Function of Matrix Addition ,Sub and Multiplication in C++ using classes

Functions  of Matrix using C++ Classes

#include<iostream>
using namespace std;
class matrics
{
private:
int rows;
int columns;
int m[100][100];
public:
matrics();
matrics add(matrics);
matrics sub(matrics);
matrics mul(matrics);
};


matrics matrics::add(matrics b)
{
matrics r;
int i,j;
for(i=0;i<rows;i++)
for(j=0;j<columns;j++)
{
r.m[i][j]= m[i][j]+b.m[i][j];
}
r.rows=rows;
r.columns=columns;
return r;
}
matrics matrics::sub(matrics b)
{
matrics r;
int i,j;
for(i=0;i<rows;i++)
for(j=0;j<columns;j++)
{
r.m[i][j]= m[i][j]-b.m[i][j];
}
r.rows=rows;
r.columns=columns;
return r;
}
matrics matrics::mul(matrics b)
{
matrics r;
for(int i=0;i<rows;i++)
{
for(int j=0;j<b.columns;j++)
{
r.m[i][j]=0;
        for(int k=0;k<rows;k++)
{
r.m[i][j]=r.m[i][j]+(m[i][k] * b.m[k][j]);
}
}
}
r.rows=rows;
r.columns=b.columns;

return r;
}