You can leave out the explicit return type. auto func = [] (std::complex<double> z, std::complex<double> c) { return z * z + c; }; You could also typedef th

You can leave out the explicit return type. auto func = [] (std::complex<double> z, std::complex<double> c) { return z * z + c; }; You could also typedef the number type somewhere: typedef std::complex<double> Complex; // ... auto func = [] (Complex z, Complex c) { return z * z + c; }; I think that last one looks ok.