span lite: A single-file header-only version of a C++20-like span for C++98, C++11 and later

Contents

• Example usage
• In a nutshell
• License
• Dependencies
• Installation and use
• Synopsis
• Reported to work with
• Building the tests
• Other implementations of span
• Notes and references
• Appendix

Example usage

#include "nonstd/span.hpp"
#include <array>
#include <vector>
#include <iostream>

std::ptrdiff_t size( nonstd::span<const int> spn )
{
    return spn.size();
}

int main()
{
    int arr[] = { 1, };

    std::cout << 
        "C-array:" << size( arr ) <<
        " array:"  << size( std::array <int, 2>{ 1, 2, } ) <<
        " vector:" << size( std::vector<int   >{ 1, 2, 3, } );
}

Compile and run

prompt> g++ -std=c++11 -Wall -I../include -o 01-basic.exe 01-basic.cpp && 01-basic.exe
C-array:1 array:2 vector:3

In a nutshell

span lite is a single-file header-only library to provide a bounds-safe view for sequences of objects. The library provides a C++20-like span for use with C++98 and later. If available, std::span is used, unless configured otherwise. span-lite can detect the presence of byte-lite and if present, it provides as_bytes() and as_writable_bytes() also for C++14 and earlier.

Features and properties of span lite are ease of installation (single header), freedom of dependencies other than the standard library. To compensate for the class template argument deduction that is missing from pre-C++17 compilers, nonstd::span can provide make_span functions. See configuration.

License

span lite is distributed under the Boost Software License.

Dependencies

span lite has no other dependencies than the C++ standard library.

Installation and use

span lite is a single-file header-only library. Put span.hpp in the include folder directly into the project source tree or somewhere reachable from your project.

Synopsis

Contents
Documentation of std::span
Later additions
Non-standard extensions
Configuration

Documentation of std::span

Depending on the compiler and C++-standard used, nonstd::span behaves less or more like std::span. To get an idea of the capabilities of nonstd::span with your configuration, look at the output of the tests, issuing span-main.t --pass @. For std::span, see its documentation at cppreference.

Later additions

back() and front()

span lite can provide back() and front() member functions for element access. See the table below and section configuration.

Non-standard extensions

Construct from std::initializer_list (p2447)

span lite can provide construction from a std::initializer_list<> as a constant set of values as proposed in p2447. See the table below and section configuration.

Construct from container

To construct a span from a container with compilers that cannot constrain such a single-parameter constructor to containers, span lite provides a constructor that takes an additional parameter of type with_container_t. Use with_container as value for this parameter. See the table below and section configuration.

Construct from std::array with const data

span lite can provide construction of a span from a std::array with const data. See the table below and section configuration.

operator()

span lite can provide member function call operator() for element access. It is equivalent to operator[] and has been marked [[deprecated]]. Its main purpose is to provide a migration path.

at()

span lite can provide member function at() for element access. Unless exceptions have been disabled, at() throws std::out_of_range if the index falls outside the span. With exceptions disabled, at(index_t) delegates bounds checking to operator[](index_t). See the table below and sections configuration and disable exceptions.

swap()

span lite can provide a swap()member function. See the table below and section configuration.

operator==() and other comparison functions

span lite can provide functions to compare the content of two spans. However, C++20's span will not provide comparison and span lite will omit comparison at default in the near future. See the table below and section configuration. See also Revisiting Regular Types.

same()

span lite can provide function same() to determine if two spans refer as identical spans to the same data via the same type. If same() is enabled, operator==() incorporates it in its comparison. See the table below and section configuration.

first(), last() and subspan()

span lite can provide functions first(), last() and subspan() to avoid having to use the dot template syntax when the span is a dependent type. See the table below and section configuration.

make_span()

span lite can provide make_span() creator functions to compensate for the class template argument deduction that is missing from pre-C++17 compilers. See the table below and section configuration.

byte_span()

span lite can provide byte_span() creator functions to represent an object as a span of bytes. This requires the C++17 type std::byte to be available. See the table below and section configuration.

Configuration

Tweak header

If the compiler supports __has_include(), span lite supports the tweak header mechanism. Provide your tweak header as nonstd/span.tweak.hpp in a folder in the include-search-path. In the tweak header, provide definitions as documented below, like #define span_CONFIG_NO_EXCEPTIONS 1.

Standard selection macro

-Dspan_CPLUSPLUS=199711L
Define this macro to override the auto-detection of the supported C++ standard, if your compiler does not set the __cplusplus macro correctly.

Select std::span or nonstd::span

At default, span lite uses std::span if it is available and lets you use it via namespace nonstd. You can however override this default and explicitly request to use std::span or span lite's nonstd::span as nonstd::span via the following macros.

-Dspan_CONFIG_SELECT_SPAN=span_SPAN_DEFAULT
Define this to span_SPAN_STD to select std::span as nonstd::span. Define this to span_SPAN_NONSTD to select nonstd::span as nonstd::span. Default is undefined, which has the same effect as defining to span_SPAN_DEFAULT.

Select extent type

-Dspan_CONFIG_EXTENT_TYPE=std::size_t
Define this to std::ptrdiff_t to use the signed type. The default is std::size_t, as in C++20 (since v0.7.0).

Select size type

-Dspan_CONFIG_SIZE_TYPE=std::size_t
Define this to std::ptrdiff_t to use the signed type. The default is std::size_t, as in C++20 (since v0.7.0). Note span_CONFIG_SIZE_TYPE replaces span_CONFIG_INDEX_TYPE which is deprecated.

Disable exceptions

-Dspan_CONFIG_NO_EXCEPTIONS=0
Define this to 1 if you want to compile without exceptions. If not defined, the header tries and detect if exceptions have been disabled (e.g. via -fno-exceptions). Disabling exceptions will force contract violation to use termination, see contract violation macros. Default is undefined.

Provide construction from std::initializer_list (p2447)

-Dspan_FEATURE_WITH_INITIALIZER_LIST_P2447=0
Define this to 1 to enable constructing a span from a std::initializer_list<> as a constant set of values. See proposal p2447. Default is undefined.

Provide construction using with_container_t

-Dspan_FEATURE_WITH_CONTAINER=0
Define this to 1 to enable constructing a span using with_container_t. Note that span_FEATURE_WITH_CONTAINER takes precedence over span_FEATURE_WITH_CONTAINER_TO_STD. Default is undefined.

-Dspan_FEATURE_WITH_CONTAINER_TO_STD=n
Define this to the highest C++ language version for which to enable constructing a span using with_container_t, like 98, 03, 11, 14, 17, 20. You can use 99 for inclusion with any standard, but prefer to use span_FEATURE_WITH_CONTAINER for this. Note that span_FEATURE_WITH_CONTAINER takes precedence over span_FEATURE_WITH_CONTAINER_TO_STD. Default is undefined.

Provide construction from std::array with const data

-Dspan_FEATURE_CONSTRUCTION_FROM_STDARRAY_ELEMENT_TYPE=0
Define this to 1 to enable constructing a span from a std::array with const data. Default is undefined.

Provide operator() member function

-Dspan_FEATURE_MEMBER_CALL_OPERATOR=0
Define this to 1 to provide member function operator()for element access. It is equivalent to operator[] and has been marked [[deprecated]]. Its main purpose is to provide a migration path. Default is undefined.

Provide at() member function

-Dspan_FEATURE_MEMBER_AT=0
Define this to 1 to provide member function at(). Define this to 2 to include index and size in message of std::out_of_range exception. Default is undefined.

Provide back() and front() member functions

-Dspan_FEATURE_MEMBER_BACKFRONT=1 (on since v0.5.0)_
Define this to 0 to omit member functions back() and front(). Default is undefined.

Provide swap() member function

-Dspan_FEATURE_MEMBER_SWAP=0
Define this to 1 to provide member function swap(). Default is undefined.

Provide operator==() and other comparison functions

-Dspan_FEATURE_COMPARISON=0
Define this to 1 to include the comparison functions to compare the content of two spans. C++20's span does not provide comparison and span lite omits comparison from v0.7.0. Default is undefined.

Provide same() function

-Dspan_FEATURE_SAME=0
Define this to 1 to provide function same() to test if two spans refer as identical spans to the same data via the same type. If same() is enabled, operator==() incorporates it in its comparison. Default is undefined.

Provide first(), last() and subspan() functions

-Dspan_FEATURE_NON_MEMBER_FIRST_LAST_SUB=0
Define this to 1 to enable both span_FEATURE_NON_MEMBER_FIRST_LAST_SUB_SPAN and span_FEATURE_NON_MEMBER_FIRST_LAST_SUB_CONTAINER. Default is undefined.

-Dspan_FEATURE_NON_MEMBER_FIRST_LAST_SUB_SPAN=0
Define this to 1 to provide functions first(), last() and subspan() that take a span<> (work with C++98). This implies span_FEATURE_MAKE_SPAN to provide functions make_span() that are required for this feature. Default is undefined.

-Dspan_FEATURE_NON_MEMBER_FIRST_LAST_SUB_CONTAINER=0
Define this to 1 to provide functions first(), last() and subspan() that take a compatible container (requires C++11). This implies span_FEATURE_MAKE_SPAN to provide functions make_span() that are required for this feature. Default is undefined.

Provide make_span() functions

-Dspan_FEATURE_MAKE_SPAN=0
Define this to 1 to provide creator functions nonstd::make_span(). This feature is implied by using span_FEATURE_NON_MEMBER_FIRST_LAST_SUB=1. Note that span_FEATURE_MAKE_SPAN takes precedence over span_FEATURE_MAKE_SPAN_TO_STD. Default is undefined.

-Dspan_FEATURE_MAKE_SPAN_TO_STD=n
Define this to the highest C++ language version for which to provide creator functions nonstd::make_span(), like 98, 03, 11, 14, 17, 20. You can use 99 for inclusion with any standard, but prefer to use span_FEATURE_MAKE_SPAN for this. Note that span_FEATURE_MAKE_SPAN takes precedence over span_FEATURE_MAKE_SPAN_TO_STD. Default is undefined.

Provide byte_span() functions

-Dspan_FEATURE_BYTE_SPAN=0
Define this to 1 to provide creator functions nonstd::byte_span(). Default is undefined.

Contract violation response macros

span-lite provides contract violation response control as suggested in proposal N4415.

-Dspan_CONFIG_CONTRACT_LEVEL_ON (default)
Define this macro to include both span_EXPECTS and span_ENSURES in the code. This is the default case.

-Dspan_CONFIG_CONTRACT_LEVEL_OFF
Define this macro to exclude both span_EXPECTS and span_ENSURES from the code.

-Dspan_CONFIG_CONTRACT_LEVEL_EXPECTS_ONLY
Define this macro to include span_EXPECTS in the code and exclude span_ENSURES from the code.

-Dspan_CONFIG_CONTRACT_LEVEL_ENSURES_ONLY
Define this macro to exclude span_EXPECTS from the code and include span_ENSURES in the code.

-Dspan_CONFIG_CONTRACT_VIOLATION_TERMINATES (default)
Define this macro to call std::terminate() on a contract violation in span_EXPECTS, span_ENSURES. This is the default case.

-Dspan_CONFIG_CONTRACT_VIOLATION_THROWS
Define this macro to throw an exception of implementation-defined type that is derived from std::runtime_exception instead of calling std::terminate() on a contract violation in span_EXPECTS and span_ENSURES. See also disable exceptions.

Reported to work with

The table below mentions the compiler versions span lite is reported to work with.

Building the tests

To build the tests you need:

• CMake, version 3.0 or later to be installed and in your PATH.
• A suitable compiler.

The lest test framework is included in the test folder.

The following steps assume that the span lite source code has been cloned into a directory named ./span-lite.

1. Create a directory for the build outputs.

   cd ./span-lite
   md build && cd build

2. Configure CMake to use the compiler of your choice (run cmake --help for a list).

   cmake -G "Unix Makefiles" -DSPAN_LITE_OPT_BUILD_TESTS=ON ..

3. Optional. You can control above configuration through the following options:

   -DSPAN_LITE_OPT_BUILD_TESTS=ON: build the tests for span, default off
   -DSPAN_LITE_OPT_BUILD_EXAMPLES=OFF: build the examples, default off

4. Build the test suite.

   cmake --build .

5. Run the test suite.

   ctest -V

All tests should pass, indicating your platform is supported and you are ready to use span lite.

Other implementations of span

• gsl-lite span.
• Microsoft GSL span.
• Google Abseil span.
• Marshall Clow's libc++ span snippet.
• Tristan Brindle's Implementation of C++20's std::span for older compilers.
• Search span c++ on GitHub.

Notes and references

Interface and specification

• span on cppreference.
• p0122 - C++20 Proposal.
• span in C++20 Working Draft.

Presentations

• TBD

Proposals

• p0122 - span: bounds-safe views for sequences of objects.
• p1024 - Usability Enhancements for std::span.
• p1419 - A SFINAE-friendly trait to determine the extent of statically sized containers.
• p0805 - Comparing Containers.
• p1085 - Should Span be Regular?.
• p0091 - Template argument deduction for class templates.
• p0856 - Restrict Access Property for mdspan and span.
• p1428 - Subscripts and sizes should be signed.
• p1089 - Sizes Should Only span Unsigned.
• p1227 - Signed size() functions.
• p1872 - span should have size_type, not index_type.
• p2447 - std::span and the missing constructor.
• lwg 3101 - span's Container constructors need another constraint.
• Reddit - 2018-06 Rapperswil ISO C++ Committee Trip Report
• Reddit - 2018-11 San Diego ISO C++ Committee Trip Report.
• Reddit - 2019-02 Kona ISO C++ Committee Trip Report.
• Reddit - 2019-07 Cologne ISO C++ Committee Trip Report
• Reddit - 2019-11 Belfast ISO C++ Committee Trip Report
• Titus Winters. Revisiting Regular Types. Abseil Blog. 31 May 2018.

Appendix

A.1 Compile-time information

The version of span lite is available via tag [.version]. The following tags are available for information on the compiler and on the C++ standard library used: [.compiler], [.stdc++], [.stdlanguage] and [.stdlibrary].

A.2 Span lite test specification