Select a template class based on a string

DigestMatcher.cpp

// For motivation see stackoverflow
// http://stackoverflow.com/questions/13612832/template-for-referencing-classes-themselves-rather-than-a-class-that-uses-templa


#include <iostream>
#include <string>
#include <typeinfo>
#include <cstring>


// The example uses CryptoAPI
// Below are some typedefs and dummy structures so we don't need cryptoapi just to test
typedef unsigned char byte;

struct DummyBase{
	enum{DIGESTSIZE = 16};
	void CalculateDigest( byte* , byte*, int){
	}
};

struct MD5:DummyBase{};
struct SHA256:DummyBase{};
struct SHA512:DummyBase{};


template<class Digest, class... T>
struct DigestMatcher{
	template<class U, class ... V>
	static byte* doDigest(std::string input, int& digestSize,std::string algorithm,U firstalgorithm, U nextalgorithm, V... algorithms){
		if(algorithm==firstalgorithm){
			return doDigest(input,digestSize);
		}
		else{
			return DigestMatcher<T...>::doDigest(input,digestSize,algorithm,nextalgorithm,algorithms...);

		}
	}

	template<class U>
	static byte* doDigest(std::string input, int& digestSize,std::string algorithm, U firstalgorithm){
		if(algorithm==firstalgorithm){
			return doDigest(input,digestSize);
		}
		else{
			std::cout << algorithm << " invalid" << std::endl;
			return 0;
		}
	}

	static byte* doDigest(std::string input, int& digestSize){

			// Cout for debug purposes
			std::cout << "using " << typeid(Digest).name() << std::endl;
			digestSize = Digest::DIGESTSIZE;
			byte* digest = new byte[ digestSize ];
			Digest().CalculateDigest( digest, (byte*) input.c_str(), input.length() );
			return digest;
	}

};


// A mock structure of the signagature class from the example

struct signatureAlgorithm{
	void createDigest(std::string input);
	std::string _algorithm;
	int _digestSize;
	byte* _digest;
	bool _ready;


};

void signatureAlgorithm::createDigest(std::string input)
{
	_digest = DigestMatcher<MD5,SHA256,SHA512>::doDigest(input,_digestSize,_algorithm,"md5","sha256","sha512");
	if(_digest)
		_ready = true;
	else
		_ready = false;
}



int main(){

	std::string input;
	signatureAlgorithm alg;
	alg._algorithm = "sha256";
	alg.createDigest(input);
}