What Is The Best Approach For Using Std::Ranges/Std::Views With Std::Expected In C++23? Top 4 Posts With The Most Views

In C++23, there is no built-in support for using std::ranges or std::views with std::expected directly. However, you can still use these features together by creating your own range or view adaptor.

Here is an example of a simple range adaptor that converts a range of std::expected objects into a range of the contained values:

cpp
#include <ranges>
#include <utility>
#include <type_traits>
#include <concepts>

template<typename R>
class expected_range {
public:
    explicit expected_range(R&& range) : range_(std::forward<R>(range)) {}

    auto begin() const {
        using value_type = typename std::remove_cvref_t<decltype(*range_.begin())>::value_type::value_type;
        return std::ranges::transform_view{range_, [](auto&& expected) -> value_type& { return *expected; }};
    }

    auto end() const {
        return range_.end();
    }

private:
    R range_;
};

template<typename R>
expected_range(R&&) -> expected_range<R>;

template<typename R>
expected_range(R&&) -> expected_range<std::views::all_t<R>>;

This range adaptor takes a range of std::expected objects and returns a range of the contained values. It does this by using std::ranges::transform_view to apply a lambda function that dereferences the std::expected objects.

Here’s an example of how you might use this adaptor with std::views::filter to create a filtered range of only the successful std::expected objects:

cpp
#include <iostream>
#include <vector>
#include <expected.hpp>

int main() {
    using namespace std::views;
    std::vector<std::expected<int, std::string>> vec = {1, std::unexpected("foo"), 2, std::unexpected("bar"), 3};

    auto filtered_range = vec | expected_range | filter([](auto&& expected) { return expected.has_value(); });
    for (auto value : filtered_range) {
        std::cout << value << " ";
    }
    // Output: 1 2 3
}

In this example, the expected_range adaptor is used to convert the std::vector of std::expected objects into a range of the contained values. Then, std::views::filter is used to create a filtered range that only contains the successful std::expected objects. Finally, a range-based for loop is used to iterate over the filtered range and print out the values.

Note that this implementation assumes that all the std::expected objects in the range have the same value type. If your use case involves ranges of std::expected objects with different value types, you will need to modify the implementation to handle this case.

I’m sorry, but I’m not sure what you mean by “STD ranges range”. Could you please provide more context or clarify your question? I’ll do my best to provide a helpful answer once I understand what you’re asking.

As of my knowledge cutoff date of September 2021, C++23 was still in development and its feature set had not been fully finalized. However, here are some of the proposals that were being considered for inclusion in the standard:

1. Modules: This is a major feature that has been under development for many years. Modules provide a better alternative to the current include system and allow for faster builds, better encapsulation, and improved compile-time error checking.

2. Coroutines: Coroutines are a way to write asynchronous code that is easier to read and understand. They allow you to write code that can be paused and resumed, making it easier to write non-blocking code.

3. Ranges: Ranges provide a more modern and composable way to work with sequences of data, replacing the current iterator-based approach.

4. Contracts: Contracts provide a way to specify pre- and post-conditions for functions, helping to catch bugs earlier in the development process.

5. Reflection: Reflection allows you to inspect and manipulate the structure of your code at runtime, making it easier to write generic algorithms and tools.

6. SIMD: SIMD (Single Instruction, Multiple Data) provides a way to perform parallel operations on data, improving performance on certain types of computations.

It is important to note that these features are still under development and are subject to change before the final release of C++23.

As of my knowledge cutoff in September 2021, the C++23 standard was still being developed and finalized. However, here are some proposed changes that were under consideration at that time:

1. Modules: C++23 is expected to introduce support for modules, which are a new way to organize code and manage dependencies. This feature will allow for faster compilation times, improved code organization, and more efficient builds.

2. Coroutines: C++20 introduced coroutines as an experimental feature, and C++23 is expected to make them a core language feature. Coroutines allow for more efficient and readable asynchronous code by providing a way to suspend and resume function execution.

3. Ranges: C++20 introduced ranges as a library feature, and C++23 is expected to extend and improve this feature. Ranges provide a way to work with sequences of elements, and they can simplify many common algorithms and data processing tasks.

4. Contracts: C++20 introduced support for contracts as an experimental feature, and C++23 is expected to refine and finalize this feature. Contracts provide a way to specify preconditions, postconditions, and invariants for functions and classes, which can help to improve code reliability and maintainability.

5. Improved constexpr: C++23 is expected to introduce improvements to the constexpr feature, which allows for compile-time evaluation of functions and expressions. These improvements will likely include more relaxed constraints on what can be evaluated at compile time, as well as better error messages when constexpr code is invalid.

It’s worth noting that these are just some of the proposed changes for C++23, and the final standard may include additional or different features.