What are C Spans and When Should You Use Them?

Understanding the Concept of a Span

In the realm of C , a span is a unique and lightweight abstraction that represents a contiguous sequence of values stored in memory. Essentially, it is akin to a struct containing two essential members: a pointer to the first element (ptr) and the length of the sequence (length).

Unlike traditional C-style arrays, a span provides enhanced functionality while inheriting the structural simplicity of a pointer-based approach. It is crucial to note that a span does not acquire or manage the memory it references; rather, it acts as a "borrowed view" of that memory.

When to Utilize a Span

The use of spans is particularly beneficial in situations where both the pointer and length information are relevant. Consider the following scenario:

void read_into(int* buffer, size_t buffer_size);

This function prototype expects a pointer to an integer array (buffer) and the size of that array (buffer_size) as input. Using a span, this function call can be simplified and made more concise:

void read_into(span buffer);

By utilizing a span, we can effectively convey both the pointer and length information required by the function.

Advantages of Employing Spans

The implementation of spans brings forth an array of compelling advantages:

• Contingency: Spans empower developers to work with a pointer and length combination as effortlessly as with standard library containers, unlocking various operations:

• for (auto& x : my_span) { / do stuff / }
• std::find_if(my_span.cbegin(), my_span.cend(), some_predicate);
• std::ranges::find_if(my_span, some_predicate); (in C 20)

• Compiler Assistance: Spans enable the compiler to perform additional tasks, such as simplifying function calls while preserving desired functionality, as seen in the following example:

int buffer[BUFFER_SIZE];
read_into(buffer, BUFFER_SIZE);

becomes:

int buffer[BUFFER_SIZE];
read_into(buffer);

• Recommended Alternative: Spans present a viable alternative to passing constant vector references (const vector&) when contiguous memory storage is anticipated, thereby avoiding critiques from knowledgeable C veterans.
• Static Analysis Assistance: Spans enhance static code analysis capabilities, assisting the compiler in identifying potential errors and ensuring code robustness.
• Runtime Bounds Checking: Debug builds of spans can incorporate runtime bounds checking, offering safeguards against accessing memory beyond the designated range.
• Ownership Indication: Spans convey that the code using them does not have ownership of the pointed-to memory, providing crucial information to developers and analyzers.

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