c++ initialize template class constructor

Question

How do I initialize pointer class B foo inside class A? I am new to C++.

Header.h

namespace Core
{
    enum type
    {
        Left, Right
    };

    template<type t>
    class B
    {
    public:
        B(int i);
    private:
        type dir;
        int b = 12;
    };

    class A
    {
    public:
        B<Left> *foo;
    };
}

Source.cpp

namespace Core
{
    template<type t>
    B<t>::B(int i)
    {
        dir = t;
        b = i;
    }
}

int main()
{
    Core::A *a = new Core::A;
    a->foo = new Core::B<Core::Left>(10);

    return 0;
}

Answer 1

Source.cpp needs a #include "Header.h" statement, and Header.h needs a header guard.

Also, you need to move the implemention of B's constructor into the header file. See Why can templates only be implemented in the header file?.

Try this:

Header.h:

#ifndef HeaderH
#define HeaderH

namespace Core
{
    enum type
    {
        Left, Right
    };

    template<type t>
    class B
    {
    public:
        B(int i);
    private:
        type dir;
        int b = 12;
    };

    class A
    {
    public:
        B<Left> *foo;
    };

    template<type t>
    B<t>::B(int i)
    {
        dir = t;
        b = i;
    }
}

#endif

Source.cpp

#include "Header.h"

int main()
{
    Core::A *a = new Core::A;
    a->foo = new Core::B<Core::Left>(10);
    //...
    delete a->foo;
    delete a;
    return 0;
}

I would suggest taking it a step further by inlining B's constructor and giving A a constructor to initialize foo:

Header.h:

#ifndef HeaderH
#define HeaderH

namespace Core
{
    enum type
    {
        Left, Right
    };

    template<type t>
    class B
    {
    public:
        B(int i)
        {
            dir = t;
            b = i;
        }

    private:
        type dir;
        int b = 12;
    };

    class A
    {
    public:
        B<Left> *foo;

        A(int i = 0)
            : foo(new B<Left>(i))
        {
        }

        ~A()
        {
            delete foo;
        }
    };
}

#endif

Source.cpp

#include "Header.h"

int main()
{
    Core::A *a = new Core::A(10);
    //... 
    delete a;
    return 0;
}

Answer 2

How do I initialize pointer class B foo inside class A?

Option 1

Construct a B<Left> with an assumed value.

class A
{
   public:
    B<Left> *foo = new B<Left>(0);
};

Option 2

Add a constructor of A that accepts an int that can be used to construct a B<Left> .

class A
{
   public:
      A(int i) : foo(new B<Left>(i)) {}
    B<Left> *foo;
};

Word of caution

Before you down too far into using pointers to objects in your classes, consider the following:

1. What is The Rule of Three?
2. https://en.cppreference.com/w/cpp/memory, specially shared_ptr and unique_ptr.