Modern C++ offers a lightweight, non-owning view into contiguous sequences called std::span. Introduced in C++20, std::span provides a safer and more expressive alternative to raw pointers and manual size handling. In this article, we explore why std::span is valuable, how to use it in practice, and a few common pitfalls to avoid.
1. What is std::span?
`std::span
` is essentially a pair of a pointer and a length that references a contiguous block of memory without taking ownership. It is template‑parameterized on the element type `T`, and it can be constructed from: – Raw pointers and length: `std::span s(ptr, n);` – C-style arrays: `int arr[5]; std::span s(arr);` – `std::array`, `std::vector`, `std::string`, and any other container with contiguous storage: `std::span s(vec);` – Another `std::span`: `std::span sub(s.subspan(2, 3));` Because it holds no ownership, `std::span` is cheap to copy and can be passed around like any other value type. ### 2. Why use std::span? 1. **Safety** – `std::span` carries size information, eliminating the risk of out‑of‑bounds access that plagues raw pointers. 2. **Expressiveness** – Function signatures that accept a contiguous block become clearer: `void process(std::span data)`. 3. **Interoperability** – It can seamlessly accept containers and raw arrays, making it a flexible bridge between legacy APIs and modern code. 4. **Performance** – Being a lightweight view, there is no additional allocation; it typically incurs no runtime cost beyond the pointer and size fields. ### 3. Example: Implementing a Generic Sum Function “`cpp #include #include #include #include template T sum(std::span values) { return std::accumulate(values.begin(), values.end(), T{}); } int main() { std::vector vec{1, 2, 3, 4, 5}; int arr[] = {10, 20, 30}; std::cout << "Sum of vector: " << sum(vec) << '\n'; std::cout << "Sum of array: " << sum(arr) << '\n'; // Subspan example std::span sub = std::span(vec).subspan(1, 3); // elements 2,3,4 std::cout << "Sum of subspan: " << sum(sub) << '\n'; } “` Output: “` Sum of vector: 15 Sum of array: 60 Sum of subspan: 9 “` ### 4. Working with Mutable Data `std::span` can be mutable or const. A mutable span allows element modification: “`cpp void double_values(std::span values) { for (int& v : values) v *= 2; } int main() { std::array data{1, 2, 3, 4}; double_values(data); // data now contains {2, 4, 6, 8} } “` ### 5. Common Pitfalls – **Dangling Spans** – Never keep a span after the underlying container is destroyed or resized beyond its original capacity. The span does not own the data. “`cpp std::span s; { std::vector temp{1, 2, 3}; s = temp; // OK } // temp is destroyed; s is now dangling “` – **Aliasing with Containers** – If you pass a span to a function that stores it beyond the scope of the caller, ensure the container outlives the span. – **Misusing Subspan** – `subspan(offset, count)` counts elements from the offset; an incorrect count can silently create an empty or partially overlapping view. ### 6. Advanced Usage: Constexpr and Compile‑Time Span `std::span` is `constexpr`‑friendly, enabling compile‑time manipulation of fixed arrays: “`cpp constexpr std::array nums{10, 20, 30, 40, 50}; constexpr std::span whole(nums); constexpr auto slice = whole.subspan(2); // {30, 40, 50} static_assert(slice.size() == 3); “` ### 7. When Not to Use std::span – **Non‑contiguous Data** – `std::span` is unsuitable for sparse or linked structures. – **Ownership Semantics** – If you need ownership transfer or deep copies, consider smart pointers or containers instead. – **Performance Critical Loops** – In extremely hot paths where even the two-word span might be a concern, manual pointers can sometimes be more efficient; however, profiling is essential. ### 8. Summary `std::span` brings a level of safety and clarity to C++ code by representing contiguous memory blocks in a self‑describing, lightweight form. It is ideal for generic algorithms, interoperation between APIs, and improving function signatures. While it is not a silver bullet—care must still be taken to avoid dangling references—the benefits in expressiveness and safety make `std::span` a staple in modern C++20 and beyond. Happy spanning!