TIL-3: final, Small Integer operations, boolean vector

final

• final is a way to forbid you overriding a virtual function again
  • It should not be used with override as it implies override
• final allows compilers to do optimization^[1]

Base virtual destructor

1. Base virtual destructor implies children virtual destructor, so we can use rule of 0 here.

Small Integer Operations

• When doing arithmetic operations on small unsigned integers (those with rank less than int), they will be promoted to int^[2]
  • the deducted return type with auto will become int
  • any further operations on this value (e.g. right shift) are operating on int, which can yield unexpected impact
• To solve this problem, when doing arithmetic operations on small unsigned integer, we need to use static_cast to change their type back to the original unsigned type

See Jason Turner’s video C++ Weekly - Ep 310 - Your Small Integer Operations Are Broken! for more details.

Weird vector bool

• C++ standard “recommends” a space-optimized specialization of std::vector<bool>
  • This optimization works by actually placing bool inside a bit
• However, if we need to access the member inside the list, the operator[] returns a proxy object for us
  • that allows us to view/change the value
• But the proxy project cannot be bound to lvalue reference auto & while other instantiation of vector all allow you to auto &ref = v[idx]
  • To store the proxy object, you need to use auto sth = v[idx]
  • If you want to use bit field by yourself, you cannot bind the member by reference either

See Jason Turner’s video C++ Weekly - Ep 325 - Why vector of bool is Weird for more details.

1. Compilers are allowed to call function directly instead of following vtable. See this answer for more details. ↩

2. cppreference provides a detailed explanation about arithmetic conversions and integer promotions ↩