Error passing map to template function in C++

Question

I have been hitting my head against a brick wall for a bit and decided to submit and just post my error.

I have a template (shown below) that passes a std::map, which is inside a namespace. This compiles fine.

My problem occurs when trying to call the function template. I get the below error:

error: no matching function for call to 
    'getValuePointer(Muon*&, std::map<int, MET*, std::less<int>, 
                     std::allocator<std::pair<const int, MET*> > >*&)'

What am I doing wrong?

Here is the template code:

#ifndef OBJECTMAPMATCH_H
#define OBJECTMAPMATCH_H

#include <map>
#include <utility>
#include <typeinfo>
#include <iostream>
#include <stdlib.h>

using namespace std;

namespace ObjectMapMatch {


  template< class A, class B, class C >
    C  getValuePointer( A &x , map< B,C> &y )
  {

    if( y.find(x.Index()) != y.end() ){

      return y.find(x.Index()).second;

    }else{
      cout << "ERROR:ObjectMapMatch::getValuePointer:Can not Find " 
           <<  typeid(y).name() << " FOR " << typeid(x).name() << endl;
      exit(1);
    }

  }

}
#endif

Here is an example call to the template function

C = ObjectMapMatch::getValuePointer<ClassC*, int, ClassD*>(A, B);

Where:

C is ClassC*
A is ClassC*
B is std::map<int,ClassD*>*

What am I doing wrong?

I constructed a [similar example](http://ideone.com/Relqo), and it runs fine. — Björn Pollex, Nov 30 '11 at 14:30
Not a solution, but just a helpful note - unless I'm mistaken, you can use what's called "template type inference" in this case and simplify your function call to `C = ObjectMapMatch::getValuePointer(A, B);`. The types `` will be inferred automatically — Nate, Nov 30 '11 at 14:32
I would add to Nate, that you not just can, but *SHOULD* use template type inference (applies to function templates only, of course). — Jan Hudec, Nov 30 '11 at 14:36
The fact that you use A,B,C with two completely different meanings here, and that you use different class names for the same thing (e.g. Muon and ClassC) makes this question a lot more confusing than it should be. — interjay, Nov 30 '11 at 14:41
@interjay Yes That was stupid of me. Would edit but the answers would then not make sense. — MWright, Nov 30 '11 at 14:50

interjay · Accepted Answer · 2011-11-30T14:43:21.217

3

According to the error message, you passed a pointer to a map as the second parameter, while the function expects a reference to a map.

You can see this because of the * at the end here:

no matching function for call to 
    'getValuePointer(Muon*&, std::map<...>*&)'
                                          ^

There will be additional issues once you fix this though: Since x is of type ClassC*, calling x.Index() will give an error. Maybe the first template parameter should be ClassC instead of ClassC*.

edited Nov 30 '11 at 14:43

answered Nov 30 '11 at 14:28

interjay

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A more to-the-point way of saying this is that `B` should not be declared as a pointer. – Nate Nov 30 '11 at 14:37
@Nate: True, but originally the question said that B wasn't a pointer (which was obviously false). It has been edited now. – interjay Nov 30 '11 at 14:41
You sir are absolutely right. Don't know how I missed that. +1 all the same. – Nate Nov 30 '11 at 14:46

score 0 · Answer 2 · answered Nov 30 '11 at 14:31

Your template definition says:

 C  getValuePointer( A &x , map< B,C> &y )

And you are calling this function with:

ObjectMapMatch::getValuePointer<ClassC*, int, ClassD*>(A, B);

... and you say that B is:

std::map<int,ClassD*>

So, for your template B is an int, and C is a ClassD*... That's why there is not a match.

Or maybe not, I mean, the name you chose for the values is the same for the classes in your template, which is quite confusing.

Hope this helps.

score 0 · Answer 3 · answered Nov 30 '11 at 14:33

your signature is wrong

template< class A, class B, class C >
C* getValuePointer( A* x , map<B, C*> &y )
{
    if( y.find(x->Index()) != y.end() ){
        return y.find(x->Index()).second;
    }else{
        cout << "ERROR:ObjectMapMatch::getValuePointer:Can not Find " 
             <<  typeid(y).name() << " FOR " << typeid(x).name() << endl;
        exit(1);
    }
}

and the call it

ObjectMapMatch::getValuePointer<ClassC, int, ClassD>(A, B);

but C is ClassD*, not ClassC* right?

score 0 · Answer 4 · answered Nov 30 '11 at 14:45

In your last edit you say that

B is std::map<int,ClassD*>*

but your function is

template< class A, class B, class C >
C  getValuePointer( A &x , map< B,C> &y )

thus accepting

std::map<int,ClassD*> &

for the second argument. So pointer to map vs. reference to map. I suppose you should either be passing *B, or B should not be pointer (STL is there so you don't need to work with pointers much so try avoiding them as much as possible).

Error passing map to template function in C++

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