7 Expressions [expr]

7.6 Compound expressions [expr.compound]

7.6.2 Unary expressions [expr.unary] Sizeof [expr.sizeof]

The sizeof operator yields the number of bytes occupied by a non-potentially-overlapping object of the type of its operand.
The operand is either an expression, which is an unevaluated operand ([expr.prop]), or a parenthesized type-id.
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.
The result of sizeof applied to any of the narrow character types is 1.
The result of sizeof applied to any other fundamental type ([basic.fundamental]) is implementation-defined.
[Note 1:
In particular, the values of sizeof(bool), sizeof(char16_­t), sizeof(char32_­t), and sizeof(wchar_­t) are implementation-defined.72
— end note]
[Note 2:
See [intro.memory] for the definition of byte and [basic.types] for the definition of object representation.
— end note]
When applied to a reference type, the result is the size of the referenced type.
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.
The result of applying sizeof to a potentially-overlapping subobject is the size of the type, not the size of the subobject.73
When applied to an array, the result is the total number of bytes in the array.
This implies that the size of an array of n elements is n times the size of an element.
The lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) standard conversions are not applied to the operand of sizeof.
If the operand is a prvalue, the temporary materialization conversion is applied.
The identifier in a sizeof... expression shall name a pack.
The sizeof... operator yields the number of elements in the pack ([temp.variadic]).
A sizeof... expression is a pack expansion ([temp.variadic]).
[Example 1: template<class... Types> struct count { static const std::size_t value = sizeof...(Types); }; — end example]
The result of sizeof and sizeof... is a prvalue of type std​::​size_­t.
[Note 3:
A sizeof expression is an integral constant expression ([expr.const]).
The type std​::​size_­t is defined in the standard header <cstddef> ([cstddef.syn], [support.types.layout]).
— end note]
sizeof(bool) is not required to be 1.
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.