CODEWITHSUNDEEP
c++templates
Alex Chapp
Alex Chapp

Reputation: 137

Confusion with c++ templates

I am working on a homework assignment that involved converting matrix and array classes supplied by my professor into templates. I am getting an error '=': cannot convert from 'Array *' to 'int' in my line m[i] = new Array < Type >(cols); I would assume this is because m[i] is returning an int but I don't think it should be if I wrote my templates correctly and I can't quite figure out why it is giving back an int instead of a array pointer since m is an array of array pointers here is the code to my array template minus some overloads for << not used in this code.

template
< typename Type >
class Array
{
private:
    int len;
    Type * buf;
public:
    Array(int newLen)
        : len(newLen), buf(new Type[newLen])
    {
    }
    Array(const Array & l)
        : len(l.len), buf(new Type[l.len])
    {
        for (int i = 0; i < l.len; i++)
            buf[i] = l.buf[i]; 
    }
    int length()
    {
        return len;
    }
    int & operator [] (int i)
    {
        assert(0 <= i && i < len);
        return buf[i];
    }
}

Here is my matrix template the error occurs in minus the same << overloads

#pragma once
#include "Array.h"

template
< typename Type >
class Matrix
{
private:
    int rows, cols;
    Array< Array<Type> * > m;
public:
    Matrix(int newRows, int newCols)
        : rows(newRows), cols(newCols), m(rows)
    {
        for (int i = 0; i < rows; i++)
            m[i] = new Array < Type >(cols);
    }
    int numRows()
    {
        return rows;
    }
    int numCols()
    {
        return cols;
    }
    Array < Type > & operator [] (int row)
    {
        return *m[row];
    }
}

Upvotes: 0

Views: 123

Answers (2)

Barry
Barry

Reputation: 304102

The proximal issue is that Array isn't fully templated:

int & operator [] (int i)

Indexing into an Array<T> shouldn't give you an int&, it should give you a T&!

The other issue with your code is that you have a new in your Array constructor - where's the corresponding delete? You're leaking memory!

Same for the Matrix constructor, which additionally has the potential of double deleting memory if you happen to copy it.

See Rule of Three

Upvotes: 1

Brandon Haston
Brandon Haston

Reputation: 434

The [] operator overload in the array class is what's messing you up. 

 int & operator [] (int i)
{
    assert(0 <= i && i < len);
    return buf[i];
}

It clearly returns an int as its type and when you try to use it such that: m[i] = new Array < Type >(cols);

m[i] will return an int type which you're trying to assign a new Array to.

Upvotes: 2

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