How Do I Effectively Use Range-Based For Loops in C 11?

Range-based for Loops in C 11

Syntax and Usage

C 11 introduced range-based for loops as a concise and expressive way to iterate over containers. The syntax is:

for (range_declaration : container) {
  // body of the loop
}

where:

• range_declaration declares a variable to hold each element of the container.
• container is the container you want to iterate over.

Capturing by Reference or Value

The variable in the range_declaration can be captured by reference or value using the following syntax:

• auto captures the element by value.
• auto& captures the element by non-const reference.
• const auto& captures the element by const reference.

For observing the elements, where you don't need to modify them, you should use const auto&. This prevents unnecessary copies and ensures that the original elements are not modified.

If you want to modify the elements, use auto&.

Note that for containers with proxy iterators (like std::vector), you need to use auto&& to capture the elements by value.

Guidelines for Using Range-based For

Consider the following guidelines when using range-based for:

• For observing elements:

  • for (const auto& elem : container) (capture by const reference)
  • If elements are cheap to copy (e.g., ints), you can use for (auto elem : container) (capture by value).

• For modifying elements:

  • for (auto& elem : container) (capture by non-const reference)
  • For proxy iterators (e.g., std::vector), use for (auto&& elem : container) (capture by &&).

Generic Code Considerations

In generic code, where you don't know the type of the elements being iterated over, use:

• for (const auto& elem : container) for observing elements.
• for (auto&& elem : container) for modifying elements (works for both regular and proxy iterators).

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