Modern C++ Programming Cookbook

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These functions can be pipelined, that is, they can call one function with the result of another. The following example maps a range of integers into a range of positive integers by applying the std::abs() function to its elements. The result is then mapped into another range of squares. These are then summed together by applying a left fold on the range:

    auto vnums = std::vector<int>{ 0, 2, -3, 5, -1, 6, 8, -4, 9 };     auto s = funclib::foldl(       std::plus<>(),       funclib::mapf(         [](int const i) {return i*i; },          funclib::mapf(           [](int const i) {return std::abs(i); },           vnums)),       0); // s = 236

As an exercise, we could implement the fold function as a variadic function template, in the manner seen in a previous recipe. The function ...