On Tue, 15 Jul 2025 at 15:12, Avi Kivity via Std-Proposals

<std-proposals@lists.isocpp.org> wrote:

Previous thread: https://lists.isocpp.org/std-proposals/2024/12/11641.php

git: https://github.com/avikivity/cpp-std-proposals/blob/main/std-construct.md

Proposal for std::construct Function Object

I. Motivation

C++ users can convert member functions to funtion objects with std::mem_fn(), partially bind arguments to functions or function objects with std::bind, std::bind_front(), and std::bind_back(), type-erase them with std::function<>, and use them to transform ranges with std::views::transform. But none of that can be done directly to class constructors; a helper function must be used to "downgrade" the constructor into a function.

The proposed std::construct<> is a utility function object that provides a convenient, generic mechanism to convert a constructor overload set into a function object, thereby allowing all the existing tooling for function objects to be brought to bear on it.

II. Example Problem

Imagine you have a range of size_t and you wish to return a vector of vectors, with the sizes given from the given range. Naive code can look like:

    std::vector<std::vector<int>> result;

    result.reserve(std::distance(input));

    for (auto sz : input) {

         result.emplace_back(sz);

    }

However, this is unsatisfying. The input range may be an input_range, which does not afford two passes (one for std::distance, one for the for loop). The emplace_back loop is less efficient than constructing the vector from a range.

A modern range-based solution would look like

   auto result = input

       | std::views::transform([] (size_t sz) {

           return std::vector<int>(sz);

       }

       | std::ranges::to<std::vector>();

This is still unsatisfying, as the lambda is not concise.

I don't quite follow what this means. The lambda is not concise? So what?

Perhaps show what the solution looks like with std::construct applied?

    // Updated example from above
    auto result = input
        | std::views::transform(std::construct<std::vector<int>>)
        | std::ranges::to<std::vector>();