sizeof operator

Queries size of the object or type.

Used when actual size of the object must be known.

Syntax

`sizeof(` type `)`	(1)
`sizeof` expression	(2)

Both versions are constant expressions of type std::size_t.

Explanation

1) Yields the size in bytes of the object representation of type.

2) Yields the size in bytes of the object representation of the type of expression, if that expression is evaluated.

Notes

Depending on the computer architecture, a byte may consist of 8 or more bits, the exact number being recorded in CHAR_BIT.

The following sizeof expressions always evaluate to 1:

sizeof(char)
sizeof(signed char)
sizeof(unsigned char)

sizeof(std::byte)

(since C++17)

sizeof(char8_t)

(since C++20)

sizeof cannot be used with function types, incomplete types, or bit-field lvalues (until C++11)glvalues (since C++11).

When applied to a reference type, the result is the size of the referenced type.

When applied to a class type, the result is the number of bytes occupied by a complete object of that class, including any additional padding required to place such object in an array. The number of bytes occupied by a potentially-overlapping subobject may be less than the size of that object.

The result of sizeof is always nonzero, even if applied to an empty class type.

When applied to an expression, sizeof does not evaluate the expression, and even if the expression designates a polymorphic object, the result is the size of the static type of the expression. Lvalue-to-rvalue, array-to-pointer, or function-to-pointer conversions are not performed. Temporary materialization, however, is (formally) performed for prvalue arguments: the program is ill-formed if the argument is not destructible. (since C++17).

Keywords

sizeof.

Example

The example output corresponds to a system with 64-bit pointers and 32-bit int.

#include <iostream>
 
struct Empty {};
struct Base { int a; };
struct Derived : Base { int b; };
struct Bit { unsigned bit: 1; };
struct CharChar      { char c; char c2; };
struct CharCharInt   { char c; char c2; int i; };
struct IntCharChar   { int i;  char c;  char c2; };
struct CharIntChar   { char c; int i;   char c2; };
struct CharShortChar { char c; short s; char c2; };
 
int main()
{
    Empty e;
    Derived d;
    Base& b = d;
    [[maybe_unused]] Bit bit;
    int a[10];
 
    std::cout 
      << "1) sizeof empty class:              " << sizeof e         << '\n'
      << "2) sizeof pointer:                  " << sizeof &e        << '\n'
//    << "3) sizeof function:                 " << sizeof(void())   << '\n' // error
//    << "4) sizeof incomplete type:          " << sizeof(int[])    << '\n' // error
//    << "5) sizeof bit field:                " << sizeof bit.bit   << '\n' // error
      << "6) sizeof(Bit) class:               " << sizeof(Bit)      << '\n'
      << "7) sizeof(int[10]) array of 10 int: " << sizeof(int[10])  << '\n'
      << "8) sizeof a        array of 10 int: " << sizeof a         << '\n'
      << "9) length of array of 10 int:       " << ((sizeof a) / (sizeof *a))   << '\n'
      << "A) length of array of 10 int (2):   " << ((sizeof a) / (sizeof a[0])) << '\n'
      << "B) sizeof the Derived class:        " << sizeof d         << '\n'
      << "C) sizeof the Derived through Base: " << sizeof b         << '\n'
      << "D) sizeof(unsigned)                 " << sizeof(unsigned) << '\n'
      << "E) sizeof(int)                      " << sizeof(int)      << '\n'
      << "F) sizeof(short)                    " << sizeof(short)    << '\n'
      << "G) sizeof(char)                     " << sizeof(char)     << '\n'
      << "H) sizeof(CharChar)                 " << sizeof(CharChar) << '\n'
      << "I) sizeof(CharCharInt)              " << sizeof(CharCharInt) << '\n'
      << "J) sizeof(IntCharChar)              " << sizeof(IntCharChar) << '\n'
      << "K) sizeof(CharIntChar)              " << sizeof(CharIntChar) << '\n'
      << "L) sizeof(CharShortChar)            " << sizeof(CharShortChar) << '\n';
}

Possible output:

1) sizeof empty class:              1
2) sizeof pointer:                  8
6) sizeof(Bit) class:               4
7) sizeof(int[10]) array of 10 int: 40
8) sizeof a        array of 10 int: 40
9) length of array of 10 int:       10
A) length of array of 10 int (2):   10
B) sizeof the Derived class:        8
C) sizeof the Derived through Base: 4
D) sizeof(unsigned)                 4
E) sizeof(int)                      4
F) sizeof(short)                    2
G) sizeof(char)                     1
H) sizeof(CharChar)                 2
I) sizeof(CharCharInt)              8
J) sizeof(IntCharChar)              8
K) sizeof(CharIntChar)              12
L) sizeof(CharShortChar)            6

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 1553	C++11	`sizeof` could be used with bit-field xvalues	prohibited

`alignof` operator(C++11)	queries alignment requirements of a type
`sizeof...` operator(C++11)	queries the number of elements in a parameter pack
C documentation for `sizeof`

Name	C++
Version
Badge
Last Updated	2022-06-02T22:02:33Z