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)
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 toget sample numbers due to random server load on godbolt)
Changing the buffer size to32 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.
Interesting idea, but your trick is never really going to help (you can store up to 255 bytes instead of 254). Also always using 256 bytes for every string seems wasteful.
22 characters is significantly less useful than 255 characters. I use this for resource name keys, asset file paths, and a few other scenarios. The max size is configurable, so I know that nothing I am going to store is ever going to require heap allocations (really bad to be doing every frame in a game engine).
I developed this specifically after benchmarking a simpler version and noticed a significant amount of time being spent in strlen(), and it had real benefits in my case.
Admittedly just storing a struct with a static buffer and separate size would have worked pretty much the same and eliminated the 255 char limitation, but it was fun to build.
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.Interesting idea, but your trick is never really going to help (you can store up to 255 bytes instead of 254). Also always using 256 bytes for every string seems wasteful.
I think LLVM’s small string optimisation is always going to be a better option: https://joellaity.com/2020/01/31/string.html
22 characters is significantly less useful than 255 characters. I use this for resource name keys, asset file paths, and a few other scenarios. The max size is configurable, so I know that nothing I am going to store is ever going to require heap allocations (really bad to be doing every frame in a game engine).
I developed this specifically after benchmarking a simpler version and noticed a significant amount of time being spent in strlen(), and it had real benefits in my case.
Admittedly just storing a struct with a static buffer and separate size would have worked pretty much the same and eliminated the 255 char limitation, but it was fun to build.