Curiously Recurring Template Pattern

Static Polymorphism

Typically, the base class template will take advantage of the fact that member function bodies (definitions) are not instantiated until long after their declarations, and will use members of the derived class within its own member functions, via the use of a cast in the case of multiple inheritance e.g.:

template struct Base { void interface { // ... static_cast(this)->implementation; // ... } static void static_func { // ... Derived::static_sub_func; // ... } }; struct Derived : Base { void implementation; static void static_sub_func; };

This technique achieves a similar effect to the use of virtual functions, without the costs (and some flexibility) of dynamic polymorphism. This particular use of the CRTP has been called "simulated dynamic binding" by some. This pattern is used extensively in the Windows ATL and WTL libraries.

To elaborate on the above example, consider a base class with no virtual functions. Whenever the base class calls another member function, it will always call its own base class functions. When we derive a class from this base class, we inherit all the member variables and member functions that weren't overridden (no constructors or destructors). If the derived class calls an inherited function that then calls another member function, that function will never call any derived or overridden member functions in the derived class.

However, if base class member functions use CRTP for all member function calls, the overridden functions in the derived class will be selected at compile time. This effectively emulates the virtual function call system at compile time without the costs in size or function call overhead (VTBL structures, and method lookups, multiple-inheritance VTBL machinery) at the disadvantage of not being able to make this choice at runtime.

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Curiously Recurring Template Pattern - Static Polymorphism