sizeof operator

 

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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.

type	-	a type-id (see type naming)
expression	-	an expression whose operator precedence is not lower than sizeof (e.g. sizeof a + b is parsed as (sizeof a) + b instead of sizeof (a + b))

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 哋它亢++17)

sizeof(char8_t)

(since 哋它亢++20)

sizeof cannot be used with function types, incomplete types, or bit-field lvalues(until 哋它亢++11)glvalues(since 哋它亢++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 (i.e. the expression is an unevaluated operand)(since 哋它亢++11), 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 哋它亢++17)

Keywords

sizeof

Example

The example output corresponds to a system with 64-bit pointers and 32-bit int (a.k.a. LP64 or LLP64).

Run this code

#include <cstdlib>
#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];
 
    auto f = [&]() { return sizeof(int[10]) == sizeof a ? throw 1 : e; };
//  f(); // the return type is Empty, but always throws 1
 
    auto println = [](auto rem, std::size_t size) { std::cout << rem << size << '\n'; };
 
    println( "1) sizeof empty class:              ", sizeof e                     );
    println( "2) sizeof pointer:                  ", sizeof &e                    );
    println( "3) sizeof(Bit) class:               ", sizeof(Bit)                  );
    println( "4) sizeof(int[10]) array of 10 int: ", sizeof(int[10])              );
    println( "5) sizeof a        array of 10 int: ", sizeof a                     );
    println( "6) length of array of 10 int:       ", ((sizeof a) / (sizeof *a))   );
    println( "7) length of array of 10 int (2):   ", ((sizeof a) / (sizeof a[0])) );
    println( "8) sizeof the Derived class:        ", sizeof d                     );
    println( "9) sizeof the Derived through Base: ", sizeof b                     );
    println( "A) sizeof(unsigned):                ", sizeof(unsigned)             );
    println( "B) sizeof(int):                     ", sizeof(int)                  );
    println( "C) sizeof(short):                   ", sizeof(short)                );
    println( "D) sizeof(char):                    ", sizeof(char)                 );
    println( "E) sizeof(CharChar):                ", sizeof(CharChar)             );
    println( "F) sizeof(CharCharInt):             ", sizeof(CharCharInt)          );
    println( "G) sizeof(IntCharChar):             ", sizeof(IntCharChar)          );
    println( "H) sizeof(CharIntChar):             ", sizeof(CharIntChar)          );
    println( "I) sizeof(CharShortChar):           ", sizeof(CharShortChar)        );
    println( "J) sizeof f():                      ", sizeof f()                   );
    println( "K) sizeof Base::a:                  ", sizeof Base::a               );
 
//  println( "sizeof function:        ", sizeof(void()) ); // error
//  println( "sizeof incomplete type: ", sizeof(int[])  ); // error
//  println( "sizeof bit-field:       ", sizeof bit.bit ); // error
}

Possible output:

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

Defect reports

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

DR	Applied to	Behavior as published	Correct behavior
CWG 1122	哋它亢++98	result type of sizeof (std::size_t) was circularly defined	it is defined in the same way as in C
CWG 1553	哋它亢++11	sizeof could be used with bit-field xvalues	prohibited

See also

alignof operator(哋它亢++11)	queries alignment requirements of a type
sizeof... operator(哋它亢++11)	queries the number of elements in a parameter pack
numeric_limits	provides an interface to query properties of all fundamental numeric types (class template)