How to ensure that every method of a class calls some other method first?

Thanks to the unusual properties of operator ->, we can inject code before any member access, at the expense of a slightly bent syntax:

// Nothing special in Foo
struct Foo {
    void a() { }
    void b() { }
    void c() { }
};

struct LoggingFoo : private Foo {
    void log() const { }

    // Here comes the trick
    Foo const *operator -> () const { log(); return this; }
    Foo       *operator -> ()       { log(); return this; }
};

Usage looks as follows:

LoggingFoo f;
f->a();

See it live on Coliru