c++ - Initialisation of static class member. Why constexpr? -

when want have static pointer member of class need constexprfor initialisation nullptr.

class application {     private:         constexpr static application* app = nullptr; } 

can explain me why need that? cannot find exact reason why it`s necessary static variable has exist @ compile time.

that's because you're initialising inside class definition. that's allowed constant integral , enumeration types (always) , constexpr data members (since c++11). normally, you'd initialise define (outside class), this:

application.h

class application {     private:         static application* app; } 

application.cpp

application* application::app = nullptr; 

note need provide out-of-class definition in constexpr case, must not contain initialiser then. still, believe second case want.