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.
| 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
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)
|
(since C++17) |
|
(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 (i.e. the expression is an unevaluated operand) (since C++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 C++17)
Keywords
Example
The example output corresponds to a system with 64-bit pointers and 32-bit int (a.k.a. LP64 or LLP64).
#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 C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| CWG 1122 | C++98 | result type of sizeof (std::size_t) was circularly defined | it is defined in the same way as in C |
| CWG 1553 | C++11 | sizeof could be used with bit-field xvalues
|
prohibited |
See also
alignof operator(C++11)
|
queries alignment requirements of a type |
sizeof... operator(C++11)
|
queries the number of elements in a parameter pack |
| provides an interface to query properties of all fundamental numeric types. (class template) |