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