Default parameters in template functions (C++)

Question

I'm trying to use default parameters in a template function. The following is a minimal example of the thing I'm trying to do:

In sort.h

template <typename T0, typename T1>
void sort(Vector<T0> &v0,Vector<T1> &v1=0)
{
    //sort v0, if (v1 != 0) sort it according to v0
}

In main.cpp

#include "sort.h"
Vector<int> v0;
sort(v0);

This does not compile; the compiler gives the error "no matching function to call to 'sort'".

Basically this function should sort the vector v0 (arbitrary datatype). In addition, a second vector v1 (arbitrary) which is sorted in the same way as vector v0 can be given as parameter. Of course I could solve this problem simply by using overloaded functions, but since I'd like to expand the list of additional vectors up to 5, I would need like hundreds of different functions.

Update: Thanks for your responses so far. I have modified my problem description to give you a better idea of what I'm trying to do.

std::sort is of no use since it sorts only one single vector. but i want sort one vector and rearrange multiple others in the same way. — Paul Reuter
– Paul Reuter, Commented Dec 10, 2010 at 7:33

wilhelmtell · Accepted Answer · 2010-12-09 16:58:46Z

8

A reference can't point at nothing.

answered Dec 9, 2010 at 16:58

wilhelmtell

59k20 gold badges98 silver badges131 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

roalz · Accepted Answer · 2017-04-06 11:59:28Z

6

You cannot default a reference to 0. You'd have to use pointers to do this (if you insist).

There's also no way for the compiler to intuit what T1 is without an actual Vector<X> to work from. A concrete parameter type would be needed for parameter 2. If the second parameter was Vector<T0>*, for example, that would be OK since the type can be divined from parameter 1 (which is not defaulted). Otherwise, you'll have to specify the template parameters on every call to testfunction.

Adding info on what you are trying to achieve might enable better answers to be provided. This seems a little convoluted.

edited Apr 6, 2017 at 11:59

roalz

2,8063 gold badges29 silver badges44 bronze badges

answered Dec 9, 2010 at 16:58

Steve Townsend

54.4k9 gold badges100 silver badges145 bronze badges

1 Comment

Steve Townsend Over a year ago

@wilhelmtell - don't you mean your crossbow?

chris__allen · Accepted Answer · 2010-12-09 17:27:38Z

2

I would just simply overload the function.

template <typename T0, typename T1>
void sort(Vector<T0> &v0,Vector<T1> &v1)
{
    //do something
}
void sort(Vector<T0> &v0)
{
    //do something
}

answered Dec 9, 2010 at 17:27

chris__allen

4392 gold badges6 silver badges14 bronze badges

1 Comment

Pawel Zubrycki Over a year ago

You forgot template<class T0> before second sort.

Prasoon Saurav · Accepted Answer · 2010-12-09 17:00:16Z

1

I would prefer

template <typename T0, typename T1>
void testfunction(Vector<T0> &v0,const Vector<T1> &v1 = Vector<T1>() )
{
    //do something
}

answered Dec 9, 2010 at 17:00

Prasoon Saurav

93.3k51 gold badges245 silver badges348 bronze badges

1 Comment

wilhelmtell Over a year ago

but you'd have to instantiate the function then.

Community · Accepted Answer · 2017-05-23 12:17:45Z

1

Just because you're giving a default value for the second argument in testfunction, the compiler has no way of deriving T1 if you don't give it something to go on. You need to explicitly state the type. Something like:

testfuntion<int, float>(v0);

Also, as others have pointed out, you need to find some way of properly declaring the second argument since a reference can't have "null" value. I personally like this way.

edited May 23, 2017 at 12:17

CommunityBot

11 silver badge

answered Dec 9, 2010 at 17:02

andand

17.6k11 gold badges61 silver badges84 bronze badges

Comments

Zac Howland · Accepted Answer · 2010-12-09 16:59:57Z

0

You'll have to change it to use pointers instead:

template <typename T0, typename T1>
void testfunction(Vector<T0>* v0, Vector<T1>* v1 = 0)
{
    //do something
}

answered Dec 9, 2010 at 16:59

Zac Howland

15.9k1 gold badge28 silver badges43 bronze badges

4 Comments

Steve Townsend Over a year ago

Right - if you are going to start using pointers, why not just go all out?

Zac Howland Over a year ago

Pretty much. If you pass in the first vector by reference and the second by pointer (you're essentially passing pointers to both anyway), so make the syntax consistent.

wilhelmtell Over a year ago

@ZacHowland references and pointers are two very different beasts. Also: what @SteveTownsend is trying to hint (if I understand correctly) is that you're taking "consistency" waay out of context here. If I'm getting Steve wrong then I'll sign my own name under that.

Zac Howland Over a year ago

@wilhelmtell: You're taking it too literally. A reference is (essentially) a pointer that has a couple extra guarantees. That is, what gets passed to your function is a memory address not an object. That is what I was getting at (not that pointers and references are identical). Since you passed in both by non-const reference, I'm assuming you are planning to manipulate their data in some way, so having one passed in as vector<T0>& and the other as vector<T1>* would force code to look like v0.push_back((*v1)[3]) or conversely v1->push_back(v0[3]). Its just a preference.

Collectives™ on Stack Overflow

Default parameters in template functions (C++)

In sort.h

In main.cpp

6 Answers 6

Comments

1 Comment

1 Comment

1 Comment

Comments

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

In sort.h

In main.cpp

6 Answers 6

Comments

1 Comment

1 Comment

1 Comment

Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related