CODEWITHSUNDEEP
c++templates
JimmidyJoo
JimmidyJoo

Reputation: 11281

How can I use the template parameter of a class as the template parameter of a class member?

I'm trying to implement the following in C++:

template <class T>
class Inner
{
public:
    Inner(T inData){data = inData;};
    ~Inner(void){};

    T data;
};

template <class T>
class Outer
{
public:
    Outer(Inner<T> in){inner = in;};
    ~Outer(void){};

    Inner<T> inner;
};

int main(void)
{
    Inner<int> in (10);
    Outer<int> out (in);

    std::cout << out.inner.data;
}

Compiled in MSVS 2010, I get the following error:

c:\...\main.cpp(15): error C2512: 'Inner<T>' : no appropriate default constructor available
          with
          [
              T=int
          ]
          c:\users\gaff\documents\visual studio 2010\projects\pgp(c++)\pgp(c++)\main.cpp(15) : while compiling class template member function 'Outer<T>::Outer(Inner<T>)'
          with
          [
              T=int
          ]
          c:\users\gaff\documents\visual studio 2010\projects\pgp(c++)\pgp(c++)\main.cpp(24) : see reference to class template instantiation 'Outer<T>' being compiled
          with
          [
              T=int
          ]

Clearly I'm missing some basic understanding here but I have no clue how to fix this, or even if what I'm trying to achieve is possible.

For what it's worth, here is a Java implementation that compiles without any problems:

public class Inner<T> {
    public T data;

    public Inner(T inData) {
        data = inData;
    }
}

public class Outer<T> {

    public Inner<T> inner;

    public Outer(Inner<T> in) {
        inner = in;
    }
}

public class Main {
    public static void main(String args[]){
        Inner<Integer> in = new Inner<Integer>(10);
        Outer<Integer> out = new Outer<Integer>(in);

        System.out.println(out.inner.data);
    }
}

Any suggestions?

Cheers

JimmidyJoo

Upvotes: 2

Views: 223

Answers (2)

GWW
GWW

Reputation: 44093

The problem is that you don't have a default constructor for inner. When the compiler initializes the object it calls the default constructor of inner before the assignment.

EDIT:

It's probably a good idea to properly implement a copy constructor and instead initialize inner like so:

Outer(Inner<T> in) : inner(in) { };

You could use the copy constructor like so:

Inner(const Inner<T> & in) : data(in.data){
}

Upvotes: 3

Neil G
Neil G

Reputation: 33202

You need to use an initializer list in the constructor:

Outer(Inner<T> in): inner(in) {}

otherwise the constructor has to default construct inner before it gets into the constructor. That's impossible since there is no default constructor.

Upvotes: 2

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