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Weekly C++ Challenge #1 - Template Virtuals


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I don't know if I'll have enough steam or gather enough interest to really keep this weekly, but lets try.


I'm going to try and keep all of these challenges moderately practical, compliant with C++ standards, and hopefully interesting. None of them will require extensive knowledge of algorithms or large amounts of code to solve. Just various non-trivial ways to use the language.



Without further distraction, lets jump into this week's challenge. Anyone who has worked with or studied C++ had to deal with virtual functions. They can be fantastic ways to distinguish derived classes. For example, I might have a base class Foo with a virtual method Foo::IsInt() that tells me if some derived class is specialized to deal with integer types. But what if derived class is templated? What if you want to be able to distinguish between many base types a function could be using? Wouldn't it be fantastic if we could just call Foo::IsType<int>() instead? Unfortunately, C++ forbids declaring template member function virtual. And yet, this is precisely what I want you to overcome in this challenge. Write implementations for base class Foo and template class Bar derived from it which support the following code.



#include <iostream>class Foo;template <typename T>class Bar;void CheckInt(Foo* x){	if (x->IsType<int>())		std::cout << "is int.\n";	else		std::cout << "is not int.\n";}int main(){	Bar<int> x;	Bar<float> y;	std::cout << "x ";	CheckInt(&x);	std::cout << "y ";	CheckInt(&y);	return 0;}
Desired output.


x is int.y is not int.
Small hint.


Template specialization is done at compile time. Virtual functions are selected at run time. If there was only a way to bridge that gap...



Bigger hint.


Static variables inside template functions get initialized for each template initialization and will persist through run time.





While there are several ways, in theory, to get type information at run time, by far the simplest, and perhaps most elegant, is to simply have a template function that generates unique type identifiers.



class Foo{    public:	template <typename T>	bool IsType()	{		return IsTypeImpl(GetTypeId<T>());	}	virtual bool IsTypeImpl(size_t id) = 0;    protected:	static size_t GetNextTypeId()	{		static size_t id = 0;		return id++;	}	template <typename T>	size_t GetTypeId()	{		static size_t id = GetNextTypeId();		return id;	}};template <typename T>class Bar : public Foo{    public:	bool IsTypeImpl(size_t id)	{		return id == GetTypeId<T>();	}};


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