference between :

Answer 1

The latter is defined in terms of the former. Given an expression, it returns the size of the type of the expression. So sizeof(vec) translates to sizeof(std::vector).

Both are evaluated at compile-time; the only run-time sizeof is in C, not C++, with variable-length arrays. The operand to sizeof is unevaluated, so there isn't any real possibly the expression itself could generate code anyway.

I prefer to use expressions over types, because chances are stating the type is redundant in some fashion.