template parameter passing

[aseemgoyal]

I have a function like below

template <class T , class compare>  // compare will be GreaterAge<Person> here
void Merge(T *a, int p, int q, int r , compare comp)
{
   .... code
}

Now , instead of passing int , i want to make this also as template.
so i tried something like :

template <class T , class compare> 
void Merge<typename Q>(T *a, Q p, Q q, Q r , compare comp)
{
   ... code
}

and called like :

Merge<int>(a,p,q,r,comp);

But it is giving error .

Question 1 :
what is correct way to define it?

Question 2 :
suppose i wanted to pass iterator ,say of a vector or map instead of int .
what would be the correct declaration for that?

[MiiNiPaa]

1)

template <typename T , typename  Q, typename compare> 
void Merge(T *a, Q p, Q q, Q r , compare comp)
{
  // ... code
}

2) Same. Just modify code to work with iterators.

[keskiverto]

You have a function that depends on two templated types (T and compare) and you would like to make a function that depends on three templated types? Add to the list?


If the int's are used as indices to an array, then iterators do not fit in transparently.

int x = 3;
vector<Foo> arr;
vector<Foo>::iterator y = arr.begin() + 3;

arr[ x ]; // Ok
arr[ y ]; // Error 

[aseemgoyal]

since i used p,q,r , how does the compiler come to know about the number of instances of Q?
I mean i have passed 5 arguments , but typename is used 3 times.

[MiiNiPaa]

I mean i have passed 5 arguments , but typename is used 3 times.

But you have three different types inside.

It is common to have less types than arguments. For example this standard function: http://en.cppreference.com/w/cpp/algorithm/reverse

[aseemgoyal]

So, from http://en.cppreference.com/w/cpp/language/function_template :
The list of template arguments does not have to be supplied if it can be deduced from context.

made few runs of sample program and found out one more rule :
1) Order of specifying template type do not matter.

Thankyou

[JLBorges]

> Question 1 :
> what is correct way to define it?
> Question 2 :
> suppose i wanted to pass iterator ,say of a vector or map instead of int .
> what would be the correct declaration for that?

Read up on std::merge() and std::inplace_merge; that should lead you in the right direction.
http://en.cppreference.com/w/cpp/algorithm/merge
http://en.cppreference.com/w/cpp/algorithm/inplace_merge
(There are usage examples at the bottom of the pages).

[keskiverto]

The list of template arguments does not have to be supplied if it can be deduced from context.

That refers to instantiation.

[aseemgoyal]

@JLBorges Right to the point !

thankyou