---
title: "C++ Bit Fields"
ms.date: "11/19/2018"
helpviewer_keywords: ["bitfields [C++]", "fields [C++], bit", "bit fields"]
ms.assetid: 6f4b62e3-cc1d-4e5d-bf34-05904104f71a
---
# C++ Bit Fields
Classes and structures can contain members that occupy less storage than an integral type. These members are specified as bit fields. The syntax for bit-field *member-declarator* specification follows:
## Syntax
*declarator* **:** *constant-expression*
## Remarks
The (optional) *declarator* is the name by which the member is accessed in the program. It must be an integral type (including enumerated types). The *constant-expression* specifies the number of bits the member occupies in the structure. Anonymous bit fields — that is, bit-field members with no identifier — can be used for padding.
> [!NOTE]
> An unnamed bit field of width 0 forces alignment of the next bit field to the next **type** boundary, where **type** is the type of the member.
The following example declares a structure that contains bit fields:
```cpp
// bit_fields1.cpp
// compile with: /LD
struct Date {
unsigned short nWeekDay : 3; // 0..7 (3 bits)
unsigned short nMonthDay : 6; // 0..31 (6 bits)
unsigned short nMonth : 5; // 0..12 (5 bits)
unsigned short nYear : 8; // 0..100 (8 bits)
};
```
The conceptual memory layout of an object of type `Date` is shown in the following figure.
Memory Layout of Date Object
Note that `nYear` is 8 bits long and would overflow the word boundary of the declared type, **unsigned** **short**. Therefore, it is begun at the beginning of a new **unsigned** **short**. It is not necessary that all bit fields fit in one object of the underlying type; new units of storage are allocated, according to the number of bits requested in the declaration.
**Microsoft Specific**
The ordering of data declared as bit fields is from low to high bit, as shown in the figure above.
**END Microsoft Specific**
If the declaration of a structure includes an unnamed field of length 0, as shown in the following example,
```cpp
// bit_fields2.cpp
// compile with: /LD
struct Date {
unsigned nWeekDay : 3; // 0..7 (3 bits)
unsigned nMonthDay : 6; // 0..31 (6 bits)
unsigned : 0; // Force alignment to next boundary.
unsigned nMonth : 5; // 0..12 (5 bits)
unsigned nYear : 8; // 0..100 (8 bits)
};
```
then the memory layout is as shown in the following figure:
Layout of Date Object with Zero-Length Bit Field
The underlying type of a bit field must be an integral type, as described in [Built-in types](../cpp/fundamental-types-cpp.md).
If the initializer for a reference of type `const T&` is an lvalue that refers to a bit field of type `T`, the reference is not bound to the bit field directly. Instead, the reference is bound to a temporary initialized to hold the value of the bit field.
## Restrictions on bit fields
The following list details erroneous operations on bit fields:
- Taking the address of a bit field.
- Initializing a non-**const** reference with a bit field.
## See also
[Classes and Structs](../cpp/classes-and-structs-cpp.md)