C++ STD Unordered Set/Map vs Boost Unordered Set/Map

December 22, 2021

What are the differences between them, and when should you use each?

I have tried a few tests on an old laptop and there seems to be no significant performance difference for storing basic types like ints and longs. I think one of the main difference is boost container emplace methods dont support std::piecewise_construct and tuples, which causes additional overhead.

Edit: the stuff I’m working on already has a lot of boost features, so I’m not worried about compatibility issues with boost libraries.

>Solution :

The Boost ones have some features that do not exist in the standard library. Off the top of my head:

Boost Hash, which is more flexible and easier to customize than specializing std::hash<> (though specializing boost::hash<> is also supported; the easier route is to implement a inline friend size_t hash_value(T const&) which will "magically" be picked up by the default hash<> implementation
Boost tends to support heterogeneous lookup better (look for extended find/insert signatures)
The ordered versions may have extra constructors to efficiently construct over known ordered sequences
In general Boost containers (including others from the Boost Container library) have more guarantees/options:
- (better) support for stateful allocators (including scoped_allocator_adaptor, so with full uses_allocator/allocator_arg_t support)
- constructors don’t allocate
- some support for incomplete types in the the template arguments

As far as I know piecewise construction is perfectly fine in Boost. E.g. Change notes lists for 1.48.0:

* `emplace` used to emulate the variadic pair constructors that
  appeared in early C++0x drafts. Since they were removed it no
  longer does so. It does emulate the new `piecewise_construct`
  pair constructors - only you need to use
  `boost::piecewise_construct`. To use the old emulation of
  the variadic constructors define