I seem to have some misconceptions about `static` in C++ -

i seem have misconceptions how static keyword works in c++; specifically, need understand problem following code:

#include <iostream>  struct a{     int a;      a(int ain) : a(ain) { }     a() : a(1) { }      static geta()     {         return a(2);     } };  struct b {     static a = a::geta(); };   int main() {     std::cout << b::a.a << std::endl; } 

had static worked expected to, above code sample have printed 2 , exited - instead, following compiler error message:

g++    -c -o syntax.o syntax.cpp syntax.cpp:17:21: error: ‘a::geta()’ cannot appear in constant-expression      static a = a::geta();                      ^ syntax.cpp:17:26: error: function call cannot appear in constant-expression      static a = a::geta();                           ^ syntax.cpp:17:26: error: invalid in-class initialization of static data member of non-integral type ‘a’ syntax.cpp:17:26: error: initializer invalid static member constructor syntax.cpp:17:26: error: (an out of class initialization required) syntax.cpp:17:26: error: ‘b::a’ cannot initialized non-constant expression when being declared make: *** [syntax.o] error 1 

i read error messages , realize there's static haven't understood, find hard pinpoint what. it's because i've done fair amount of work in languages java , c#, have static keyword it, apparently, works differently.

will please explain me why above code invalid?

static member functions

there 2 ways of invoking static member function of given t, either;

• you use scope resolution operator ::, or;

• you invoke on instance of said type t using operator. (as if invoking other member-function on variable).

---

legal constructs

struct obj {   static void func () {      /* ... */   } }; 

the 2 snippets below both call static member function func within obj.

obj a; a.func (); 

obj::func (); 

---

illegal constructs

obj a;  obj.func (); // illegal, `obj` type , not instance ::func ();  // illegal, `a` instance of `obj`, not type 

---

---

static data members

static data members used declare variables shared between every instance of given type.

in snippet provided in question trying initialize static data member a of type a returned value from, corrected, a::geta (). not allowed since trying initialize static data member without making constexpr illegal construct in-class initialization.

struct b {   static a = a::geta(); // illegal, `static a` not `constexpr` }; 

more in-class initialization of static data members can found here:

• stackoverflow.com - c++ initialize static variables in class?