Comment by aw1621107
1 year ago
This style of small string optimization tries to take up the same amount of space on the stack as a "normal" heap-allocated string. On 64-bit platforms that is 24 bytes: 8 bytes for the pointer to the heap allocation, 8 bytes for the number of characters/bytes in the string, and 8 bytes for the allocation capacity.
It's quite possible to make the small string buffer larger, but that comes at the cost of the large string representation taking up more space than necessary on the stack. IIRC libstdc++ does this, which makes its std::string take up 32 bytes on the stack.
> It's quite possible to make the small string buffer larger, but that comes at the cost of the large string representation taking up more space than necessary on the stack. IIRC libstdc++ does this, which makes its std::string take up 32 bytes on the stack.
Though to follow through on that, 24 bytes is also more than necessary. You don't have to be very clever to shrink your string size to 16 bytes (6 for the pointer, 6 for the size, 4 to store either capacity or spare capacity as floating point).
True, though as always that comes with tradeoffs of its own.
Oh! So the string itself is still on the heap? I assumed it was all on the stack.
No, let me try to explain differently. If `compact_str` was not used, then your normal `String` would take 24 bytes of stack space (regardless of the string size) + heap space. What `compact_str` is trying to do is not use heap when string content is less than 24.
Thank you. That aligns with what I originally thought.