default constructor does not exist in the class

[Cplusc]

anybody can help with this piece of code;
I am using class to build matrices nut when I define my object it says the default constructor does not exist. how can I fix it?
Thanks in advance

#include <iostream>;
#include <string>;
#include<cstdlib>;
using namespace std;
class Matrix {
public:
	Matrix(int rows, int cols);
	~Matrix();
private:
	const int ROWS;
	const int COLS;
	double *vals;
};

Matrix::Matrix(int rows, int cols) :
	ROWS(rows), COLS(cols)
{
	vals = new double[ROWS * COLS];
}
Matrix::~Matrix()
{
	delete[] vals;
}

int main()
{

	Matrix m1;
	return 0;
}

[salem c]

Well a default constructor has no parameters, so

Matrix::Matrix() :
	ROWS(0), COLS(0), vals(nullptr)
{
}

[lastchance]

when I define my object it says the default constructor does not exist. how can I fix it?

Either:
(1) write a default constructor (one with no arguments);
or
(2) match your statement in main() to the constructor that you have supplied (takes two ints as parameters);
or
(3) give the parameters in the current constructor some default values (e.g. Matrix::Matrix(int rows=1, int cols=1)

[jlb]

You either need to construct your matrix with the correct number of parameters, and you really should declare the no argument constructor (the "default" constructor) as deleted.

#include <iostream>
#include <string>

using namespace std;

class Matrix {
public:
        Matrix() = delete;
	Matrix(int rows, int cols);
	~Matrix();
private:
	const int ROWS;
	const int COLS;
	double *vals;
};

Matrix::Matrix(int rows, int cols) :
	ROWS(rows), COLS(cols)
{
	vals = new double[ROWS * COLS];
}
Matrix::~Matrix()
{
	delete[] vals;
}

int main()
{

	Matrix m1(10,23);
	return 0;
}

[Cplusc]

Thank you all for your answers.

[Cplusc]

how can i set the value for m1?sorry for my naïve question

[Ganado]

Do you mean, how do you set the values of the individual array elements?
Either make vals public and access vals[i], or, you could do something like overload the () operator and let it have (row, column) input arguments.

#include <iostream>
#include <string>

class Matrix {
public:
    Matrix() = delete;
	Matrix(int rows, int cols);
	~Matrix();
	
	double& operator()(int row, int col)
	{
	    return vals[COLS * row + col];   
	}
	
	const double& operator()(int row, int col) const 
	{
	    return vals[COLS * row + col];   
	}

	const int ROWS;
	const int COLS;
	
private:
	double *vals;
};

Matrix::Matrix(int rows, int cols) :
	ROWS(rows), COLS(cols)
{
	vals = new double[ROWS * COLS];
}
Matrix::~Matrix()
{
	delete[] vals;
}

int main()
{
	Matrix m1(10,23);
	
	m1(0, 0) = 42;
	m1(5, 11) = -100;
	m1(9, 22) = 1700;
	
	for (int i = 0; i < m1.ROWS; i++)
	{
	    for (int j = 0; j < m1.COLS; j++)
	    {
	        std::cout << m1(i, j) << ' ';   
	    }
	    std::cout << '\n';
	}
	
	return 0;
}

[Cplusc]

Yes,exactly, thanks

[seeplus]

As you're not allowing the size to be changed, then the size can only be specified in the constructor. Hence a default constructor is only really valid if it defines a default sized matrix. Consider:

#include <iostream>

class Matrix {
public:
	Matrix(size_t rows = defRow, size_t cols = defCol) : ROWS(rows), COLS(cols), vals(new double[rows * cols] {}) {}
	~Matrix() { delete[] vals; }

	double& operator()(size_t row, size_t col) { return vals[COLS * row + col]; }
	const double& operator()(size_t row, size_t col) const { return vals[COLS * row + col]; }

	const size_t ROWS {};
	const size_t COLS {};

	friend std::ostream& operator<<(std::ostream& os, const Matrix& m) {
		for (size_t i = 0; i < m.ROWS; ++i) {
			for (size_t j = 0; j < m.COLS; ++j)
				os << m(i, j) << ' ';

			os << '\n';
		}

		return os;
	}

private:
	static constexpr size_t defRow {10};
	static constexpr size_t defCol {10};

	double* vals {};
};

int main()
{
	Matrix m1(10, 23);
	Matrix m2;

	m1(0, 0) = 42;
	m1(5, 11) = -100;
	m1(9, 22) = 1700;

	m2(2, 2) = 2;
	m2(4, 4) = 44;

	std::cout << m1 << '\n' << m2 << '\n';
}

[Cplusc]

Actually I want to use this class to produce the local and global stifness matrix in finite element method and the size should be chargeable. And also I want to pass another argument in the class which is the area of each element. The area of each element is obtained by a struct. Since I'm very new in objective oriented programming this gonna be hard for me at this time. I don't know hot to pass an argument within a class which is coming from another class or struct.

[Cplusc]

@Ganado
I don't understand what these piece of code is doing?
double& operator()(int row, int col)
{
return vals[COLS * row + col];
}

const double& operator()(int row, int col) const
{
return vals[COLS * row + col];
}

[keskiverto]

Ganado wrote:

overload the () operator

There are two overloads of that operator.

The first returns a reference to element in the matrix:

double& operator()(int row, int col)
{
  return vals[COLS * row + col];
}

You can both read and write with it:

Matrix m1(10, 23);
m1( 1, 2 ) = 42; // write
std::cout << m1( 1, 2 ); // read 

However, if the matrix is const, the that operator cannot be used.
The other can be called on const objects:

const double& operator()(int row, int col) const
{
  return vals[COLS * row + col];
}

Note that although it too returns a reference, the reference is const. Therefore, one can read elements but not modify them.

[Ganado]

Admittedly the const version is a bit silly without a way to initialize non-zero array values in a constructor.