How Can I Correctly Use Range-Based For Loops in C 11?

How to correctly use the range-based for loop in C 11

The range-based for loop is a concise and powerful Iterator traversal mechanism that has been introduced in C 11 introduced in. It is used to iterate over a container or other iterable sequence and reference elements in the sequence through one or more variables. Here's how to use a range-based for loop correctly:

Syntax

for (auto elem : container) {
  // 代码体
}

Where:

• The auto keyword means that the compiler will automatically infer the element type.
• elem is the iterator variable for the elements in the loop.
• container is the container or iterable sequence to be iterated over.

Element Capture Mode

When using a range-based for loop, you can choose one of the following three element capture modes:

1. Press Value capture (auto elem): Creates a local copy of an element. This works for types whose elements are cheap to copy (e.g. int, double, etc.).
2. Capture by reference (auto&elem): Directly reference the element in the container. This is used for modifying elements or types where copying needs to be avoided.
3. Capture proxy by reference (auto&&elem): For traversing a container using a "proxy iterator" (e.g. std::vector).

Observe or modify

Range-based for loops can be used for two main purposes:

1. Observe elements: Use const auto&elem captures elements to ensure the original element is not modified.
2. Modify element: Use auto&elem to capture the element to modify the original element directly.

Guidelines

To correctly use range-based for For loops, consider the following guidelines:

• For observed elements, use:

for (const auto& elem : container) // 按引用捕获常量

• For modified elements, use:

for (auto& elem : container) // 按引用捕获

• If the container uses "proxy Iterator", use:

for (auto&& elem : container) // 按引用捕获 proxy

Notes in Generic Code

In generic code, it is recommended to use const auto&elem to capture elements for observation, and auto&&elem captures elements for modification, as these methods work with various types of containers and elements.

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