Is there a way to initialize an array with non-constant variables? (C++)

Question

I am trying to create a class as such:

class CLASS
{
public:
    //stuff
private:
    int x, y;
    char array[x][y];
};

Of course, it doesn't work until I change int x, y; to

const static int x = 10, y = 10;

Which is impractical, because I am trying to read the values of x and y from a file. So is there any way to initialize an array with non-contant values, or declare an array and declare its size on different statements? And I know this would probably require the creation of an array class, but I'm not sure where to start on this, and I don't want to create a 2D dynamic list when the array itself is not dynamic, just the size is not known at compile-time.

Answer 1

You can't allocate or initialize a global or static array declaratively using non-constant values (compile-time). It's possible for local arrays though (C99 variable sized arrays, as their initializer essentially runs at runtime every time the function is executed).

For your situation, I suggest using a pointer instead of an array and create the actual array dynamically at runtime (using new):

class CLASS
{
public:
    CLASS(int _x, int _y) : x(_x), y(_y) {
       array = new char*[x];
       for(int i = 0; i < x; ++i)
           array[i] = new char[y];
    }
    ~CLASS() {
       for (int i = 0; i < x; ++i)
           delete[] array[i];
       delete[] array;
    }
    //stuff
private:
    int x, y;
    char **array;
};

Answer 2

If the size is not known at compile time, the array is dynamic. What you could do to keep it static is to make them larger than your largest expected size.

Answer 3

If you want a dynamically sized array as a class member, you need to array new it and assign that value to a pointer. The char array[size] syntax is only for statically-sized arrays.

Better yet, you really should use an std::vector< std::vector<char> >, there are very few good reasons to manually work with dynamically sized arrays these days.

Answer 4

use vector.

#include <vector>
class YourClass
{
public:
    YourClass()
    : x(read_x_from_file()), y(read_y_from_file())
    {
        my_array.resize(x);
        for(int ix = 0; ix < x; ++ix)
            my_array[ix].resize(y);
    }

    //stuff

private:
    int x, y;
    std::vector<std::vector<char> > my_array;
};

Answer 5

Not in that manner, as in c++, c-style array sizes have to be known at compile time, with some vendor specific extensions allowing certain runtime sizes (to enhance compatibility with C99), but not in the situation you are describing (if you are interested, here's a description). The easiest thing to do would be:

std::vector< std::vector<char> > array;

And apply the size in the constructor:

array.resize(x);
for(std::vector< std::vector<char> >::iterator curr(array.begin()),end(array.end());curr!=end;++curr){
   curr->resize(y);
}

There are many advantages of vector over c style arrays, see here

Answer 6

The compiler need to have the exact size of the class when compiling, you will have to use the new operator to dynamically allocate memory.

Switch char array[x][y]; to char** array; and initialize your array in the constructor, and don't forget to delete your array in the destructor.

class MyClass
{
public:
    MyClass() {
        x = 10; //read from file
        y = 10; //read from file
        allocate(x, y);
    }

    MyClass( const MyClass& otherClass ) {
        x = otherClass.x;
        y = otherClass.y;
        allocate(x, y);

        // This can be replace by a memcopy
        for( int i=0 ; i<x ; ++i )
            for( int j=0 ; j<x ; ++j )
                array[i][j] = otherClass.array[i][j];
    }

    ~MyClass(){
        deleteMe();
    }

    void allocate( int x, int y){
        array = new char*[x];
        for( int i = 0; i < y; i++ )
            array[i] = new char[y];
    }

    void deleteMe(){
        for (int i = 0; i < y; i++)
           delete[] array[i];
        delete[] array;
    }

    MyClass& operator= (const MyClass& otherClass)
    {
        if( this != &otherClass )
        {
            deleteMe();
            x = otherClass.x;
            y = otherClass.y;
            allocate(x, y);
            for( int i=0 ; i<x ; ++i )
                for( int j=0 ; j<y ; ++j )
                    array[i][j] = otherClass.array[i][j];            
        }
        return *this;
    }
private:
    int x, y;
    char** array;
};

*EDIT: I've had the copy constructor and the assignment operator

Answer 7

Put all the memory into one block.
Because it is private you can then get your access methods to retrieve the correct value.

Quick example:

#include <vector>
#include <iostream>

class Matrix
{
    public:
    class Row
    {
        public:
        Row(Matrix& p,unsigned int x)
            :parent(p)
            ,xAxis(x)
        {}
        char& operator[](int yAxis)
        {
            return parent.data(xAxis,yAxis);
        }
        private:
            Matrix&         parent;
            unsigned int    xAxis;
    };

    Matrix(unsigned int x,unsigned int y)
        :xSize(x)
        ,ySize(y)
        ,dataArray(x*y)
    {}

    Matrix::Row operator[](unsigned int xAxis)
    {
        return Row(*this,xAxis);
    }
    char& data(unsigned int xAxis,unsigned int yAxis)
    {
        return dataArray[yAxis*xSize + xAxis];
    }
    private:
        unsigned int xSize;
        unsigned int ySize;
        std::vector<char>   dataArray;
};


int main()
{
    Matrix      two(2,2);

    two[0][0]   = '1';
    two[0][1]   = '2';
    two[1][0]   = '3';
    two[1][1]   = '4';

    std::cout <<  two[1][0] << "\n";
    std::cout <<  two.data(1,0) << "\n";
}

Answer 8

Take a look at boost::multi_array.

Answer 9

You can allocate memory to your 2-dimensional array in the constructor and free it in the destructor. The simplest way:

array = (char **)malloc(sizeof(char *) * x);
if (array) {
    for (i = 0; i < x; i++) {
        array[i] = (char *)malloc(sizeof(char) * y);
        assert(array[i]);
    }
}