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I am trying to use transform_reduce to transform a set of numbers and then reduce it to the maximum of those elements.

auto lambda = [](uint64_t x) {
     return function(x);
 };

 counting_iterator<uint64_t> start(1);
 counting_iterator<uint64_t> finish(bound);


 auto max_val = *transform_reduce(start, finish, lambda, std::max_element(start, finish));

My logic is that it will change all the values from 1 to bound to function(1) to function(bound), then with those values, give me the largest, but I am having issues, can anyone help me get to want I want to achieve?

Andrew O'Rourke
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1 Answers1

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Signature of std::transform_reduce (which takes one range) is:

template<class InputIt, class T, class BinaryOp, class UnaryOp>
T transform_reduce(InputIt first, InputIt last,
               T init, BinaryOp binop, UnaryOp unary_op);

So, in your case:

auto max_val =
    std::transform_reduce(start, finish,
                          lambda(*start),
                          [](const auto& lhs, const auto& rhs){ return std::max(lhs, rhs); },
                          lambda);

if legal (but it is not), [](const auto& lhs, const auto& rhs){ return std::max(lhs, rhs); } could be replaced by using lambda_t = decltype(lambda(*start)); static_cast<const lambda_t & (*)(const lambda_t&, const lambda_t&)>(&std::max<lambda_t>) as std::max<T> would be ambiguous (might be max(const T&, const T&) or max(std::initializer_list<T>)), so the additional cast.

But anyway, we are not allowed to take address of most std functions, including std::max.

Jarod42
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  • Are you sure about the last paragraph? Which compiler are you using? – Nikolai Dec 10 '19 at 21:57
  • @Nikolai: I generally use coliru or godbold, but they don't support `std::transform_reduce` :( (even cppreference example)... – Jarod42 Dec 11 '19 at 00:24
  • The substitution suggested in the last paragraph is not legal because std::max is still ambiguous even with the template argument specified. See https://stackoverflow.com/questions/47625376/auto-variable-to-store-function-pointer-to-stdmax – RyanCu Nov 23 '21 at 01:04
  • @RyanCu: Clarified the fact that taking address of `std::max` is illegal, even when selecting right overload (and the way to select right overload). – Jarod42 Nov 23 '21 at 08:41