How do you create a private dynamic array in a c++ class?

Question

Hello I am having trouble with using dynamic arrays within a class. I am instructed that "the class VectorDouble will have a private member variable for a dynamic array of doubles." I am only as far as writing the header file for this program, but I have not gotten past that. This class needs to be able to double in size once it has reached capacity. Here is my code:

#include <iostream>
using namespace std;

// VectorDouble class header file
class VectorDouble
{
    public:
    // constructor for an empty vector of type double
        VectorDouble();
    // constructor that take an int argument for an integer size of and array
        VectorDouble(int initial_size);
    // copy constructor
        VectorDouble(VectorDouble &object);
    // destructor to return dynamic memory to the freestore
        ~VectorDouble();
    // overloaded operator = for VectorDouble class
        VectorDouble& operator =(const VectorDouble &source);
    // overloaded operator == for VectorDouble class
        friend bool& operator ==(const VectorDouble &this_vector,
                                 VectorDouble &other_vector);
    // adds a new element at the end of the array
        void push_back();
    // returns allocated size of VectorDouble dynamic array
        int capacity();
    // returns used size of the VectorDouble object
        int size();
    // allocates a specified block of memory for specified number of double
    // type values
        void reserve(int new_reserve);
    // changes the size of the dynamic array
        void resize(int new_size);
    // returns the value at the specified index
        double value_at(int index);
    // changes the value at the specified index to double value d
        void change_value_at(double d, int index);

    private:

        int count;
        int max_count;
        int index_pointer;
        *index_pointer = new double[100]; 

}; 

The errors I am getting are all on this line:*index_pointer = new double[100];

• `new' cannot appear in a constant-expression

• ISO C++ forbids declaration of `index_pointer' with no type

• ISO C++ forbids initialization of member `index_pointer'

• making `index_pointer' static

• invalid in-class initialization of static data member of non-integral type `int*'

Solution 1

Your pointer needs a type. Change that line to

double* index_pointer;

And in your constructor add the line

index_pointer = new double[100];

And so on for your other constructors and assignment operator.

But this is also a naming conflict because you have another private int member named index_pointer. I'm not sure what that member is for, but if you do actually need it then you'll have to name it or the pointer something else.

Remember to delete[] index_pointer; in your destructor.

Solution 2

You should use std::vector<double>.

Comments

[πάντα ῥεῖ]

Is this homework or excercise? Otherwise I would simply recommend using std::vector<double> instead of providing your handrolled class.

[nszejna]

Yes it is homework, I have learned how to use vectors, but we are supposed to use dynamic arrays to make a class similar to vectors but with the type double only.

[leemes]

@g-makulik "I'm instructed that..." sounds like homework. So I think they are learning RAII in C++, including writing a copy constructor and stuff.

[andre]

The code should be double* index_pointer; no '*index_pointer = new double[100];'

[Jacob]

this is obviously homework to implement a naive vector<double>, so I doubt that's an option

[πάντα ῥεῖ]

In case you need to supply this class as surrogate for std::vector<double> because you can't use STL in your environment or simply asked to implement this as an excercise, this answer will probably solve your problem.

[matthew3r]

Maybe, but he never said that. If not an option, then he will not use it :) - Now I see it's a homework, but otherwise vector is the answer.

[nszejna]

Oh so only the pointer is in the private section then the part where the array is created is in the constructor?

[πάντα ῥεῖ]

@user1678621 No. The problem is that you'll have to declare index_pointerin the 1st place. Creating the array dynamically happens in the constructor code, yes.