It could be done as you say, in 16 bytes (assuming 64 bit pointers): struct not_too_long_string { char* data; uint32_t size; uint32

It could be done as you say, in 16 bytes (assuming 64 bit pointers): struct not_too_long_string { char* data; uint32_t size; uint32_t capacity; }; Of course, if you do it that way then the longest string you can store with the small string optimization is probably ~15 bytes instead of ~23. So although you do save 1/3 on the size of each string, on average you're probably still going to end up doing a greater number of dynamic allocations because of the reduced small string capacity. Unless of course you know a priori that a sufficiently large portion of your strings will be > 15 bytes anyway, which of course the implementors of std::string almost certainly don't know. Edit: I failed to notice the part about the length being in the data block (doh). I guess the disadvantage to putting the length there would be that an extra indirection is required to get the length, a rather common operation. And as others have pointed out, that only saves 4 bytes, which will be used anyway for alignment..