C++17 STL Cookbook

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In order to relate how much constexpr-if constructs are an improvement to C++, we can have a look at how the same thing could have been implemented before C++17:

template <typename T>class addable{    T val;public:    addable(T v) : val{v} {}    template <typename U>    std::enable_if_t<!std::is_same<T, std::vector<U>>::value, T>    add(U x) const { return val + x; }    template <typename U>    std::enable_if_t<std::is_same<T, std::vector<U>>::value,                      std::vector<U>>    add(U x) const {        auto copy (val);        for (auto &n : copy) {             n += x;        }        return copy;    }};

Without using constexpr-if, this class works for all different types we wished for, but it looks super complicated. How does it work?

The implementations alone of the two different add functions ...