如何使用函数初始化静态常量成员

Question

In a very simplified case, I have the following setup in which, I simply want to initialize a constant static member(of class foo) from functions of class A (singleton and instance are unrelated to this question):

class A
{
public:

    static A instance;
    A & getInstance() { return instance; }
    int i(){ return 10;}
    int j(){ return 20;}
};

class foo {
public:

    static const int ii = A::getInstance().i() * A::getInstance().j(); 
};
const int foo::ii;

int main()
{
    foo f;
    return 1;
}

the aim is to initialize member ii using some function as above. but it generates the following error:

$ c++ static_constant.cpp 
static_constant.cpp:14:30: error: ‘A::getInstance()’ cannot appear in a constant-expression
static_constant.cpp:14:42: error: a function call cannot appear in a constant-expression
static_constant.cpp:14:44: error: ‘.’ cannot appear in a constant-expression
static_constant.cpp:14:46: error: a function call cannot appear in a constant-expression
static_constant.cpp:14:53: error: ‘A::getInstance()’ cannot appear in a constant-expression
static_constant.cpp:14:65: error: a function call cannot appear in a constant-expression
static_constant.cpp:14:67: error: ‘.’ cannot appear in a constant-expression
static_constant.cpp:14:69: error: a function call cannot appear in a constant-expression

could you please help me out? will Appreciate it.

Answer 1

You are using getInstance as a static function, but it's not declared static .

Change the declaration of getInstance :

static A & getInstance() { return instance; }

Answer 2

Several issues with the code, but here's a full compilable sample:

class A
{
public:

    static A instance;
    static A & getInstance() { return instance; }
    int i(){ return 10;}
    int j(){ return 20;}
};

class foo {
public:

    static const int ii;
};
const int foo::ii = A::getInstance().i() * A::getInstance().j(); 
A A::instance;