Conditionally swallow comma in constructor initializer list macro

Question

I have the following macros that conditionally generate and initialize a member for classes:

#ifdef LIFETIME_TRACKING

#define DEFINE_TRACKER_MEMBER() mutable ::LifetimeTracker m_tracker
#define INITIALIZE_TRACKER_MEMBER(x) m_tracker(x)

#else

#define DEFINE_TRACKER_MEMBER() using dummy = void
#define INITIALIZE_TRACKER_MEMBER(x) /* ??? */ 

#endif

They are meant to be used as follows:

struct Example
{
    int data;
    Dependency dep;
    DEFINE_TRACKER_MEMBER();

    Example() : data(42), INITIALIZE_TRACKER_MEMBER(dep) { }
};

When LIFETIME_TRACKING is defined, everything is fine. When it is not defined, I could not figure out a way of making the constructor initializer list compile the , INITIALIZE_TRACKER_MEMBER(dep) syntax without either artificially increasing the class's size or making the syntax ugly and not friendly for clang-format (e.g. including the comma inside the macro, as INITIALIZE_TRACKER_MEMBER(, dep)).

The best I came up with is to have DEFINE_TRACKER_MEMBER create a dummy empty-struct member:

#define DEFINE_TRACKER_MEMBER() struct { } m_tracker
#define INITIALIZE_TRACKER_MEMBER m_tracker{}

However that increases the size of the parent class.

Is there any way I can keep the desired syntax and not incur a class size increase?

Answer 1

What about simply moving the comma into the macro itself?

#ifdef LIFETIME_TRACKING

#define DEFINE_TRACKER_MEMBER() mutable ::LifetimeTracker m_tracker;
#define INITIALIZE_TRACKER_MEMBER(x) ,m_tracker(x)

#else

#define DEFINE_TRACKER_MEMBER()
#define INITIALIZE_TRACKER_MEMBER(x)

#endif

struct Example
{
    int data;
    Dependency dep;
    DEFINE_TRACKER_MEMBER()

    Example() : data(42) INITIALIZE_TRACKER_MEMBER(dep) { }
};