default value for templates

[pchahar]

Hi,
I need to implement a method fun() inside a class classA.
The method fun() should be capable of receiving at most 3 arguments with any data type.
I was following the approach as
class A
{
public:
template<class T1, class T2, class T3>
void fun(T1, T2, T3);
};

template<class T1, class T2, class T3>
void A::fun(T1 t1, T2 t2, T3 t3)
{
}

when the method fun is called from main, it is working only if I'm providing 3 arguments. When I give 2 or 1 arguments it is giving me error as follows.

int main()
{
int i=10;
char c='A';
float f=1.0;
A obj;
obj.fun(i,c,f);// working fine
obj.fun(i,c);// giving an error that fun() does not take 2 arguments
}
can anyone tell me how can we give default value in this case, so that the call for fun() with two arguments can work.

[jsmith]

Can you just write a second fun() method that takes only two parameters?

[pchahar]

It is required not to create overloaded functions.
We need to provide single function that can work for 2 or 3 arguments.

[anders43]

I was not aware of that function templates can have default values, thought only class templates could have.

[pchahar]

Yes function template cannot have default values. This is what causing an issue here. But, is there any other way of implementation such that a function can receive any data type any default values.

[Bazzy]

I think you can give the template value but set an argument to be optional

template<class T1, class T2, class T3>
    void A::fun(T1 t1, T2 t2, T3 t3=0)//default value
    {
		//...
    }
//...
obj.fun<int,char,char>(i,c);//specify types 

[jsmith]

The above example would be slightly more correct this way:

template< class T1, class T2, class T3>
void A::fun( T1 t1, T2 t2, T3 t3 = T3() ) { /* */ }

Bazzy's example makes the requirement that T3 be constructible from int.
This code makes only the requirement that T3 be default constructible.

[pchahar]

Thanks Bazzy and jsmith.
Well it solves problem in a single file.

But if the templates are in different files then we have linking error as
error LNK2019: unresolved external symbol ....
for the function fun()

If I have header file with class declaration and cpp file for function definition. Then linking error will come from the calling function which is present in some third file.

[jsmith]

You will need to put the implementation of the template function in the header file.

[anders43]

template< class T1, class T2, class T3>
void A::fun( T1 t1, T2 t2, T3 t3 = T3() ) { /* */ }

that doesn't compile on VS2008

'could not deduce template argument for T3'

when

obj.fun(i,c)

so no, i dont think that solves his problem.

[Bazzy]

Since you can't have default template values for function templates, you should tell which types you are using:
obj.fun<int,char,char>(i,c);

[pchahar]

I'm not sure for the data types.
Hence, the issue is still open with templates.