| Endian Arithmetic Types

Endian Home Conversion Functions Arithmetic Types Buffer Types Choosing Approach

Contents
Introduction Example Limitations Feature set Enums and typedefs Class template `endian` Synopsis Members Stream inserter Stream extractor FAQ Design Experience Motivating use cases C++11 Compilation Acknowledgements

## Introduction

Header boost/endian/arithmetic.hpp provides integer binary types with control over byte order, value type, size, and alignment. Typedefs provide easy-to-use names for common configurations.

These types provide portable byte-holders for integer data, independent of particular computer architectures. Use cases almost always involve I/O, either via files or network connections. Although data portability is the primary motivation, these integer byte-holders may also be used to reduce memory use, file size, or network activity since they provide binary integer sizes not otherwise available.

Such integer byte-holder types are traditionally called endian types. See the Wikipedia for a full exploration of endianness, including definitions of big endian and little endian.

Boost endian integers provide the same full set of C++ assignment, arithmetic, and relational operators as C++ standard integral types, with the standard semantics.

Unary arithmetic operators are `+`, `-`, `~`, `!`, plus both prefix and postfix `--` and `++`. Binary arithmetic operators are `+`, `+=`, `-`, `-=`, `*`, `*=`, `/`, `/=`, `&`, `&=`, `|`, `|=`, `^`, `^=`, `<<`, `<<=`, `>>`, and `>>=`. Binary relational operators are `==`, `!=`, `<`, `<=`, `>`, and `>=`.

Implicit conversion to the underlying value type is provided. An implicit constructor converting from the underlying value type is provided.

## Example

The endian_example.cpp program writes a binary file containing four-byte, big-endian and little-endian integers:

>     #include <iostream>
>     #include <cstdio>
>     #include <boost/endian/arithmetic.hpp>
>     #include <boost/static_assert.hpp>

>     using namespace boost::endian;

>     namespace
>     {
>       //  This is an extract from a very widely used GIS file format.
>       //  Why the designer decided to mix big and little endians in
>       //  the same file is not known. But this is a real-world format
>       //  and users wishing to write low level code manipulating these
>       //  files have to deal with the mixed endianness.

>       struct header
>       {
>         big_int32_t     file_code;
>         big_int32_t     file_length;
>         little_int32_t  version;
>         little_int32_t  shape_type;
>       };

>       const char* filename = "test.dat";
>     }

>     int main(int, char* [])
>     {
>       header h;

>       BOOST_STATIC_ASSERT(sizeof(h) == 16U);  // reality check

>       h.file_code   = 0x01020304;
>       h.file_length = sizeof(header);
>       h.version     = 1;
>       h.shape_type  = 0x01020304;

>       //  Low-level I/O such as POSIX read/write or <cstdio>
>       //  fread/fwrite is sometimes used for binary file operations
>       //  when ultimate efficiency is important. Such I/O is often
>       //  performed in some C++ wrapper class, but to drive home the
>       //  point that endian integers are often used in fairly
>       //  low-level code that does bulk I/O operations, <cstdio>
>       //  fopen/fwrite is used for I/O in this example.

>       std::FILE* fi = std::fopen(filename, "wb");  // MUST BE BINARY

>       if (!fi)
>       {
>         std::cout << "could not open " << filename << '\n';
>         return 1;
>       }

>       if (std::fwrite(&h, sizeof(header), 1, fi)!= 1)
>       {
>         std::cout << "write failure for " << filename << '\n';
>         return 1;
>       }

>       std::fclose(fi);

>       std::cout << "created file " << filename << '\n';

>       return 0;
>     }

After compiling and executing endian_example.cpp, a hex dump of `test.dat` shows:

>     01020304 00000010 01000000 04030201

Notice that the first two 32-bit integers are big endian while the second two are little endian, even though the machine this was compiled and run on was little endian.

## Limitations

Requires `<climits>` `CHAR_BIT == 8`. If `CHAR_BIT` is some other value, compilation will result in an `#error`. This restriction is in place because the design, implementation, testing, and documentation has only considered issues related to 8-bit bytes, and there have been no real-world use cases presented for other sizes.

In C++03, `endian_arithmetic` does not meet the requirements for POD types because it has constructors, private data members, and a base class. This means that common use cases are relying on unspecified behavior in that the C++ Standard does not guarantee memory layout for non-POD types. This has not been a problem in practice since all known C++ compilers lay out memory as if `endian` were a POD type. In C++11, it is possible to specify the default constructor as trivial, and private data members and base classes no longer disqualify a type from being a POD type. Thus under C++11, `endian_arithmetic` will no longer be relying on unspecified behavior.

## Feature set

- Big endian| little endian | native endian byte ordering.
- Signed | unsigned
- Unaligned | aligned
- 1-8 byte (unaligned) | 1, 2, 4, 8 byte (aligned)
- Choice of value type

## Enums and typedefs

Two scoped enums are provided:

>     enum class order {big, little, native};

>     enum class align {no, yes};

One class template is provided:

>     template <order Order, typename T, std::size_t n_bits,
>       align Align = align::no>
>     class endian_arithmetic;

Typedefs, such as `big_int32_t`, provide convenient naming conventions for common use cases:

> Name | Alignment | Endianness | Sign | Sizes in bits (n)
> `big_int` n`_t` | `no` | `big` | signed | 8,16,24,32,40,48,56,64
> `big_uint` n`_t` | `no` | `big` | unsigned | 8,16,24,32,40,48,56,64
> `little_int` n`_t` | `no` | `little` | signed | 8,16,24,32,40,48,56,64
> `little_uint` n`_t` | `no` | `little` | unsigned | 8,16,24,32,40,48,56,64
> `native_int` n`_t` | `no` | `native` | signed | 8,16,24,32,40,48,56,64
> `native_uint` n`_t` | `no` | `native` | unsigned | 8,16,24,32,40,48,56,64
> `big_int` n`_at` | `yes` | `big` | signed | 8,16,32,64
> `big_uint` n`_at` | `yes` | `big` | unsigned | 8,16,32,64
> `little_int` n`_at` | `yes` | `little` | signed | 8,16,32,64
> `little_uint` n`_at` | `yes` | `little` | unsigned | 8,16,32,64

The unaligned types do not cause compilers to insert padding bytes in classes and structs. This is an important characteristic that can be exploited to minimize wasted space in memory, files, and network transmissions.

Warning: Code that uses aligned types is possibly non-portable because alignment requirements vary between hardware architectures and because alignment may be affected by compiler switches or pragmas. For example, alignment of an 64-bit integer may be to a 32-bit boundary on a 32-bit machine. Furthermore, aligned types are only available on architectures with 8, 16, 32, and 64-bit integer types.

Recommendation: Prefer unaligned arithmetic types.

Recommendation: Protect yourself against alignment ills. For example:

>     static_assert(sizeof(containing_struct) == 12, "sizeof(containing_struct) is wrong");

Note: Note: One-byte arithmetic types have identical layout on all platforms, so they never actually reverse endianness. They are provided to enable generic code, and to improve code readability and searchability.

## Class template `endian``_arithmetic`

An endian is an integer byte-holder with user-specified endianness, value type, size, and alignment. The usual operations on integers are supplied.

### Synopsis

    namespace boost
    {
      namespace endian
      {
        //  C++11 features emulated if not available

        enum class order
        {
          big,                             // big-endian
          little,                          // little-endian
          native = implementation-defined  // same as order::big or order::little
        };

        enum class align {no, yes};

        template <order Order, class T, std::size_t n_bits,
          align Align = align::no>
        class endian_arithmetic
          : public endian_buffer<Order, T, n_bits, Align>
        {
        public:
          typedef T value_type;

          // if BOOST_ENDIAN_FORCE_PODNESS is defined && C++11 PODs are not
          // available then these two constructors will not be present
          endian_arithmetic() noexcept = default;
          endian_arithmetic(T v) noexcept;

          endian_arithmetic& operator=(T v) noexcept;
          operator value_type() const noexcept;
          value_type value() const noexcept; // for exposition; see endian_buffer
          const char* data() const noexcept; // for exposition; see endian_buffer

          // arithmetic operations
          //   note that additional operations are provided by the value_type
          value_type operator+(const endian& x) noexcept;
          endian& operator+=(endian& x, value_type y) noexcept;
          endian& operator-=(endian& x, value_type y) noexcept;
          endian& operator*=(endian& x, value_type y) noexcept;
          endian& operator/=(endian& x, value_type y) noexcept;
          endian& operator%=(endian& x, value_type y) noexcept;
          endian& operator&=(endian& x, value_type y) noexcept;
          endian& operator|=(endian& x, value_type y) noexcept;
          endian& operator^=(endian& x, value_type y) noexcept;
          endian& operator<<=(endian& x, value_type y) noexcept;
          endian& operator>>=(endian& x, value_type y noexcept;
          value_type operator<<(const endian& x, value_type y) noexcept;
          value_type operator>>(const endian& x, value_type y) noexcept;
          endian& operator++(endian& x) noexcept;
          endian& operator--(endian& x) noexcept;
          endian operator++(endian& x, int) noexcept;
          endian operator--(endian& x, int) noexcept;

          // Stream inserter
          template <class charT, class traits>
          friend std::basic_ostream<charT, traits>&
            operator<<(std::basic_ostream<charT, traits>& os, const T& x);

          // Stream extractor
          template <class charT, class traits>
          friend std::basic_istream<charT, traits>&
            operator>>(std::basic_istream<charT, traits>& is, T& x);
        };

        // typedefs

        // unaligned big endian signed integer types
        typedef endian<order::big, int_least8_t, 8>        big_int8_t;
        typedef endian<order::big, int_least16_t, 16>      big_int16_t;
        typedef endian<order::big, int_least32_t, 24>      big_int24_t;
        typedef endian<order::big, int_least32_t, 32>      big_int32_t;
        typedef endian<order::big, int_least64_t, 40>      big_int40_t;
        typedef endian<order::big, int_least64_t, 48>      big_int48_t;
        typedef endian<order::big, int_least64_t, 56>      big_int56_t;
        typedef endian<order::big, int_least64_t, 64>      big_int64_t;

        // unaligned big endian unsigned integer types
        typedef endian<order::big, uint_least8_t, 8>       big_uint8_t;
        typedef endian<order::big, uint_least16_t, 16>     big_uint16_t;
        typedef endian<order::big, uint_least32_t, 24>     big_uint24_t;
        typedef endian<order::big, uint_least32_t, 32>     big_uint32_t;
        typedef endian<order::big, uint_least64_t, 40>     big_uint40_t;
        typedef endian<order::big, uint_least64_t, 48>     big_uint48_t;
        typedef endian<order::big, uint_least64_t, 56>     big_uint56_t;
        typedef endian<order::big, uint_least64_t, 64>     big_uint64_t;

        // unaligned little endian signed integer types
        typedef endian<order::little, int_least8_t, 8>     little_int8_t;
        typedef endian<order::little, int_least16_t, 16>   little_int16_t;
        typedef endian<order::little, int_least32_t, 24>   little_int24_t;
        typedef endian<order::little, int_least32_t, 32>   little_int32_t;
        typedef endian<order::little, int_least64_t, 40>   little_int40_t;
        typedef endian<order::little, int_least64_t, 48>   little_int48_t;
        typedef endian<order::little, int_least64_t, 56>   little_int56_t;
        typedef endian<order::little, int_least64_t, 64>   little_int64_t;

        // unaligned little endian unsigned integer types
        typedef endian<order::little, uint_least8_t, 8>    little_uint8_t;
        typedef endian<order::little, uint_least16_t, 16>  little_uint16_t;
        typedef endian<order::little, uint_least32_t, 24>  little_uint24_t;
        typedef endian<order::little, uint_least32_t, 32>  little_uint32_t;
        typedef endian<order::little, uint_least64_t, 40>  little_uint40_t;
        typedef endian<order::little, uint_least64_t, 48>  little_uint48_t;
        typedef endian<order::little, uint_least64_t, 56>  little_uint56_t;
        typedef endian<order::little, uint_least64_t, 64>  little_uint64_t;

        // unaligned native endian signed integer types
        typedef implementation-defined_int8_t   native_int8_t;
        typedef implementation-defined_int16_t  native_int16_t;
        typedef implementation-defined_int24_t  native_int24_t;
        typedef implementation-defined_int32_t  native_int32_t;
        typedef implementation-defined_int40_t  native_int40_t;
        typedef implementation-defined_int48_t  native_int48_t;
        typedef implementation-defined_int56_t  native_int56_t;
        typedef implementation-defined_int64_t  native_int64_t;

        // unaligned native endian unsigned integer types
        typedef implementation-defined_uint8_t   native_uint8_t;
        typedef implementation-defined_uint16_t  native_uint16_t;
        typedef implementation-defined_uint24_t  native_uint24_t;
        typedef implementation-defined_uint32_t  native_uint32_t;
        typedef implementation-defined_uint40_t  native_uint40_t;
        typedef implementation-defined_uint48_t  native_uint48_t;
        typedef implementation-defined_uint56_t  native_uint56_t;
        typedef implementation-defined_uint64_t  native_uint64_t;

        // aligned big endian signed integer types
        typedef endian<order::big, int8_t, 8, align::yes>       big_int8_at;
        typedef endian<order::big, int16_t, 16, align::yes>     big_int16_at;
        typedef endian<order::big, int32_t, 32, align::yes>     big_int32_at;
        typedef endian<order::big, int64_t, 64, align::yes>     big_int64_at;

        // aligned big endian unsigned integer types
        typedef endian<order::big, uint8_t, 8, align::yes>      big_uint8_at;
        typedef endian<order::big, uint16_t, 16, align::yes>    big_uint16_at;
        typedef endian<order::big, uint32_t, 32, align::yes>    big_uint32_at;
        typedef endian<order::big, uint64_t, 64, align::yes>    big_uint64_at;

        // aligned little endian signed integer types
        typedef endian<order::little, int8_t, 8, align::yes>    little_int8_at;
        typedef endian<order::little, int16_t, 16, align::yes>  little_int16_at;
        typedef endian<order::little, int32_t, 32, align::yes>  little_int32_at;
        typedef endian<order::little, int64_t, 64, align::yes>  little_int64_at;

        // aligned little endian unsigned integer types
        typedef endian<order::little, uint8_t, 8, align::yes>   little_uint8_at;
        typedef endian<order::little, uint16_t, 16, align::yes> little_uint16_at;
        typedef endian<order::little, uint32_t, 32, align::yes> little_uint32_at;
        typedef endian<order::little, uint64_t, 64, align::yes> little_uint64_at;

        // aligned native endian typedefs are not provided because
        // <cstdint> types are superior for that use case

      } // namespace endian
    } // namespace boost

The `implementation-defined` text above is either `big` or `little` according to the endianness of the platform.

### Members

    endian() = default;  // C++03: endian(){}

> Effects: Constructs an uninitialized object of type `endian_arithmetic<E, T, n_bits, A>`.

    endian(T v);

> Effects: Constructs an object of type `endian_arithmetic<E, T, n_bits, A>`.

> Postcondition: `x == v,` where `x` is the constructed object.

    endian& operator=(T v);

> Postcondition: `x == v,` where `x` is the constructed object.

> Returns: `*this`.

    operator T() const;

> Returns: The current value stored in `*this`, converted to `value_type`.

    const char* data() const;

> Returns: A pointer to the first byte of the endian binary value stored in `*this`.

### Other operators

Other operators on endian objects are forwarded to the equivalent operator on `value_type`.

### Stream inserter

    template <class charT, class traits>
    friend std::basic_ostream<charT, traits>&
      operator<<(std::basic_ostream<charT, traits>& os, const T& x);

> Returns: `os << +x`.

### Stream extractor

    template <class charT, class traits>
    friend std::basic_istream<charT, traits>&
      operator>>(std::basic_istream<charT, traits>& is, T& x);

> Effects: As if:

> >     T i;
> >     if (is >> i)
> >       x = i;

> Returns: `is`.

## FAQ

See the Endian home page FAQ for a library-wide FAQ.

Why not just use Boost.Serialization? Serialization involves a conversion for every object involved in I/O. Endian integers require no conversion or copying. They are already in the desired format for binary I/O. Thus they can be read or written in bulk.

Are endian types PODs? Yes for C++11. No for C++03, although several macros are available to force PODness in all cases.

What are the implications of endian integer types not being PODs with C++03 compilers? They can't be used in unions. Also, compilers aren't required to align or lay out storage in portable ways, although this potential problem hasn't prevented use of Boost.Endian with real compilers.

What good is native endianness? It provides alignment and size guarantees not available from the built-in types. It eases generic programming.

Why bother with the aligned endian types? Aligned integer operations may be faster (as much as 10 to 20 times faster) if the endianness and alignment of the type matches the endianness and alignment requirements of the machine. The code, however, will be somewhat less portable than with the unaligned types.

Why provide the arithmetic operations? Providing a full set of operations reduces program clutter and makes code both easier to write and to read. Consider incrementing a variable in a record. It is very convenient to write:

        ++record.foo;

Rather than:

        int temp(record.foo);
        ++temp;
        record.foo = temp;

## Design considerations for Boost.Endian types

- Must be suitable for I/O - in other words, must be memcpyable.
- Must provide exactly the size and internal byte ordering specified.
- Must work correctly when the internal integer representation has more bits that the sum of the bits in the external byte representation. Sign extension must work correctly when the internal integer representation type has more bits than the sum of the bits in the external bytes. For example, using a 64-bit integer internally to represent 40-bit (5 byte) numbers must work for both positive and negative values.
- Must work correctly (including using the same defined external representation) regardless of whether a compiler treats char as signed or unsigned.
- Unaligned types must not cause compilers to insert padding bytes.
- The implementation should supply optimizations with great care. Experience has shown that optimizations of endian integers often become pessimizations when changing machines or compilers. Pessimizations can also happen when changing compiler switches, compiler versions, or CPU models of the same architecture.

## Experience

Classes with similar functionality have been independently developed by several Boost programmers and used very successful in high-value, high-use applications for many years. These independently developed endian libraries often evolved from C libraries that were also widely used. Endian types have proven widely useful across a wide range of computer architectures and applications.

## Motivating use cases

Neil Mayhew writes: "I can also provide a meaningful use-case for this library: reading TrueType font files from disk and processing the contents. The data format has fixed endianness (big) and has unaligned values in various places. Using Boost.Endian simplifies and cleans the code wonderfully."

## C++11

The availability of the C++11 Defaulted Functions feature is detected automatically, and will be used if present to ensure that objects of `class endian_arithmetic` are trivial, and thus PODs.

## Compilation

Boost.Endian is implemented entirely within headers, with no need to link to any Boost object libraries.

Several macros allow user control over features:

- BOOST_ENDIAN_NO_CTORS causes `class endian_arithmetic` to have no constructors. The intended use is for compiling user code that must be portable between compilers regardless of C++11 Defaulted Functions support. Use of constructors will always fail,
- BOOST_ENDIAN_FORCE_PODNESS causes BOOST_ENDIAN_NO_CTORS to be defined if the compiler does not support C++11 Defaulted Functions. This is ensures that objects of `class endian_arithmetic` are PODs, and so can be used in C++03 unions. In C++11, `class endian_arithmetic` objects are PODs, even though they have constructors, so can always be used in unions.

## Acknowledgements

Original design developed by Darin Adler based on classes developed by Mark Borgerding. Four original class templates combined into a single `endian_arithmetic` class template by Beman Dawes, who put the library together, provided documentation, added the typedefs, and also added the `unrolled_byte_loops` sign partial specialization to correctly extend the sign when cover integer size differs from endian representation size.

Last revised: 25 March, 2015

© Copyright Beman Dawes, 2006-2009, 2013

Distributed under the Boost Software License, Version 1.0. See www.boost.org/ LICENSE_1_0.txt