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I am practicing c++ template programming and got a very confusing question. My purpose is to write a program to output the data of classes. If the the class is an POD (plain old data), use a global function (say Serialize) for output, if it's not, use the class's own Serialize function (assume every non-pod class in this program has implemented its own Serialize method). To better illustrate this idea, I have a little program to do that:

C++

#include <assert.h>
#include <iostream>

using namespace std;

class myclass
{
public:
	myclass(void):_nID(10) { _nOldID = _nID; };

	void Serialize(std::ostream &os)	
	{
		os << _nID;
	}

	void Serialize(std::istream &is)
	{
		is >> _nOldID;
	}

private:
	int _nID, _nOldID;
};

template <typename T>
void Serialize(std::ostream &os, const T& Obj)
{
	os << Obj;
}


/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
//
int main(int argc, char* argv[])
{
//	typedef myclass	T;
	typedef unsigned int	T;
	T Obj;

	// Do some initialization for Obj

	if (!std::is_pod<T>::value)			// T is not a POD
	{
		std::cout << "T != POD";
		Obj.Serialize(cout);			// Use its own serializer
	}
	else								// T is a POD
	{
		Serialize(cout, Obj);			// Use default global serializer
		std::cout << "T = POD";
	}

	return 0;
}

The problem is that the compiler giving an error that error C2228: left of '.Serialize' must have class/struct/union in the if clause, which I think it shouldn't because T is defined as unsigned int which is of POD type. On the other hand, if T is defined as myclass, the compiler complains the global Serialize function saying "binary '<<' : no operator found which takes a right-hand operand of type 'const T' (or there is no acceptable conversion)", which is in the "else" clause which should not be reached.

Anybody can explain this?

Re: A template question

pasztorpisti13-Aug-13 12:22

pasztorpisti

13-Aug-13 12:22

You want to do this if I'm right: You want to generate a call to the Serialize() method of the object if the method is implemented in the object otherwise you want to call a fallback method or maybe you want to generate a compile time error. Then you are looking for SFINAE[^]. The solution I provided is much simpler than the one found on the wiki page but I made use of C++11.

C++

#include <assert.h>
#include <iostream>

using namespace std;

class myclass
{
public:
    myclass():_nID(10) { _nOldID = _nID; };

    void Serialize(std::ostream &os)
    {
        os << _nID;
    }

    void Deserialize(std::istream &is)
    {
        is >> _nOldID;
    }

private:
    int _nID, _nOldID;
};


class otherclass
{

};


namespace Serialize_SFINAE
{
    template <typename T>
    auto Serialize(T& obj, std::ostream &os) -> decltype(obj.Serialize(os),void())
    {
        obj.Serialize(os);
    }

    // fallback method
    template <typename T>
    void Serialize(T& obj, ...)
    {
        // replace this with valid code if you want to handle this error at runtime instead of compile time
        obj.this_class_doesnt_have_a_serialize_method;
    }
}


template <typename T>
void Serialize(std::ostream &os, T& obj)
{
    // Call the serialize method of T if present, call our fallback method otherwise
    Serialize_SFINAE::Serialize(obj, os);
}

// overloads for all primitive C/C++ types that can't have methods
void Serialize(std::ostream &os, int obj)
{
    os << obj;
}

void Serialize(std::ostream &os, bool obj)
{
    os << obj;
}


/////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
//
int test_code()
{
    myclass Obj;
    //otherclass Obj;
    //bool Obj = false;
    //int Obj = 0;

    Serialize(cout, Obj);
    return 0;
}

Re: A template question

EQ Learner14-Aug-13 4:08

EQ Learner

14-Aug-13 4:08

Thanks for the code and explanation. That's exactly what I intended to do. Your codes get compiled and worked fine.

One more question: If for any class/struct object without Serialize method implemented, I want to use the default serialization for them such as os << Obj or call the global Serialize template function. I just cant use a specialization method for every such class because I can possibly have many such classes.

modified 14-Aug-13 10:30am.

Re: A template question

pasztorpisti14-Aug-13 5:21

pasztorpisti

14-Aug-13 5:21

The fallback sfinae method (void Serialize(T& obj, ...)) is for that case. You can put there a compile error (I did that) but you can put there a default handler code too as an alternative for the case when the struct/class doesn't have a Serialize() method.

I almost forget to mention: When you have to handle primitive types (int, bool, ...) and structs/classes together then it can usually be done by calling an overloaded function/method that has overloads for the primitive types and a template method for the rest (structs/classes). In the template method you can do different kind of magic with the type it is called with, you can call one of its methods, sfinae,... When creating the overload for primitive types there is one strange thing to be aware of: char, signed char, and unsigned char are 3 different types and you need 3 overloads!

Re: A template question

EQ Learner14-Aug-13 5:55

EQ Learner

14-Aug-13 5:55

I did learn a lot from what you have explained. Thanks again for your help and patience.

For POD type of data, I used a std::is_class<> predicate to handle and I only need two specializations.

Within your namespace Serialize_SFINAE {...}, I just cannot simply add a default Serialize function which compiler will complain an ambiguous call. Could you be more specific?

Re: A template question

pasztorpisti14-Aug-13 6:03

pasztorpisti

14-Aug-13 6:03

You are welcome!
But I can not be more specific because I don't know what you wanna do from what you described. I never said "adding a new method" in my previous posts to create the "ambiuous call" condition.

Re: A template question

EQ Learner14-Aug-13 6:13

EQ Learner

14-Aug-13 6:13

Ok, to be more specific, I have redefined the otherclass to enable it has an << operator:

C++

class otherclass
{
public:
	otherclass(void):i(0)	{}

	friend std::ostream & operator << (std::ostream & os, otherclass &Obj)
	{
		os << Obj.i;
		return (os);
	}

	int i;
};

In the Serialize_SFINAE name space, I modified it to handle the otherclass serialization:

C++

namespace Serialize_SFINAE
{
    template <typename T>
    auto Serialize(T& obj, std::ostream &os) -> decltype(obj.Serialize(os),void())
    {
        obj.Serialize(os);
    }
 
    // fallback method
    template <typename T>
    void Serialize( T& obj, std::ostream &os, ...)
    {
        // replace this with valid code if you want to handle this error at runtime instead of compile time
//        obj.this_class_doesnt_have_a_serialize_method;

		os << obj;
    }
}

So now the program can handle the otherclassserialization. However, when switch back to myclass, compiler generate an ambiguous call message: error C2668: 'Serialize_SFINAE::Serialize' : ambiguous call to overloaded function

So what should the second Serialize(..) within that name space be in order to handle the object without Serialize internally implemented?

Re: A template question

pasztorpisti14-Aug-13 8:29

pasztorpisti

14-Aug-13 8:29

Well, you actually changed the signature of the Serialize method. You are not allowed to do that. If you want to be able to access a stream for this purpose then put both of these methods into a class instead, store a stream reference/pointer in the class, create an instance of that class and call these sfinae methods of the instance. This way you don't have to change the signature of the sfinae methods. A simpler less nice alternative is not changing the methods but accessing the stream from a global variable.

Re: A template question

EQ Learner14-Aug-13 8:57

EQ Learner

14-Aug-13 8:57

Good. That's clarify what I thought and is what I planed to do after the last post.

Thanks again for your kind explanations.

Re: A template question

pasztorpisti14-Aug-13 9:29

pasztorpisti

14-Aug-13 9:29

You are welcome!

how can i improve my c programming in different ways

Member 985748713-Aug-13 2:18