Mapbox Variant

An alternative to boost::variant for C++11.

Why use Mapbox Variant?

Mapbox variant has the same speedy performance of boost::variant but is faster to compile, results in smaller binaries, and has no dependencies.

For example on OS X 10.9 with clang++ and libc++:

Test	Mapbox Variant	Boost Variant
Size of pre-compiled header (release / debug)	2.8/2.8 MB	12/15 MB
Size of simple program linking variant (release / debug)	8/24 K	12/40 K
Time to compile header	185 ms	675 ms

(Numbers from an older version of Mapbox variant.)

Goals

Mapbox variant has been a very valuable, lightweight alternative for apps that can use c++11 or c++14 but that do not want a boost dependency. Mapbox variant has also been useful in apps that do depend on boost, like mapnik, to help (slightly) with compile times and to majorly lessen dependence on boost in core headers. The original goal and near term goal is to maintain external API compatibility with boost::variant such that Mapbox variant can be a "drop in". At the same time the goal is to stay minimal: Only implement the features that are actually needed in existing software. So being an "incomplete" implementation is just fine.

Currently Mapbox variant doesn't try to be API compatible with the upcoming variant standard, because the standard is not finished and it would be too much work. But we'll revisit this decision in the future if needed.

If Mapbox variant is not for you, have a look at these other implementations.

Want to know more about the upcoming standard? Have a look at our overview.

Depends

• Compiler supporting -std=c++11 or -std=c++14

Tested with:

• g++-4.7
• g++-4.8
• g++-4.9
• g++-5
• clang++-3.5
• clang++-3.6
• clang++-3.7
• clang++-3.8
• Visual Studio 2015

Usage

There is nothing to build, just include variant.hpp and recursive_wrapper.hpp in your project. Include variant_io.hpp if you need the operator<< overload for variant.

Unit Tests

On Unix systems compile and run the unit tests with make test.

On Windows run scripts/build-local.bat.

Limitations

• The variant can not hold references (something like variant<int&> is not possible). You might want to try std::reference_wrapper instead.

Deprecations

• The included implementation of optional is deprecated and will be removed in a future version. See mapbox#64.
• Old versions of the code needed visitors to derive from static_visitor. This is not needed any more and marked as deprecated. The static_visitor class will be removed in future versions.

Benchmarks

The benchmarks depend on:

• Boost headers (for benchmarking against boost::variant)
• Boost built with --with-timer (used for benchmark timing)

On Unix systems set your boost includes and libs locations and run make test:

export LDFLAGS='-L/opt/boost/lib'
export CXXFLAGS='-I/opt/boost/include'
make bench

Check object sizes

make sizes /path/to/boost/variant.hpp