c++ - Implicit cast of null pointer to bool

Question

Suppose I have Foo* foo = nullptr;

If I'm checking whether or not foo is nullptr, am I permitted to write

if (!foo)

or should I write

if (foo == nullptr)

Answer 1

See this standard reference (bold emphasis mine):

C++11 §4.12 Boolean conversions

A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true. A prvalue of type std::nullptr_t can be converted to a prvalue of type bool; the resulting value is false.

The middle sentence is relevant: it is telling you that the null pointer value (foo = nullptr) can be implicitly cast to false which itself has type bool. Therefore if (!foo) is well-defined.