How can I use CRTP in C++ to avoid the overhead of virtual member functions?
There are two ways.
The first one is by specifying the interface statically for the structure of types:
template <class Derived>
struct base {
void foo() {
static_cast<Derived *>(this)->foo();
};
};
struct my_type : base<my_type> {
void foo(); // required to compile.
};
struct your_type : base<your_type> {
void foo(); // required to compile.
};
The second one is by avoiding the use of the reference-to-base or pointer-to-base idiom and do the wiring at compile-time. Using the above definition, you can have template functions that look like these:
template <class T> // T is deduced at compile-time
void bar(base<T> & obj) {
obj.foo(); // will do static dispatch
}
struct not_derived_from_base { }; // notice, not derived from base
// ...
my_type my_instance;
your_type your_instance;
not_derived_from_base invalid_instance;
bar(my_instance); // will call my_instance.foo()
bar(your_instance); // will call your_instance.foo()
bar(invalid_instance); // compile error, cannot deduce correct overload
So combining the structure/interface definition and the compile-time type deduction in your functions allows you to do static dispatch instead of dynamic dispatch. This is the essence of static polymorphism.