I came up with a kind of clever data type for storing short strings in a fixed size struct so they can be stored on the stack or inline without any allocations.
It's always null-terminated so it can be passed directly as a C-style string, but it also stores the string length without using any additional data (Getting the length would normally have to iterate to find the end).
The trick is to store the number of unused bytes in the last character of the buffer. When the string is full, there are 0 unused bytes and the size byte overlaps the null terminator.
(Only works for strings < 256 chars excluding null byte)

Implementation in C++ here: https://github.com/frustra/strayphotons/blob/master/src/common/common/InlineString.hh

Edit: Since a couple people don't seem to understand the performance impact of this vs regular std::string, here's a demo: https://godbolt.org/z/34j7obnbs This generates 10000 strings like "Hello, World! 00001" via concatenation. The effect is huge in debug mode, but still has performance benefits with optimizations turned on:

With -O3 optimization
std::string: 0.949216ms
char[256] with strlen: 0.88104ms
char[256] without strlen: 0.684734ms

With no optimization:
std::string: 3.5501ms
char[256] with strlen: 0.885888ms
char[256] without strlen: 0.687733ms

(You may need to run it a few times to get sample numbers due to random server load on godbolt)
Changing the buffer size to 32 bytes has a negligible performance improvement over 256 bytes in this case, but might be slightly faster due to the whole string fitting in a cache line.