Removing code duplicate with if constexpr - c++

Question

I have some code as follows:
Matrix<T, N> res = zeros<T, N>(dims);
if constexpr (N1 == 1) {
MatrixView<U, 2> m1_view ({1, m1.dims[0]}, m1.dataView(), {m1.dims[0], 1});
if constexpr (N2 == 1) {
MatrixView<V, 2> m2_view ({m2.dims[0], 1}, m2.dataView(), {1, 1});
// duplicate code! yuck..
res.applyFunctionWithBroadcast(m1_view, m2_view, MatmulTo<U, V, T,
std::min(std::max(N1, 2lu), N - 1),
std::min(std::max(N2, 2lu), N - 1), N - 1>);
} else {
MatrixView<V, N2> m2_view (m2);
res.applyFunctionWithBroadcast(m1_view, m2_view, MatmulTo<U, V, T,
std::min(std::max(N1, 2lu), N - 1),
std::min(std::max(N2, 2lu), N - 1), N - 1>);
}
} else {
MatrixView<U, N1> m1_view (m1);
if constexpr (N2 == 1) {
MatrixView<V, 2> m2_view ({m2.dims[0], 1}, m2.dataView(), {1, 1});
// duplicate code! yuck..
res.applyFunctionWithBroadcast(m1_view, m2_view, MatmulTo<U, V, T,
std::min(std::max(N1, 2lu), N - 1),
std::min(std::max(N2, 2lu), N - 1), N - 1>);
} else {
MatrixView<V, N2> m2_view (m2);
res.applyFunctionWithBroadcast(m1_view, m2_view, MatmulTo<U, V, T,
std::min(std::max(N1, 2lu), N - 1),
std::min(std::max(N2, 2lu), N - 1), N - 1>);
}
}
return res;
which can be simplified in languages like Python (but not in C++):
if constexpr (N1 == 1) {
MatrixView<U, 2> m1_view ({1, m1.dims[0]}, m1.dataView(), {m1.dims[0], 1});
} else {
MatrixView<U, N1> m1_view (m1);
}
if constexpr (N2 == 1) {
MatrixView<V, 2> m2_view ({m2.dims[0], 1}, m2.dataView(), {1, 1});
} else {
MatrixView<V, N2> m2_view (m2);
}
// doesn't compile
res.applyFunctionWithBroadcast(m1_view, m2_view, MatmulTo<U, V, T,
std::min(std::max(N1, 2lu), N - 1),
std::min(std::max(N2, 2lu), N - 1), N - 1>);
return res;
MatrixView is not default constructible. (It is basically a non-owning view for some Matrix, therefore allowing so will break design)
The code above is logically valid but it won't compile, so I wrote the first ugly, verbose code.
Is there any way to remove code duplication in situations like this?

Answer 1

you might do:
auto m1_view = [&](){
if constexpr (N1 == 1) {
return MatrixView<U, 2>({1, m1.dims[0]}, m1.dataView(), {m1.dims[0], 1});
} else {
return MatrixView<U, N1>(m1);
}
}(); // notice extra () to immediate call
auto m2_view = [&](){
if constexpr (N2 == 1) {
return MatrixView<V, 2>({m2.dims[0], 1}, m2.dataView(), {1, 1});
} else {
return MatrixView<V, N2>(m2);
}
}();
Matrix<T, N> res = zeros<T, N>(dims);
res.applyFunctionWithBroadcast(m1_view, m2_view, MatmulTo<U, V, T,
std::min(std::max(N1, 2lu), N - 1),
std::min(std::max(N2, 2lu), N - 1), N - 1>);
return res;