Template class constructor issues - designing a container for multidim arrays

问题

I'm trying to create my own container for an array of any dimension for numerical computing. I would like to do this using templates so that I could overload the subscript operator [] so that it works like normal arrays and vectors e.g. access entries like a[10][10][10] etc.

I am having trouble getting the constructor to work when trying to create containers to hold multidimensional arrays. Please help!

#include <iostream>
#include <iterator>
#include <algorithm>
#include <vector>
using namespace std;

template <class T>
class container{
public:
 inline T& operator[](int i){return data[i];}
 container(int si, T initval){
  size=si; 
  data=new T[size]; 
  transform(data,data+size,data, [initval] (T d) {return initval;});
        // transform fills array with the initial value. 
 }
 ~container(){delete [] data;}
private:
 T* data;
 int size;
};

int main(){
 //For example:
 vector<vector<int>> v1(10,vector<int>(10,0)); //2D 10x10
 vector<vector<vector<int>>> v2(10,vector<vector<int>>(10,vector<int>(10,0))); 
    //3D 10x10x10

 container<int> c1(10,0); //1D 10x1 works!
 container<container<int>> c2(10,container<int>(10,0)); //2D 10x10 fails!

 system("pause");
 return 0;
}

VS10 error output:

error C2512: 'container<T>' : no appropriate default constructor available

      with
      [
          T=int
      ]
      c:\users\jack\documents\visual studio 2010\projects\ref\ref\ref.cpp(11) : while compiling class template member function 'container<T>::container(int,T)'
      with
      [
          T=container<int>
      ]
      c:\users\jack\documents\visual studio 2010\projects\ref\ref\ref.cpp(28) : see reference to class template instantiation 'container<T>' being compiled
      with
      [
          T=container<int>
      ]

Build FAILED.

I know I could just use valarray or a boost library, but I would like to understand how to create my own. Efficiency is important. Thanks!

回答1:

Your constructor uses the expression new T[size] and this requires T to be default constructible (if T is a class type).

You need to do something like: allocate raw memory (e.g. using operator new) and construct T instances "in place" using a placement new expression. Alternatively, you could just give container a default constructor.

回答2:

You are missing a lot of constructors. You need a default constructor and a copy constructor at the very least, operator= too.

回答3:

Give container a default constructor, that takes no parameters. It looks like you want to make your container non-resizable though.
You could make the dimension of container a template parameter and ignore the T initVal
If you do that you probably don't even need to use new[] and delete[] but if you do you need the full "rule of 3" of copying and assigning to be properly implemented.
You might find a const overload of operator[] useful too.