C++11/C++14 The nullptr and strong typed enumerations- 2020

"If you're an experienced C++ programmer and are anything like me, you initially approached C++11 thinking, "Yes, yes, I get it. It's C++, only more so." But as you learned more, you were surprised by the scope of the changes. auto declarations, range-based for loops, lambda expressions, and rvalue references change the face of C++, to say nothing of the new concurrency features. And then there are the idiomatic changes. 0 and typedefs are out, nullptr and alias declarations are in. Enums should now be scoped. Smart pointers are now preferable to built-in ones. Moving objects is normally better than copying them.
- Effective Modern C++ by Scott Meyers

The constant 0 has had the double role of constant integer and null pointer constant. The ambiguity inherent in the double meaning of 0 was dealt with in C by the use of the preprocessor macro NULL, which commonly expands to either ((void*)0) or 0.

C++ didn't adopt the same behavior, allowing only 0 as a null pointer constant.

This interacts poorly with function overloading:

void foo(char *);
void foo(int);

If NULL is defined as 0, which function the statement foo(NULL) will call?

Probably, it will call foo(int), which is almost certainly not what the programmer intended. In other words, there is an ambiguity.

So, C++11 corrected this by introducing a new keyword to serve as a distinguished null pointer constant (nullptr):

char *pc = nullptr;     // OK
int  *pi = nullptr;     // OK
bool   b = nullptr;     // OK. b is false.
int    i = nullptr;     // error

foo(nullptr);           // calls foo(char *), not foo(int);

In C++03, enumerations are not type-safe. They are effectively integers, even when the enumeration types are distinct. This allows the comparison between two enum values of different enumeration types.

    // C++ 03

    enum Day {Sunday, Monday, Tuesday};
    enum Month {January, February, March};

    Day d = Monday;
    Month m = February;

    if (d == m)
	std::cout << "Monday == February\n";  // This will be printed out
    else
	std::cout << "Monday != February\n";

    // C++ 11
    enum class Day { Sunday, Monday, Tuesday };
    enum class Month { January, February, March };

    Day d = Day::Monday;
    Month m = Month::February;

    // No "==" operator is defined for the object type nor overloaded
    // So, in C++ 11, this is an compile time error
    if (d == m)
	std::cout << "Monday == February\n";
    else
	std::cout << "Monday != February\n";

C++11/C++14 New Features