Overloading (+=) operator in C++

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C++

overloading

Imagine we want to do some matrix assembly with defining operator overloading. node_0,node_1,and node_2 are coming from the connectivity matrix which is n_element*4.
I've already defined the summation operator (matrix+ scalar) and summation operator (matrix+matrix) but in does not work properly in this case and I'm not sure how to define the proper overloading operator for this. I would be grateful if you could help me with this.

What I have tried:

for (int i=0;i<n_element;i++)
{

		int node_0=((connectivity[i].Node_0 - 1));
		int node_1=((connectivity[i].Node_1 - 1));
		int node_2=((connectivity[i].Node_2 - 1));

		 NN[node_0][node_0]  = NN[node_0][node_0]  + NN_e[0][0];
		 NN[node_0][node_1]  = NN[node_0][node_1]  + NN_e[0][1];
		 NN[node_0][node_2]  = NN[node_0][node_2]  + NN_e[0][2];
		 NN[node_1][node_0]  = NN[node_1][node_0]  + NN_e[1][0];
		 NN[node_1][node_1]  = NN[node_1][node_1]  + NN_e[1][1];
		 NN[node_1][node_2]  = NN[node_1][node_2]  + NN_e[1][2];
		 NN[node_2][node_0]  = NN[node_2][node_0]  + NN_e[2][0];
		 NN[node_2][node_1]  = NN[node_2][node_1]  + NN_e[2][1];
		 NN[node_2][node_2]  = NN[node_2][node_2]  + NN_e[2][2];

}