How Can I Make My Custom C Types Iterable Using Range-Based For Loops?

Iteration Simplification with Custom Types

In C 11, the introduction of range-based for loops has simplified the process of iterating over sequences. These loops follow a standardized pattern:

for (Type& v : a) { ... } // equivalent to traditional loop below
for (auto iv = begin(a); iv != end(a); ++iv) {
  Type& v = *iv;
  ...
}

You can extend this functionality to your custom types, allowing them to be used in range-based loops, by following these guidelines:

Approach 1: Member Function Declaration

• Define begin() and end() member functions in your type X that return objects with iterator-like behavior.

Approach 2: Free Function Invocation

• Create free functions begin(X&) and end(X&) in the same namespace as your type X, again returning iterator-like objects.

These functions will provide the necessary initialization (begin()) and termination (end()) points for the range-based loop.

Function Return Value Requirements

The objects returned by begin() and end() do not need to be actual iterators. They must, however, meet the following requirements:

• Overload pre- to increment the object
• Ensure valid initialization expressions
• Support binary != for boolean context
• Implement unary * to return a value assignable to the loop variable
• Provide a public destructor

Additional Considerations

• If your custom type belongs to the xml namespace, define xml::begin() to avoid potential conflicts with std::begin().
• Future C revisions may decouple the types of begin() and end(), allowing for "lazy-end" evaluation.
• C 17 introduced a new type, null_sentinal_t, which facilitates range-based loops over null-terminated character buffers.
• The range-based loop uses temporary variables stored in auto&& references, which means your begin() and end() overloads cannot detect if they are operating on temporaries.

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