A common mistake when writing const-correct code in C++ is that a const initialized object can still manipulate the values that member pointers points at. The following example illustrates the problem:
class Foo {public: Foo(int* ptr) : ptr_{ptr} {} auto set_ptr_val(int v) const { *ptr_ = v; // Compiles despite function being declared const! }private: int* ptr_{};};auto main() -> int { const auto foo = Foo{}; foo.set_ptr_val(42);}
Although the function set_ptr_val() is mutating the int value, it's valid to declared it const since the pointer ptr_ itself is not mutated, only the int object that the pointer is pointing at.
In order to prevent this in a readable way, a wrapper called std::experimental::propagate_const ...
