[expr.sizeof]

# 7 Expressions [[expr]](./#expr)

## 7.6 Compound expressions [[expr.compound]](expr.compound#expr.sizeof)

### 7.6.2 Unary expressions [[expr.unary]](expr.unary#expr.sizeof)

#### 7.6.2.5 Sizeof [expr.sizeof]

[1](#1)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/expressions.tex#L5607)

The sizeof operator yields the number of bytes
occupied by a non-potentially-overlapping object of the type
of its operand[.](#1.sentence-1)

The operand is either an expression,
which is an [unevaluated operand](expr.context#def:unevaluated_operand "7.2.3 Context dependence [expr.context]"), or a parenthesized[*type-id*](dcl.name#nt:type-id "9.3.2 Type names [dcl.name]")[.](#1.sentence-2)

The sizeof operator shall not be applied to an expression that
has function or incomplete type,
to the parenthesized name of such
types, or to a glvalue that designates a bit-field[.](#1.sentence-3)

The result of sizeof applied to any of the narrow character types is 1[.](#1.sentence-4)

The result ofsizeof applied to any other fundamental
type ([[basic.fundamental]](basic.fundamental "6.9.2 Fundamental types")) is implementation-defined[.](#1.sentence-5)

[*Note [1](#note-1)*:

In particular, the values of sizeof(bool), sizeof(char16_t),sizeof(char32_t), and sizeof(wchar_t) are
implementation-defined[.](#1.sentence-6)[57](#footnote-57 "sizeof(bool) is not required to be 1.")

— *end note*]

[*Note [2](#note-2)*:

See [[intro.memory]](intro.memory "6.8.1 Memory model") for the definition of byte
and [[basic.types.general]](basic.types.general#term.object.representation "6.9.1 General") for the definition of object representation[.](#1.sentence-7)

— *end note*]

[2](#2)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/expressions.tex#L5641)

When applied to a reference type, the result is the size
of the referenced type[.](#2.sentence-1)

When applied to a class, the result is the number of bytes in an object
of that class including any padding required for placing objects of that
type in an array[.](#2.sentence-2)

The result of applying sizeof to a
potentially-overlapping subobject is
the size of the type, not the size of the subobject[.](#2.sentence-3)[58](#footnote-58 "The actual size of a potentially-overlapping subobject can be less than the result of applying sizeof to the subobject, due to virtual base classes and less strict padding requirements on potentially-overlapping subobjects.")

When applied to an array, the result is the total number of bytes in the
array[.](#2.sentence-4)

This implies that the size of an array of n elements isn times the size of an element[.](#2.sentence-5)

[3](#3)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/expressions.tex#L5663)

The lvalue-to-rvalue ([[conv.lval]](conv.lval "7.3.2 Lvalue-to-rvalue conversion")),
array-to-pointer ([[conv.array]](conv.array "7.3.3 Array-to-pointer conversion")), and
function-to-pointer ([[conv.func]](conv.func "7.3.4 Function-to-pointer conversion")) standard conversions are not
applied to the operand of sizeof[.](#3.sentence-1)

If the operand is a prvalue,
the [temporary materialization conversion](conv.rval "7.3.5 Temporary materialization conversion [conv.rval]") is applied[.](#3.sentence-2)

[4](#4)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/expressions.tex#L5672)

The [*identifier*](lex.name#nt:identifier "5.11 Identifiers [lex.name]") in a sizeof... expression shall name a
pack[.](#4.sentence-1)

The sizeof... operator yields the number of elements
in the pack ([[temp.variadic]](temp.variadic "13.7.4 Variadic templates"))[.](#4.sentence-2)

A sizeof... expression is a pack expansion ([[temp.variadic]](temp.variadic "13.7.4 Variadic templates"))[.](#4.sentence-3)

[*Example [1](#example-1)*: template<class... Types>struct count {static constexpr std::size_t value = sizeof...(Types);}; — *end example*]

[5](#5)

[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/expressions.tex#L5686)

The result of sizeof and sizeof... is a prvalue of typestd​::​size_t[.](#5.sentence-1)

[*Note [3](#note-3)*:

A sizeof expression
is an integral constant expression ([[expr.const]](expr.const "7.7 Constant expressions"))[.](#5.sentence-2)

The [*typedef-name*](dcl.typedef#nt:typedef-name "9.2.4 The typedef specifier [dcl.typedef]") std​::​size_t is declared in the standard header[<cstddef>](cstddef.syn#header:%3ccstddef%3e "17.2.1 Header <cstddef> synopsis [cstddef.syn]") ([[cstddef.syn]](cstddef.syn "17.2.1 Header <cstddef> synopsis"), [[support.types.layout]](support.types.layout "17.2.4 Sizes, alignments, and offsets"))[.](#5.sentence-3)

— *end note*]

[57)](#footnote-57)[57)](#footnoteref-57)

sizeof(bool) is not required to be 1[.](#footnote-57.sentence-1)

[58)](#footnote-58)[58)](#footnoteref-58)

The actual size of a potentially-overlapping subobject
can be less than the result of
applying sizeof to the subobject, due to virtual base classes
and less strict padding requirements on potentially-overlapping subobjects[.](#footnote-58.sentence-1)