Template class can't find constructor

Question

I am currently working on template classes (and I am quite lost). I want to build a template class that can stores objects.

Here is my .h file :

#ifndef STORAGE_H_
#define STORAGE_H_

#include <iostream>
using namespace std;

template<class T,int maxsize>
class Storage
{
    public:
        Storage();

        int getSize();
        T get(int index);
        bool add(T element);

    private:
        T array[maxsize];
        int length;

};

template<class T,int maxsize>
Storage<T,maxsize>::Storage()
  : length(0),// Liste d'initialisation des données membres
{}

template<class T,int maxsize>
int Storage<T,maxsize>::getSize()
{
    return length;
}

template<class T,int maxsize>
T Storage<T,maxsize>::get(int index)
{
    return array[index];
}

template<class T,int maxsize>
bool Storage<T,maxsize>::add(T element)
{
    if (length>=maxsize) return false;
    array[length++]=element;
    return true;
}

#endif /* STORAGE_H_ */

It works fine when i'm dealing with int for instance. But when it comes to others objects I previously made it fails. In my main .cpp file :

Storage<Pokemon,4> pokemonArray;

Which sends me the following error :

no matching function for call to 'Pokemon::Pokemon()'

Pokemon being a class I have. To me this line means that it can't find the constructor Pokemon().

How can I make it work ?

Thank you guys !

It's trying to call the default constructor, which works fine for primitive types like int, but your Pokemon class probably has it's own constructor that expects arguments. — Trevor Hickey, Nov 03 '14 at 19:28
Can we see code for Pokemon? Does it have a default constructor? — Manny, Nov 03 '14 at 19:28
Is pokemon copyable, movable, copy-constructible, move-constructible? (We already know it is not defaultconstructible.) — Deduplicator, Nov 03 '14 at 19:44
No default ctor for Pokemon, which was the missing point of all this ! — Xema, Nov 03 '14 at 21:13

score 2 · Accepted Answer · answered Nov 03 '14 at 19:38

The following code will work, assuming that Pokemon has a default constructor:

#include <iostream>
using namespace std;

template<class T,int maxsize>
class Storage
{
    public:
        Storage();

        int getSize();
        T get(int index);
        bool add(T element);

    private:
        T array[maxsize];
        int length;

};

template<class T,int maxsize>
Storage<T,maxsize>::Storage()
  : length(0)
{}

template<class T,int maxsize>
int Storage<T,maxsize>::getSize()
{
    return length;
}

template<class T,int maxsize>
T Storage<T,maxsize>::get(int index)
{
    return array[index];
}

template<class T,int maxsize>
bool Storage<T,maxsize>::add(T element)
{
    if (length>=maxsize) return false;
    array[length++]=element;
    return true;
}

class Pokemon{

    public: Pokemon(){};
};

int main(){
    Storage<Pokemon,4> pokemonArray;
}

Note that if no constructor was given, the compiler would generate one automatically and the code would still compile.

class Pokemon{
    //no constructors is still ok
};

The default constructor is called implicitly by the Array inside your Storage class. You most likely have a non-trivial constructor in your Pokemon class which is causing the problem. Perhaps something like this:

class Pokemon{

    public: 
    Pokemon(std::string name){
        //....
    }
    //...
};

When you provide a non-trivial constructor, the compiler will not add one implicitly, and thus, you either need to change your Storage class, or give the Pokemon class a default constructor along side your current constructor(s):

class Pokemon{

    public: 
    Pokemon(){}; //<-- needed
    Pokemon(std::string name){ ... }; //<-- optional
};

I didn't have indeed a default constructor. And my constructors aren't trivial. I needed to create default constructors for all my classes related somehow to the class Pokemon. But in the end, it worked ! Thaks ! — Xema, Nov 03 '14 at 21:05

score 1 · Answer 2 · edited May 23 '17 at 12:20

Since your Storage templated class has an array of fixed size T (i.e. Pokemon objects):

template<class T, int maxsize>
class Storage
{
public:
    Storage();

    int getSize();
    T get(int index);
    bool add(T element);

private:
    T array[maxsize]; // Array
    int length;
};

when you try to instantiate an object of that class you should make sure that the Pokemon class has a visible default constructor to initialize that array of objects. The following won't work for instance:

class Pokemon {
public:
    Pokemon() = delete;
};

int main(void)
{
    Storage<Pokemon, 4> pokemonArray;

    return 0;
}

Solution: make sure the class has a default constructor or change your design (what about a std::vector of smart pointers for the internal storage?)

Indeed I didn't have any default constructors. I am working on a assignement that asks for the use of template class and not vector (well yes, but this is the next question ;)). — Xema, Nov 03 '14 at 21:08

Deduplicator · Answer 3 · 2014-11-03T19:48:11.693

1

You error is simple:

You are using initialized storage.

Use uninitialized storage to store those members, like with an anonymous union.
As you have taken over lifetime-management of the contained T elements, you need to write your own copy-ctor, move-ctor, copy-assignment, move-ctor and destructor.

An alternative is re-using a standard-container already doing that for you with dynamic storage, but it looks like you wanted to build your own and avoid the heap.

Or, you know, make sure your types are default-constructible by giving them a default-ctor.

edited Nov 03 '14 at 19:48

answered Nov 03 '14 at 19:41

Deduplicator

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Creating uninitialized storage seemed a bit too complex for me comparing to default constructors. Thank you for answering :). – Xema Nov 03 '14 at 21:07
Well, it is easier if you decide you don't want to support some of those operations, in which case you can declare them `delete`d. – Deduplicator Nov 03 '14 at 21:11

Template class can't find constructor

3 Answers3