How to specialize base class template method from derived class? - c++

Question

I'm trying to specialize a base class method from a derived class template, but I'm not sure what would be the proper syntax to achieve it.
For implementation reasons, I need a class structure similar to the one below, but need to specialize func() from the top-level class:
template < typename T >
class BaseTemplate {
public:
BaseTemplate() = default;
virtual ~BaseTemplate() = default;
virtual void func() {
std::cout << __func__ << std::endl;
}
};
class DerivedBase : public BaseTemplate< float > {
public:
DerivedBase() : BaseTemplate() { /* ... */}
virtual ~DerivedBase() = default;
};
template < typename U >
class DerivedTemplate : public DerivedBase {
public:
DerivedTemplate() : DerivedBase() { /* ... */ }
~DerivedTemplate() = default;
protected:
U _member;
};
template < >
void DerivedTemplate< int >::func() {
std::cout << __func__ << " [int]" << std::endl; // error: no member function ‘func’ declared in ‘DerivedTemplate<int>’
}
Additionaly, explicitly declaring using DerivedBase::func; in DerivedTemplate results in another error:
error: template-id ‘func<>’ for ‘void DerivedTemplate<int>::func()’ does not match any template declaration

Answer 1

There is no need to specialize base class function in your case. Since function func() is virtual you can just implement it in one of your derived classes (either in DerivedTemplate or DerivedBase or both if needed):
template < typename U >
class DerivedTemplate : public DerivedBase {
public:
DerivedTemplate() : DerivedBase() { /* ... */ }
~DerivedTemplate() = default;
virtual void func() override {
std::cout << __func__ << "(DerivedTemplate version)" << std::endl;
}
protected:
U _member;
};

Answer 2

Additionaly, explicitly declaring using DerivedBase::func; in
DerivedTemplate results in another error:
error: template-id ‘func<>’ for ‘void DerivedTemplate::func()’
does not match any template declaration
DerivedBase::func; method is defined but not declared. You need to add void func(); in DerivedTemplate.
[Demo]
#include <iostream> // cout
template <typename T>
class BaseTemplate {
public:
BaseTemplate() = default;
virtual ~BaseTemplate() = default;
virtual void func() {
std::cout << __func__ << " [BaseTemplate]\n";
}
};
class DerivedBase : public BaseTemplate<float> {
public:
DerivedBase() : BaseTemplate<float>{} {}
virtual ~DerivedBase() = default;
};
template <typename U>
class DerivedTemplate : public DerivedBase {
public:
DerivedTemplate() : DerivedBase{} {}
~DerivedTemplate() = default;
void func();
};
template <>
void DerivedTemplate<int>::func() {
std::cout << __func__ << " [DerivedTemplate<int>]\n";
}
int main() {
BaseTemplate<char> bt{}; bt.func();
DerivedTemplate<int> dti{}; dti.func();
}
// Outputs:
//
// func [BaseTemplate]
// func [DerivedTemplate<int>]
What you could do in your example is to access BaseTemplate::func from DerivedTemplate<int>.
[Demo]
#include <iostream> // cout
template <typename T>
class BaseTemplate {
public:
BaseTemplate() = default;
virtual ~BaseTemplate() = default;
virtual void func() {
std::cout << __func__ << " [BaseTemplate]";
}
};
class DerivedBase : public BaseTemplate<float> {
public:
DerivedBase() : BaseTemplate<float>{} {}
virtual ~DerivedBase() = default;
};
template <typename U>
class DerivedTemplate : public DerivedBase {
public:
DerivedTemplate() : DerivedBase{} {}
~DerivedTemplate() = default;
void func();
};
template <>
void DerivedTemplate<int>::func() {
BaseTemplate::func();
std::cout << " [int]";
}
int main() {
DerivedTemplate<int> dti{}; dti.func();
}
// Outputs:
//
// func [BaseTemplate] [int]
Also, if you plan to use runtime polymorphism with this hierarchy, you'll want to use virtual void func() override in some of your classes.
[Demo]
#include <iostream> // cout
#include <memory> // make_unique, unique_ptr
template <typename T>
class BaseTemplate {
public:
BaseTemplate() = default;
virtual ~BaseTemplate() = default;
virtual void func() {
std::cout << __func__ << " [BaseTemplate]\n";
}
};
class DerivedBase : public BaseTemplate<float> {
public:
DerivedBase() : BaseTemplate<float>{} {}
virtual ~DerivedBase() = default;
virtual void func() override {
std::cout << __func__ << " [DerivedBase]\n";
}
};
template <typename U>
class DerivedTemplate : public DerivedBase {
public:
DerivedTemplate() : DerivedBase{} {}
~DerivedTemplate() = default;
virtual void func() override {
std::cout << __func__ << " [DerivedTemplate]\n";
}
};
template <>
void DerivedTemplate<int>::func() {
std::cout << __func__ << " [DerivedTemplate<int>]\n";
}
int main()
{
std::unique_ptr<BaseTemplate<float>> dti_up{std::make_unique<DerivedTemplate<int>>()};
dti_up->func();
}
// Outputs:
//
// func [DerivedTemplate<int>]