Notes

A pointer literal meaning “points at nothing.” Introduced in C++11 to replace the older NULL and plain 0.

int* p = nullptr;

if (p == nullptr) { ... }   // explicit and clear
if (!p)            { ... }   // equivalent, more common in practice

Why not just use 0 or NULL

void f(int x);
void f(char* p);

f(NULL);     // ambiguous, or picks the wrong overload - NULL is often just `0`,
             // an int, not a pointer at all
f(nullptr);  // unambiguous - only matches the pointer overload

nullptr has its own type, std::nullptr_t, which converts to any pointer type but not to int. That’s the entire fix.

Related

Pointer basics