Using std::apply on std::expected in C++23

Using std::apply on std::expected in C++23
Using std::apply on std::expected in C++23
Lina Fontaine
Monday, July 22, 2024 at 10:21:46 AM

Streamlining Error Handling in C++23

Today's C++ development requires effective error handling and return value management. Working with functions that return {std::expected} types usually involves a lot of checks and error handling code, which can complicate the logic and make it more difficult to maintain.

This research looks into the usage of a more complex and general method to ease error handling. To decrease boilerplate code and increase readability, we'll consider creating a'magic_apply' method that aggregates the results of several {std::expected} values and delivers them to another function.

Command Description
std::expected A C++ error handling template type capable of storing both values and errors.
std::unexpected When used with std::expected, it indicates an unexpected error value.
template<typename...> Outline a variadic template that accepts a limitless number of template arguments.
decltype Used in template programming, specifically to determine the type of expression.
args.value() If a std::expected object contains a value, access it.
args.has_value() Determines whether a value is present in a std::expected object.
(... && args.has_value()) Fold the expression to see if it contains values for all std::expected objects.
func(args.value()...) To invoke the method func, use the values of the std::expected objects.
return unexpected<Err>(args.error()...) Returns an unexpected error that includes the errors from the std::expected objects.

Effective Error Management with Variable Templates

C++23 scripts use the std::expected type to simplify error handling. The primary purpose is to create a generic function named magic_apply that can send the output of multiple std::expected values to another function. This reduces the need for lengthy error-checking when dealing with a large number of std::expected values. Using variadic templates, magic_apply can accept any number of std::expected inputs, making it extremely versatile. Before invoking the function with the contents of any std::expected object, the underlying logic of magic_apply leverages a fold expression, (... && args.has_value()), to ensure all std::expected have valid values.

The first script example demonstrates this principle with simple types like int and double. It defines a compute_all function that performs fundamental computations, as well as getA and getB functions that return std::expected types. If both values from getA and getB are valid, we can call compute_all with magic_apply; otherwise, the error is propagated. By minimizing boilerplate code, this solution improves readability and maintainability. The second script presents a similar idea, but emphasizes the approach's adaptability by using string kinds and lambda functions.

Using'std::expected' to reduce error handling complexity in C++.

C++23 Script with Variadic Templates

#include <expected>
#include <string>
#include <iostream>
#include <tuple>

using namespace std;

template<typename Func, typename... Args, typename Err>
auto magic_apply(Func func, const expected<Args, Err>&... args) -> expected<decltype(func(args.value()...)), Err> {
    if ((... && args.has_value())) {
        return func(args.value()...);
    } else {
        return unexpected<Err>(args.error()...);
    }
}

expected<int, string> getA(int x) {
    if (x > 0) return x;
    return unexpected<string>("Error in getA");
}

expected<double, string> getB(double y) {
    if (y > 0) return y;
    return unexpected<string>("Error in getB");
}

double compute_all(int a, double b) {
    return a + b;
}

int main() {
    auto result = magic_apply(compute_all, getA(10), getB(20.5));
    if (result) {
        cout << "Result: " << result.value() << endl;
    } else {
        cout << "Error: " << result.error() << endl;
    }
    return 0;
}

Combining Different {std::expected} Results: C++23 values

C++23 Script using Lambda Functions

#include <expected>
#include <string>
#include <iostream>

using namespace std;

template<typename Func, typename... Args, typename Err>
auto magic_apply(Func func, const expected<Args, Err>&... args) -> expected<decltype(func(args.value()...)), Err> {
    bool all_valid = (args.has_value() && ...);
    if (all_valid) {
        return func(args.value()...);
    } else {
        return unexpected<Err>(args.error()...);
    }
}

expected<string, string> getA(bool flag) {
    if (flag) return "SuccessA";
    return unexpected<string>("Failed A");
}

expected<string, string> getB(bool flag) {
    if (flag) return "SuccessB";
    return unexpected<string>("Failed B");
}

string compute_all(const string& a, const string& b) {
    return a + " and " + b;
}

int main() {
    auto result = magic_apply(compute_all, getA(true), getB(true));
    if (result) {
        cout << "Result: " << result.value() << endl;
    } else {
        cout << "Error: " << result.error() << endl;
    }
    return 0;
}

Improving C++ Error Handling using Variadic Templates.

Another significant advantage of using std::expected in C++ is its ability to improve error handling in complex systems. When many asynchronous actions produce std::expected types, it's crucial to seamlessly combine their outcomes. In addition to simplifying the code, this solution ensures robust error handling. To develop more versatile and generic functions, combine an arbitrary number of std::expected values with variadic templates.

magic_apply is versatile enough to be used with functions that accept various types of arguments. The implementation is further simplified by using decltype, which automatically deduces the return type of the combined function call. Furthermore, this technique can be improved to handle more sophisticated jobs, such as merging std::expected values with other error types or modifying the values before delivering them to the function. Because of its versatility, the pattern can be applied to a variety of jobs, from simple computations to complicated processes.

Frequently Asked Questions regarding Variadic Templates and std::expected

  1. What is std::expected?
  2. It is a C++ template type that may store either an error or a valid value and is used for error handling.
  3. How does magic_apply work?
  4. It removes the need for repeated error checks by combining the results of many std::expected values and passing them to a function.
  5. What are variadic templates?
  6. Variable templates provide a great deal of flexibility in function design by allowing functions to accept any number of parameters.
  7. Why do we use decltype in magic_apply?
  8. The values of the std::expected objects are used to automatically determine the return type of the function called.
  9. Is magic_apply able to handle different types of errors?
  10. Yes, it can be modified to function with std::expected values having various error types with a few adjustments.
  11. What benefits does using std::expected provide?
  12. When managing errors, it provides a more expressive and cleaner method than more traditional solutions such as exceptions or return codes.
  13. Is std::unexpected associated with std::expected?
  14. In addition to std::expected, std::unexpected is an invalid value.
  15. Can asynchronous actions work with magic_apply?
  16. It is able to handle std::expected values returned by asynchronous operations.
  17. What is a fold expression?
  18. In this example, the fold expression feature in C++ is used to check if all std::expected objects have valid values in a simple manner.

Wrapping Up:

In C++23, implementing a generic function to handle multiple std::expected values significantly improves code readability and simplifies error handling. The magic_apply function saves boilerplate code and improves maintainability by utilizing variadic templates to ensure that all expected values are valid before processing. This method provides a versatile solution that may be used in a variety of circumstances, as well as a cleaner, more efficient way to manage failures in modern C++ programming.