C++ - inherit from base class with member function as template argument - c++

Question

I have this very simple class using CRTP:
template <typename DerivedType, void (DerivedType::*updateMethod)() = &DerivedType::update>
class UpdatableBase {};
class MyClass : private UpdatableBase<MyClass> {
public:
void update() {
}
};
int main() {
return 0;
}
Which, when compiled with g++ test.cpp -std=c++14, gives this:
test.cpp:2:85: error: no member named 'update' in 'MyClass'
template <typename DerivedType, void (DerivedType::*updateMethod)() = &DerivedType::update>
~~~~~~~~~~~~~^
test.cpp:5:25: note: in instantiation of default argument for 'UpdatableBase<MyClass>' required here
class MyClass : private UpdatableBase<MyClass> {
^~~~~~~~~~~~~~~~~~~~~~
1 error generated.
Why does it say there's "no member named 'update' in 'MyClass'"? There very clearly is.

Answer 1

This is a very common problem with CRTP: the compiler instantiates the base class definition before it reads the derived class definition.
The work around is to only use the definition of the derived class inside base class member definition:
#include <type_traits>
template <typename DerivedType, class DelayedParameter = void>
class UpdatableBase {
public:
//definition of members of templates are instantiated when needed!
void update(){
if constexpr(std::is_same_v<DelayedParameter,void>)
static_cast<DerivedType>(this)->update();
else
(static_cast<DerivedType*>(this)->*DelayedParameter::value)();
}
};
class MyClassDelayedParameter;
class MyClass:public UpdatableBase<MyClass,MyClassDelayedParameter>
//the compiler only instantiate the definition of the base class:
//it will only instantiate the DECLARATION of the base class members.
{
public:
void update();
};
//Here MyClass is defined so we can access its member.
struct MyClassDelayedParameter:
std::integral_constant<void(MyClass::*)(),&MyClass::update>
{};
//UpdatableBase<...>::update DEFINITION is instantiated here. At this point
//of instantiation, MyClass and MyClassDelayedParameter are defined.
int main() {
MyClass a;
UpdatableBase<MyClass,MyClassDelayedParameter>& b=a;
b.update();
return 0;
}
DEMO