How to create our own dynamic array class in c++

Question

I was trying yo create my own array class (similar to std::vector) just for fun but there is some problem... The Array class code itself works and compiles successfully but throws an error if i try to instantiate an object of Array class.

#include<iostream>

template<typename type, int size>
class Array
{
 private:
 type _mArray[size] = new type[size];
 public:
 int Access(int index)
 {
  return _mArray[index];
 }
 int Len()
 {
   return size;
 }
 void Insert(int index, type val)
 {
  _mArray[index] = val;
 }
 ~Array()
 {
  delete[] _mArray; 
 }
};//All code above compiles successfully

int main()
{
  Array<int, 2> name; //this line throws an error
}

I am a bit new to C++ so if someone can explain then I will be very thankful....

Btw here is the error Array initializer must be an initializer list

If you want to have a dynamic array like std::vector, don't make size a template parameter. Also type of size should not be an int but std::size_t (avoids having to check for index < 0). Check the input indices against the size of the actual array (or you can get out of bound access). — Pepijn Kramer
– Pepijn Kramer, Commented Dec 27, 2021 at 7:11
The Array class code itself works -- int main() { Array<int, 2> name; Array<int, 2> name2; name = name2; } -- Double deletion error at runtime. You still have work today to get this code to actually be useful. — PaulMcKenzie
– PaulMcKenzie, Commented Dec 27, 2021 at 7:18
Look more like std::array (fixed size array) than std::vector (dynamic size array). — Jarod42
– Jarod42, Commented Dec 27, 2021 at 10:30

Sam Varshavchik · Accepted Answer · 2021-12-27 04:14:47Z

2

type _mArray[size] = new type[size];

The template instantiates with: type is int, and size is 2. Therefore, this becomes:

int _mArray[2] = new int[2];

This obviously does not make much sense. If you put this, verbatim, in your main() your C++ compiler will also serve you with the same complaint.

It's clear that the intent here is, simply:

type _mArray[size];

And nothing else.

P.S. Now, let's go back and reread what the suffering C++ compiler was struggling to communicate here:

Array initializer must be an initializer list

int _mArray[2] is, obviously, an array. There's an = stuck after it. Ok, this must be array initialization. How do you initialize an array in C++? With a braced initialization list, of course. This would be something like this, for example:

int _mArray[2]={1, 2};

The C++ compiler saw nothing of that kind, and was trying to tell you that.

answered Dec 27, 2021 at 4:14

Sam Varshavchik

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2 Comments

A M Over a year ago

Very good explanation +1

Jarod42 Over a year ago

destructor has to be fixed/removed then :-)

Joma · Accepted Answer · 2021-12-27 04:20:34Z

0

#include<iostream>

template<typename type, int size>
class Array
{
 private:
 type * _mArray ;
 public:
 int Access(int index)
 {
  return _mArray[index];
 }
 int Len()
 {
   return size;
 }

 Array()
 {
   _mArray = new type[size];
 }
 ~Array()
 {
  delete[] _mArray; 
 }
 int& operator[](int index){
    return _mArray[index];
 }
};//All code above compiles successfully

int main()
{
  Array<int, 2> name; 
  name[0] = 1024;
  name[1] = 100;

  for(int i= 0; i< name.Len(); i++)
  {
     std::cout<< name[i] << std::endl;
  }

}

answered Dec 27, 2021 at 4:20

Joma

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2 Comments

A M Over a year ago

And what? Just code dump?

Jarod42 Over a year ago

rule of 3/5/0 broken.

mkrufky · Accepted Answer · 2021-12-27 04:24:40Z

0

You can get it to build using the following minimal change:

@@ -4,7 +4,7 @@ template<typename type, int size>
 class Array
 {
  private:
- type _mArray[size] = new type[size];
+ type* _mArray;
  public:
  int Access(int index)
  {
@@ -18,6 +18,10 @@ class Array
  {
   _mArray[index] = val;
  }
+ Array()
+ {
+  _mArray = new type[size]; 
+ }
  ~Array()
  {
   delete[] _mArray;

Basically, you should be initializing the array in your constructor, and store a pointer to it as a class member. The following code builds:

#include<iostream>

template<typename type, int size>
class Array
{
 private:
 type* _mArray;
 public:
 int Access(int index)
 {
  return _mArray[index];
 }
 int Len()
 {
   return size;
 }
 void Insert(int index, type val)
 {
  _mArray[index] = val;
 }
 Array()
 {
  _mArray = new type[size]; 
 }
 ~Array()
 {
  delete[] _mArray; 
 }
};//All code above compiles successfully

int main()
{
  Array<int, 2> name; //this line throws an error
}

answered Dec 27, 2021 at 4:24

mkrufky

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1 Comment

Jarod42 Over a year ago

rule of 3/5/0 broken.

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