sizeof Operator: Compile-time or Run-time?

In C/C++ we use sizeof operator in C/C++ to calculate the size of any datatype. To use the operator to the fullest it is important to understand whether this operator works at compile-time or run-time. Well, a compile-time operator means that gets evaluated entirely at the time of compilation. Let us take an example

#include <stdio.h> int main(void) {             int i = 10;             int* j = new int [i];             int k[] = {10, 2, 3, 4};             int* l = k;                         printf("size of int = %d\n", sizeof(int));             printf("size of i = %d\n", sizeof(i));             printf("size of j = %d\n", sizeof(*j));             printf("size of k = %d\n", sizeof(k));             printf("size of l = %d\n", sizeof(l));             delete [] j;             return 0; }

In C++ programming language, sizeof works at compile-time. In the code given above, all the call to sizeof [i.e. sizeof(int), sizeof(i), sizeof(k), sizeof(l)]  can be entirely evaluated at the compile-time and so the above piece of code is valid. The output of this program would be as below:

size of int = 4 size of i = 4 size of j = 4 size of k = 16 size of l = 4

Note that in above example the size of array k is known at the compile time. Whereas it is impossible for compiler to determine the size of dynamic array j.

Difference in behavior of sizeof in C and C++:

• C language(C99 standard) supports variable-length arrays (VLAs) and for which the ‘sizeof’ operator does not necessarily evaluate to a compile-time value;  it can also be evaluated at runtime.
• Whereas C++ language supports templates and for which the ‘sizeof’ need to be evaluated at compile-time only.

You can learn few more interesting points about sizeof at the following link:

• http://www.joyofprogramming.com/Docs_ColumnArticles/35-JoP-Nov-09.pdf
• http://en.wikipedia.org/wiki/Sizeof