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eigen-2.1.1: eigen3/doc/snippets/TopicAliasing_mult2.cpp

MatrixXf matA(2,2), matB(2,2); 
matA << 2, 0,  0, 2;

// Simple but not quite as efficient
matB = matA * matA;
cout << matB << endl << endl;

// More complicated but also more efficient
matB.noalias() = matA * matA;
cout << matB;