Creating Views using std::ranges::subrange

Ensuring Lifetime Management in std::ranges::subrange

How do I ensure the lifetime of the container outlives the std::ranges::subrange?

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Ensuring the lifetime of the container outlives the std::ranges::subrange is crucial since std::ranges::subrange does not own the data it views. Here are some best practices and techniques to manage lifetimes effectively.

Stack-Allocated Containers

When working with stack-allocated containers, ensure that the container's scope covers the entire usage of the subrange.

Define the container before the subrange and keep both within the same scope. For example:

#include <iostream>
#include <ranges>
#include <vector>

int main() {
  std::vector<int> Nums{1, 2, 3, 4, 5};
  {
    std::ranges::subrange view{Nums};  
    for (int n : view) {
      std::cout << n << ", ";
    }
  }  // view goes out of scope after Nums
}
1, 2, 3, 4, 5,

Heap-Allocated Containers

When using heap-allocated containers, manage the lifetime using smart pointers such as std::shared_ptr to ensure the container remains valid for the duration of the subrange. For example:

#include <iostream>
#include <memory>
#include <ranges>
#include <vector>

int main() {
  auto Nums = std::make_shared<std::vector<int>>(
    std::initializer_list<int>{1, 2, 3, 4, 5});

  std::ranges::subrange view{
    Nums->begin(), Nums->end()};

  for (int n : view) {
    std::cout << n << ", ";
  }
}
1, 2, 3, 4, 5,

Function Scope and Lifetimes

When passing containers to functions, use references or smart pointers to ensure the lifetime is managed correctly. Avoid returning subranges that reference local variables. For example:

#include <iostream>
#include <memory>
#include <ranges>
#include <vector>

std::ranges::subrange<std::vector<int>::iterator>
  get_subrange(std::shared_ptr<
    std::vector<int>> nums) {
      return std::ranges::subrange{
        nums->begin(), nums->end()};
}

int main() {
  auto Nums = std::make_shared<std::vector<int>>(
      std::initializer_list<int>{1, 2, 3, 4, 5});
  auto view = get_subrange(Nums);

  for (int n : view) {
    std::cout << n << ", ";
  }
}
1, 2, 3, 4, 5,

Avoiding Dangling References

Be cautious of creating subranges from temporary containers, as this can lead to dangling references. The following program is problematic, as it creates a dangling pointer:

#include <iostream>
#include <ranges>
#include <vector>

std::ranges::subrange<std::vector<int>::iterator>
bad_example() {
  std::vector<int> temp{1, 2, 3, 4, 5};
  
  // This is problematic as temp will be
  // deallocated when the function returns
  return std::ranges::subrange{
    temp.begin(), temp.end()
  };
}

int main() {
  auto view = bad_example();
  for (int n : view) {
    std::cout << n << ", "; 
  }
}
error: reference to local variable 'temp' returned

Summary

  • Stack Allocation: Keep containers and subranges within the same scope.
  • Heap Allocation: Use smart pointers to manage container lifetimes.
  • Function Scope: Pass containers by reference or smart pointer.
  • Avoid Dangling References: Never create subranges from temporary containers.

By following these practices, you can ensure that the lifetime of the container outlives the std::ranges::subrange, preventing undefined behavior and potential crashes.

Answers to questions are automatically generated and may not have been reviewed.

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