Pfthb

What is the best way to have a static member in a non-templated library class,
without placing the burden of defining the member on the class user?

Say I want to provide this class:

class i_want_a_static_member

 static expensive_resource static_resource_;

public:
 void foo()
 
 static_resource_.bar();
 
;

Then the user of the class must not forget to define the static member somewhere
(as already answered many times):

// this must be done somewhere in a translation unit
expensive_resource i_want_a_static_member::static_resource_;

I do have an answer below, but it has some disadvantages. Are there better and/or more elegant solutions?

You can use function local static variables.

struct Foo 
 static int& Bar() static int I; return I; 
; // ^~~~~~~~~~~~

My own solution is to use a templated holder class, as static members work fine in templates, and use this holder as a base class.

template <typename T>
struct static_holder

 static T static_resource_;
;

template <typename T>
T static_holder<T>::static_resource_;

Now use the holder class:

class expensive_resource /*...*/ ;

class i_want_a_static_member : private static_holder<expensive_resource>

public:
 void foo()
 
 static_resource_.bar();
 
;

But as the name of the member is specified in the holder class, you can't use the same holder for more than one static member.

As of C++ 17. You can now use inline variables to do this:

static const inline float foo = 1.25f;