Suppressing the inline variable warning

Question

Background
We are trying to "borrow" some code which uses an inline static variable. The code complies, without any warnings, with a C++17 compiler. However with a lower version compiler it gives the following warning:
warning: inline variables are only available with ‘-std=c++17’ or ‘-std=gnu++17’

We have been able to suppress the waning with a command-line switch but I wanted to know if there is a way to refactor the code for suppressing the warning.

I have been able to reproduce the issue with the following sample code:

---

Sample Code

#include <iostream>
using namespace std;

template <typename T>
class MyClass {
    T a;
public:
    void initialize(T value) {
        a = value;
    }
    
   friend ostream& operator<<(std::ostream &out, const MyClass &obj) {
       out << "Hello world " << obj.a << "!!!";
       return out;
   }
   
   /*inline*/ static MyClass<T> SENTINEL_VALUE;
   // Currently we have the undefined reference linker error
   // If we uncomment /*inline*/ we get the warning
   //
   // I know that static class member has to instantiated before
   // main() but in this case I am not sure how to do that
};


int main()
{
    MyClass<int>::SENTINEL_VALUE.initialize(56);
    cout << MyClass<int>::SENTINEL_VALUE << endl;

    MyClass<int> myObj;
    myObj.initialize(42);
    cout << myObj << endl;
 
    return 0;
}

Answer 1

Inlined static member is supported only since C++17, hence the compiler's warning. There is no sense in suppressing it, if you use something prior to C++17.
All you need is to initialize your static member (within the same header file):

#include <iostream>
using namespace std;

template <typename T>
class MyClass {
    T a;
public:
    void initialize(T value) {
        a = value;
    }

    friend ostream& operator<<(std::ostream &out, const MyClass &obj) {
        out << "Hello world " << obj.a << "!!!";
        return out;
    }

    static MyClass<T> SENTINEL_VALUE;
};

template <typename T>
MyClass<T> MyClass<T>::SENTINEL_VALUE{};

So, for instance, cout << MyClass<int>::SENTINEL_VALUE << endl; outputs Hello world 0!!!.

Initializing a static member of a class template within a header file is allowed and will not lead to multiple definitions error.
Note, that this applies only to member variables of class templates (or member variable templates).