Conditional type selection using C++ template specializations

One of the very basic techniques of C++ meta-programming is to provide simple selector classes that provide to refer to a type from a (public) typedef member.

The basic pattern for type selections based on a condition applied to the template parameters passed, or even independently looks like the following code demonstrates:

---

	#include <iostream>
	#include <typeinfo>
	using namespace std;
	template<typename FalseType, typename TrueType, bool condition>
	struct ConditionalTypeSelector
	{
	typedef void ResultType;
	};
	template<typename FalseType, typename TrueType>
	struct ConditionalTypeSelector<FalseType,TrueType,false>
	{
	typedef FalseType ResultType;
	};
	template<typename FalseType, typename TrueType>
	struct ConditionalTypeSelector<FalseType,TrueType,true>
	{
	typedef TrueType ResultType;
	};
	struct A {
	unsigned char member;
	};
	struct B {
	int member;
	};
	struct C {
	long long member;
	};
	int main() {
	cout << typeid
	( ConditionalTypeSelector
	< A,B,(sizeof(A) > sizeof(B))
	>::ResultType
	).name() << endl;
	cout << typeid
	( ConditionalTypeSelector
	< A,B,(sizeof(B) > sizeof(A))
	>::ResultType
	).name() << endl;
	cout << typeid
	( ConditionalTypeSelector
	< A,C,(sizeof(A) > sizeof(C))
	>::ResultType
	).name() << endl;
	cout << typeid
	( ConditionalTypeSelector
	< C,B,true>::ResultType
	).name() << endl;
	cout << typeid
	( ConditionalTypeSelector
	< C,A,false>::ResultType
	).name() << endl;
	return 0;
	}

view raw ConditionalTypeSelectorSample.cpp hosted with ❤ by GitHub

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A compiled and running example of the above code can be found here.