auto and expression templates

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auto can also cause problems where expression templates come into play:

auto mult(int c) {
    return c * std::valarray<int>{1};

auto v = mult(3);
std::cout << v[0]; // some value that could be, but almost certainly is not, 3.

The reason is that operator* on valarray gives you a proxy object that refers to the valarray as a means of lazy evaluation. By using auto, you’re creating a dangling reference. Instead of mult had returned a std::valarray<int>, then the code would definitely print 3.

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