variant-hpp
v1.1.0-alpha
Published
C++11/C++14 variant
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Mapbox Variant
An alternative to boost::variant
for C++11 and C++14
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++-5.2
- clang++-3.5
- clang++-3.6
- clang++-3.7
- clang++-3.8
- clang++-3.9
- 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 likevariant<int&>
is not possible). You might want to trystd::reference_wrapper
instead.
Deprecations
- The included implementation of
optional
is deprecated and will be removed in a future version. See https://github.com/mapbox/variant/issues/64. - Old versions of the code needed visitors to derive from
static_visitor
. This is not needed any more and marked as deprecated. Thestatic_visitor
class will be removed in future versions.
Benchmarks
make bench
Check object sizes
make sizes /path/to/boost/variant.hpp