Object Sizes and sizeof

Object Sizes and sizeof

Every object takes up memory, and the amount matters. C++ gives you a tool to ask exactly how much: the sizeof operator.

sizeof returns bytes

sizeof tells you how many bytes an object or type uses. You can ask about a variable or a type:

cout << sizeof(int);       // how big is an int? (4 on most machines)
cout << sizeof(double);    // how big is a double? (8)
cout << sizeof(x);         // how big is variable x?

• sizeof(type) - a size with parentheses around the type.
• sizeof(variable) - works on a variable too, parentheses optional.

The result is a size_t (that unsigned count type we met earlier).

A typical result

On a modern 64-bit machine, you'll often see:

Why sizes can vary

The C++ standard sets minimum sizes, not exact ones. A long might be 4 bytes on one machine and 8 on another; an int is at least 2 bytes and usually 4. That's why sizeof exists - instead of guessing, ask.

#include <iostream>
using namespace std;

int main() {
    cout << "int is " << sizeof(int) << " bytes" << endl;
    return 0;
}

sizeof is compile-time

sizeof is evaluated while compiling, not while running. The answer is baked into the program as a constant - no runtime cost at all.

TL;DR

• sizeof returns the size of a type or variable in bytes.
• sizeof(int) gives the bytes of an int; sizeof(x) works on a variable.
• Typical sizes: int 4, double 8, char and bool 1.
• Sizes vary by machine - the standard only sets minimums.
• sizeof is computed at compile time.