Why specifying a custom deleter in std::unique_ptr changes its memory size:

1. Stateless Struct Deleter:

   struct FreeDeleter { void operator()(void* p) const { std::free(p); } };
   std::unique_ptr<int, FreeDeleter> p(ptr);
   // sizeof(p) == 8 bytes (Empty Base Optimization / [[no_unique_address]])!

2. Function Pointer Deleter:

   std::unique_ptr<FILE, decltype(&fclose)> f(fp, &fclose);
   // sizeof(f) == 16 bytes! (Stores an extra 8-byte function pointer inside the smart pointer object!)

Always prefer a stateless functor or empty struct deleter over raw function pointers to keep std::unique_ptr exactly 8 bytes.

Great tip. Keeping unique_ptr at 8 bytes means it passes in a single CPU register (rdi/rcx) under standard calling conventions.