Difference between revisions of "cpp/language/sizeof"

Revision as of 11:52, 21 September 2020

Queries size of the object or type.

Used when actual size of the object must be known.

Syntax

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

Explanation

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),sizeof(unsigned char), and sizeof(std::byte) are always equal to 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.

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

Keywords

sizeof

Example

#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 Bit class:                " << sizeof(Bit)     << '\n'
      << "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';
}

size of empty class:              1
size of pointer:                  8
size of Bit class:                4
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

See also

• alignof
• sizeof...