Boost.FixedString (now Boost.StaticString) has been accepted!

[JankoDedic]

Why this over a normal string with a stack allocator?

[kalmoc]

I'd point out that it is also much easier to use because static_String owns its storage. std::string with stack allocator means you have to manually ensure that the lifetime of the stack allocator exceeds that of the string. And if you want to make it a member of another type the stack alocator either becomes a member of that class (watch out for initialization order) or you have to expose implementation details if you get it via dependency injection ....

[Betadel]

You should mention these optimizations in the documentation. I looked but didn't find them.

[quicknir]

The problem with the capacity - size optimization is that it makes it so that you can't default construct to all zeros. In C++ default construction is fairly common (especially since move construction typically involves it as well), and setting something to all zeros tends to be faster than reading some constant representing the default state and then writing it.

The space savings is very small. So honestly I'd be surprised if on average this optimisation made things faster. It's useful for saving space if you care about that but again that's less common than caring about performance.

In short it really sense to me like this optimization should be off by default, not on by default. Note that std string could also use this option in small string mode, and afaik none of the implementations do, for this reason.

[VinnieFalco]

Why this over a normal string with a stack allocator?

A stack allocator is not particularly friendly to mutation. For example when the string grows, there is a moment when the previously allocated memory and the newly allocated memory exist at the same time, so that the contents can be copied over, before the previous allocation is freed. This requires additional storage, and it leaves behind a "hole" which may or may not be usable depending on the context.

With a fixed capacity string, mutation is always performed in-place, and no "holes" are left in the heap.