Why does the string created with SecureRandom.uuid in Ruby 2.6.6 take exactly 77 bytes?
irb(main):018:0> ObjectSpace.memsize_of(SecureRandom.uuid)
=> 77
If I copy and paste the string it will take only 40.
irb(main):021:0> SecureRandom.uuid
=> "bfd59b9c-3248-409f-bcba-2df11df62c13"
irb(main):022:0> ObjectSpace.memsize_of("bfd59b9c-3248-409f-bcba-2df11df62c13")
=> 40
If you look here https://github.com/ruby/ruby/blob/b59077eecf912a16efefc0256f6e94a000ce3888/gc.c#L4066 you can see, that memsize_of returns
size + sizeof(RVALUE)
where size calculation is kinda complicated, but you should be able to find the proper branch for T_STRING easily:
size += rb_str_memsize(obj);
In turn, if you look at the latter - https://github.com/ruby/ruby/blob/8c2e5bbf58e562ea410b53c2f77e4186d5ca9da3/string.c#L1404 - you will see it returns either 0 or STR_HEAP_SIZE(str) - the actual space allocated for the string's content.
I don't fully understand the flags' meaning (don't know Ruby internals well enough), but I suppose they are just different for literals and strings created in runtime - so your last case gives you just the size of RVALUE itself (40 bytes) while the first one adds the real string data size to it...