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 return a constant of type std::size_t.

Explanation

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

2) Returns size in bytes of the object representation of the type that would be returned by expression, if evaluated.

Notes

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

sizeof(char), sizeof(char8_t), sizeof(signed char), and sizeof(unsigned char) always return 1.

sizeof cannot be used with function types, incomplete types, or bit-field glvalues.

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 size of an object of that class plus any additional padding required to place such object in an array.

When applied to an empty class type, always returns 1.

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: sizeof determines the size of the result object. (since C++17)

Keywords

sizeof

Example

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

Run this code

#include <iostream>
 
struct Empty {};
struct Base { int a; };
struct Derived : Base { int b; };
struct Bit { unsigned bit: 1; };
 
int main()
{
    Empty e;
    Derived d;
    Base& b = d;
    [[maybe_unused]] Bit bit;
    int a[10];
    std::cout << "size of empty class: "              << sizeof e          << '\n'
              << "size of pointer: "                  << sizeof &e         << '\n'
//            << "size of function: "                 << sizeof(void())    << '\n'  // error
//            << "size of incomplete type: "          << sizeof(int[])     << '\n'  // error
//            << "size of bit field: "                << sizeof bit.bit    << '\n'  // error
              << "size of array of 10 int: "          << sizeof(int[10])   << '\n'
              << "size of array of 10 int (2): "      << sizeof a          << '\n'
              << "length of array of 10 int: "        << ((sizeof a) / (sizeof *a)) << '\n'
              << "length of array of 10 int (2): "    << ((sizeof a) / (sizeof a[0])) << '\n'
              << "size of the Derived: "              << sizeof d          << '\n'
              << "size of the Derived through Base: " << sizeof b          << '\n';
 
}

Possible output:

size of empty class: 1
size of pointer: 8
size of array of 10 int: 40
size of array of 10 int (2): 40
length of array of 10 int: 10
length of array of 10 int (2): 10
size of the Derived: 8
size of the Derived through Base: 4

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

General
value categories (lvalue, rvalue, xvalue)
order of evaluation (sequence points)
constant expressions
unevaluated expressions
primary expressions
lambda-expression(C++11)
Literals
integer literals
floating-point literals
boolean literals
character literals including escape sequences
string literals
null pointer literal(C++11)
user-defined literal(C++11)
Operators
Assignment operators: `a=b`, `a+=b`, `a-=b`, `a*=b`, `a/=b`, `a%=b`, `a&=b`, `a\|=b`, `a^=b`, `a<<=b`, `a>>=b`
Increment and decrement: `++a`, `--a`, `a++`, `a--`
Arithmetic operators: `+a`, `-a`, `a+b`, `a-b`, `a*b`, `a/b`, `a%b`, `~a`, `a&b`, `a\|b`, `a^b`, `a<<b`, `a>>b`
Logical operators: `a\|\|b`, `a&&b`, `!a`
Comparison operators: `a==b`, `a!=b`, `a<b`, `a>b`, `a<=b`, `a>=b`, `a<=>b`(C++20)
Member access operators: `a[b]`, `a`, `&a`, `a->b`, `a.b`, `a->b`, `a.*b`
Other operators: `a(...)`, `a,b`, `a?b:c`
Default comparisons(C++20)
Alternative representations of operators
Precedence and associativity
Fold expression(C++17)
new-expression
delete-expression
throw-expression
alignof
sizeof
sizeof...(C++11)
typeid
noexcept(C++11)
Operator overloading
Conversions
Implicit conversions
const_cast
static_cast
reinterpret_cast
dynamic_cast
Explicit conversions `(T)a`, `T(a)`
User-defined conversion