Question

This is slightly different than the question posed just below this one. Suppose I have a container class which has two template parameters, the first of which is a type, the second of which is the size of the container.

Now we have multiple containers with a different container storage size. Essentially, the container functions (all the public ones, anyway) only really care about T; N is only used to allocate local storage (an allocator is used if N is not enough).

I have put together a simple example implementation that showcases the problem I am having.

#include <iostream>

template <typename T, size_t N = 10>
class TestArray
{
public:
    T Local[N];

    class Iterator
    {
    public:
        T* Array;
        int Index;      

        Iterator() : Array(NULL), Index(-1) { }
        Iterator(T* _array, int _index) : Array(_array), Index(_index) { }

        bool operator == (const Iterator& _other) const 
        { 
             return _other.Index == Index && _other.Array == Array; 
        }

        bool operator != (const Iterator& _other) const 
        { 
            return !(*this == _other); 
        }

        template <size_t _N>
        Iterator& operator = (const typename TestArray<T, _N>::Iterator &_other)
        {
            Array = _other.Array;
            Index = _other.Index;

            return *this;
        }

        void Next() { ++Index; }
        void Prev() { --Index; }

        T& Get() { return Array[Index]; }
    };

    T& operator [] (const int _index) { return Local[_index]; }

    Iterator Begin() { return Iterator(Local, 0); }
    Iterator End() { return Iterator(Local, N); }

    template <size_t _N>
    void Copy(const TestArray<T, _N> &_other, int _index, int _count)
    {   
        int i;

        for (i = 0; i < _count; i++)
            Local[_index + i] = _other.Local[i];
    }   
};

This is really a two part question. The first part of which I posted earlier: Template container with multiple template parameters interacting with other template containers with a different template parameter. For the second part, I’m trying to use it as follows:

int main() {

    TestArray<int> testArray1;
    TestArray<int, 25> testArray2;

    TestArray<int>::Iterator itr;

    itr = testArray1.Begin();

    for (itr = testArray1.Begin(); itr != testArray1.End(); itr.Next())
    {
        itr.Get() = itr1.Index;
    }

    testArray2.Copy(testArray1, 0, 10);

    for (itr = testArray2.Begin(); itr != testArray2.End(); itr.Next())
    {
        std::cout << itr.Get() << std::endl;
    }

    return 0;
}

Here is an IDEONE link: http://ideone.com/GlN54

When compiled with gcc-4.3.4, I get the following:

prog.cpp: In function ‘int main()’:
prog.cpp:67: error: no match for ‘operator=’ in ‘itr = testArray2.TestArray<T, N>::Begin [with T = int, unsigned int N = 25u]()’
prog.cpp:10: note: candidates are: TestArray<int, 10u>::Iterator& TestArray<int, 10u>::Iterator::operator=(const TestArray<int, 10u>::Iterator&)
prog.cpp:67: error: no match for ‘operator!=’ in ‘itr != testArray2.TestArray<T, N>::End [with T = int, unsigned int N = 25u]()’
prog.cpp:19: note: candidates are: bool TestArray<T, N>::Iterator::operator!=(const TestArray<T, N>::Iterator&) const [with T = int, unsigned int N = 10u]

In VS2010, I get the following:

1>------ Build started: Project: testunholytemplatemess, Configuration: Debug Win32 ------
1>  main.cpp
1>c:\users\james\documents\testproj\testunholytemplatemess\testunholytemplatemess\main.cpp(67): error C2679: binary '=' : no operator found which takes a right-hand operand of type 'TestArray<T,N>::Iterator' (or there is no acceptable conversion)
1>          with
1>          [
1>              T=int,
1>              N=25
1>          ]
1>          c:\users\james\documents\testproj\testunholytemplatemess\testunholytemplatemess\main.cpp(34): could be 'TestArray<T>::Iterator &TestArray<T>::Iterator::operator =(const TestArray<T>::Iterator &)'
1>          with
1>          [
1>              T=int
1>          ]
1>          while trying to match the argument list '(TestArray<T>::Iterator, TestArray<T,N>::Iterator)'
1>          with
1>          [
1>              T=int
1>          ]
1>c:\users\james\documents\testproj\testunholytemplatemess\testunholytemplatemess\main.cpp(67): error C2679: binary '!=' : no operator found which takes a right-hand operand of type 'TestArray<T,N>::Iterator' (or there is no acceptable conversion)
1>          with
1>          [
1>              T=int,
1>              N=25
1>          ]
1>          c:\users\james\documents\testproj\testunholytemplatemess\testunholytemplatemess\main.cpp(19): could be 'bool TestArray<T>::Iterator::operator !=(const TestArray<T>::Iterator &) const'
1>          with
1>          [
1>              T=int
1>          ]
1>          while trying to match the argument list '(TestArray<T>::Iterator, TestArray<T,N>::Iterator)'
1>          with
1>          [
1>              T=int
1>          ]

I thought the Iterator& operator = would make it so that this assignment operator should work, but apparently not. Perhaps I am being thick but I am failing to determine the correct solution here. Does anyone have any suggestions?

Answer 1

TestArray<T, 1> and TestArray<T, 2> are different types, and so are TestArray<T, 1>::Iterator and TestArray<T, 2>::Iterator. The assignment cannot work! (Your itr is a different type from the type of testArray2.Begin().)

The entire construction seems very dubious to me — is this really necessary? What are you trying to achieve? Have you looked at std::array?

---

Update: It works if you supply the template parameters explicitly:

  for (itr.operator=<int,25>(testArray2.Begin());
       itr.operator!=<int,25>(testArray2.End());
       itr.Next())
  {
    std::cout << itr.Get() << std::endl;
  }

I’m not entirely sure why the parameters cannot be deduced from the arguments, and I am looking forward to a good explanation — in the meantime, I put the full code on Ideone, though it doesn’t compile there, but it does with my GCC 4.6.1.