How to initialize dynamic array inside a class? - c++

Question

I wish to initialize a multidimensional, dynamic array inside a class. But, I am getting an error.
I have seen several examples on the net. They seem to be difficult. I am new to coding. I would like a simple solution if possible.
class myp
{
int ntc = 5;
public:
double** y = new double*[ntc];
for(int i = 0; i < ntc; ++i)
y[i] = new int[3];
};
int main()
{
int x;
myp mp;
mp.y[1][1] = 3;
cout<<mp.y[1][1]<<endl;;
return 0;
}
test.cpp:12:2: error: expected unqualified-id before ‘for’
for(int i = 0; i < ntc; i++)
^~~
test.cpp:12:17: error: ‘i’ does not name a type
for(int i = 0; i < ntc; i++)
^
test.cpp:12:26: error: ‘i’ does not name a type
for(int i = 0; i < ntc; i++)

Answer 1

You need to do class initialisation in the constructor function, and cleanup in the destructor.
class myp
{
int m_numColumns;
int m_numRows;
double** y;
public:
// overload array operators
double* operator [] (size_t row) { return y[row]; }
const double* operator [] (size_t row) const { return y[row]; }
// return dimensions of array
int numColumns() const { return m_numColumns; }
int numRows() const { return m_numRows; }
// constructor
myp(int nc, int nr) : m_numColumns(nc), m_numRows(nr)
{
y = new double*[m_numRows];
for(int i = 0; i < m_numColumns; ++i)
y[i] = new int[m_numColumns];
}
// destructor
~myp()
{
for(int i = 0; i < m_numColumns; ++i)
delete [] y[i];
delete [] y;
}
// be careful of the copy ctor. I'm deleting it in this case!
myp(const myp&) = delete;
// edit: as per user4581301's suggestion
myp() = delete;
myp(myp&&) = delete; // remove move ctor
myp& operator = (const myp&) = delete; // remove assignment
myp& operator = (myp&&) = delete; // remove move assignment
};
int main()
{
myp mp(5, 3);
mp[1][1] = 3;
cout << mp[1][1]<<endl;
return 0;
}

Answer 2

Just For Run.
class myp
{
int ntc = 5;
public:
double **y;
void initArray()
{
y = new double*[ntc];
for(int i = 0; i < ntc; ++i)
y[i] = new double[3]; // i change this line [new int] to [new double]tv
}
};
int main()
{
int x;
myp mp;
mp.initArray();
mp.y[1][1] = 3;
cout<<mp.y[1][1]<<endl;;
return 0;
}
using constructor & destructor
class myp
{
int ntc = 5;
public:
double **y;
myp() // run at created
{
y = new double*[ntc];
for(int i = 0; i < ntc; ++i)
y[i] = new double[3];
}
~myp() // run at the end of life cycle
{
/* free memory here */
}
};
int main()
{
int x;
myp mp; // myp() called
mp.y[1][1] = 3;
cout<<mp.y[1][1]<<endl;
return 0;
}
using constructor with parameter, for dynamic size
class myp
{
// int ntc = 5; // using at created
public:
double **y;
myp(int ntc, int size) // run at created
// if you want to use only myp mp;
// myp(int ntc = 5, int size = 3) {} will be helpful
{
y = new double*[ntc];
for(int i = 0; i < ntc; ++i)
y[i] = new double[size];
}
~myp() // run at the end of life cycle
{
/* free memory here */
}
};
int main()
{
int x;
myp mp(5, 3); // myp(int, int) called
mp.y[1][1] = 3;
cout<<mp.y[1][1]<<endl;
return 0;
}