---
title: "C++ Bit Fields | Microsoft Docs"
ms.custom: ""
ms.date: "11/04/2016"
ms.technology: ["cpp-language"]
ms.topic: "language-reference"
dev_langs: ["C++"]
helpviewer_keywords: ["bitfields [C++]", "fields [C++], bit", "bit fields"]
ms.assetid: 6f4b62e3-cc1d-4e5d-bf34-05904104f71a
author: "mikeblome"
ms.author: "mblome"
ms.workload: ["cplusplus"]
---
# 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 a date object](../cpp/media/vc38uq1.png "vc38UQ1")
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](../cpp/media/vc38uq2.png "vc38UQ2")
Layout of Date Object with Zero-Length Bit Field

The underlying type of a bit field must be an integral type, as described in [Fundamental 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)