static const in c++ class: undefined reference

std::min<int>'s arguments are both const int&(not just int), i.e. references to int. And you can't pass a reference to A::MY_CONST because it is not defined (only declared).

Provide a definition in the .cpp file, outside the class:

class A {
public:
    static const int MY_CONST = 5; // declaration
};

const int A::MY_CONST; // definition (no value needed)

// initialize static constants outside the class

class A {
public:
    static const int MY_CONST;
};

const int A::MY_CONST = 5;

void fun( int b ) {
    int j = A::MY_CONST;  // no problem
    int k = std::min<int>( A::MY_CONST, b ); // link error: 
                                            // undefined reference to `A::MY_CONST` 
}

You can also save the const value to a local variable.

class A {
public:
    static const int MY_CONST = 5;
};

void fun( int b ) {
    int j = A::MY_CONST;  // no problem
    int k = std::min<int>( A::MY_CONST, b ); // link error: undefined reference to `A::MY_CONST` 
    int l = std::min<int>( j, b);  // works
}

To explain what's happening here:

You declared static const integer inside class, this "feature" is here to be able to use it as constant expression,i.e. for local array size, template non-type parameters, etc.. If compiler wants to use this constant expression it must be able to see it's value in that translation unit.

9.5/3

If a non-volatile const static data member is of integral or enumeration type, its declaration in the class definition can specify a brace-or-equal-initializer in which every initializer-clause that is an assignment expression is a constant expression (5.19). A static data member of literal type can be declared in the class definition with the constexpr specifier; if so, its declaration shall specify a brace-or-equal-initializer in which every initializer-clause that is an assignment-expression is a constant expression. [ Note: In both these cases, the member may appear in constant expressions. — end note ] The member shall still be defined in a namespace scope if it is odr-used (3.2) in the program and the namespace scope definition shall not contain an initializer.

odr-used means to form reference to that variable or take it's address.

std::min takes it's parameters by reference, so they are odr-used.

Solution:

Define it!

class A
{
    static const int a = 5;
};

const int A::a; //definition, shall not contain initializer