The compiler throws an error saying that "Dict a(10)" has initializer but incomplete type"

Question

Possible Duplicate:
Why can templates only be implemented in the header file?

classdeclaration.h

template<typename anytype>
class List;

template<typename anytype>
class Dict;

---

classdefinition.cpp

#include "classdeclaration.h"
#include<iostream>

using std::cout;
using std::endl;

template<typename anytype>
class List{
    private:
        int size;
        anytype * list;
    public:
        List(int a);
        List(const List<anytype> &a);
        int getSize();
        ~List();
};

template<typename anytype>
 List<anytype>::List(int a){
            size=a;
            list=new anytype[size];
            cout<<"List object initialized. The address of list is "<<list<<endl;
        }

template<typename anytype>
int List<anytype>::getSize(){
        return size;
        }

template<typename anytype>
List<anytype>::List(const List<anytype> &a){
        this.size=a.getSize();
        this.list=new List<anytype>(size);
        }

template<typename anytype>
List<anytype>::~List(){ 
            delete[] list;
            cout<<"List destructor called."<<endl;

}


template<typename anytype>
class Dict{
    private:
        int dicta;
        List<anytype> *dict;
    public:
        Dict(int num);
        Dict(const Dict<anytype>& a);
        int getDicta();
        ~Dict();
};

template<typename anytype> 
Dict<anytype>::Dict(int num){
            dicta=num;
            dict=new List<anytype>(dicta);
            cout<<"Dict object initialized. The address of dict is "<<dict<<endl;
        }

template<typename anytype>
int Dict<anytype>::getDicta(){
            return dicta;
        }

template<typename anytype>
Dict<anytype>::Dict(const Dict<anytype> & a){
            this.dicta=a.getDicta;
            dict=new Dict<anytype>(dicta);
        }

template<typename anytype> 
Dict<anytype>::~Dict(){
            delete[] dict;
            cout<<"Dict destructor called."<<endl;
        }

---

test.cpp

#include<iostream>
#include "classdeclaration.h"  //A


int main()
{
    using namespace std;
    Dict<int> a(10);
    cout<<"The address of Dict<int> a is "<<&a<<endl;
}

The question here is that the file will not compile with command "g++ test.cpp classdefinition.cpp -o e:\KC" and the compiler throws an error saying that Dict<int> a(10) has initializer but incomplete type. What does it mean by incomplete type and how can I fix it?

The other question is that if I replace the preprocessor statement in line A with #include "classdefinition.cpp", the file will compile but fail to run.

Answer 1

You must place the definition of all templates in a header file. The compiler cannot find the definitions between translation units, unlike regular functions.

Also, how the hell am I supposed to know why your code will fail to run if you don't tell me what the error is?

Answer 2

You should always place the full class definition in the header files, otherwise you will always get similar errors.

When it comes to templates, the full implementation also needs to be seen by the code using the class. This is because a templated class is not fully declared until you actually use it with template parameters.