Exploring C++ std::span – Part 5: Performance and Practicality

Throughout this series, we’ve explored the capabilities and best practices for using std::span. We began by understanding its role as a lightweight, non-owning view over contiguous data and its use cases. We then delved into element access, iterators, sub-view creation, and the importance of const-correctness and type safety. Along the way, we’ve seen how std::span simplifies function interfaces, enhances code clarity, and promotes safe data handling.

You can review the previous articles first if you need to catch up your knowledge:

• Exploring C++ std::span – Part 1: An Introduction To the Non-Owning View
• Exploring C++ std::span – Part 2: Accessing Elements and Iterating
• Exploring C++ std::span – Part 3: Subviews and Slices
• Exploring C++ std::span – Part 4: Const-Correctness and Type-Safety

Now, in this final article, we’ll turn our attention to the performance characteristics and limitations of std::span. As a zero-cost abstraction, std::span offers significant efficiency benefits compared to alternatives like copying arrays or vectors. However, it’s essential to understand its constraints, such as its requirement for contiguous memory and its non-owning nature, which imposes strict lifetime considerations.

Finally, we’ll recap best practices to help you integrate std::span effectively into your production code. By the end of this article, you’ll have a comprehensive understanding of not just when and how to use std::span, but also its boundaries and trade-offs, empowering you to make informed decisions in your C++ projects.

Performance Benefits of std::span

One of the defining characteristics of std::span is its efficiency. As a zero-cost abstraction, std::span offers significant performance benefits by providing a view into existing data without the overhead of ownership, allocation, or copying. This design makes it ideal for scenarios where performance is critical, such as high-frequency function calls or operations on large datasets.

Why is std::span Lightweight?

Unlike containers such as std::vector or std::array, std::span does not manage memory. Instead, it simply stores a pointer to the beginning of the data and a size, making it comparable in size and performance to passing a raw pointer and an integer.

Key Characteristics:

1. Non-Owning Nature:
   • std::span does not allocate memory or manage the lifetime of its data.
   • It avoids the overhead associated with copying or reallocating data.
2. Compact Representation:
   • Internally, std::span is typically implemented as a pair: a pointer and a size.
   • This minimal structure makes it highly efficient to create, copy, and pass by value.

Example: Comparing Memory Usage of std::span vs. std::vector

#include <span>
#include <vector>
#include <iostream>

void printSpan(std::span<int> sp) {
    for (int value : sp) {
        std::cout << value << " ";
    }
    std::cout << "\n";
}

void printVector(const std::vector<int>& vec) {
    for (int value : vec) {
        std::cout << value << " ";
    }
    std::cout << "\n";
}

int main() {
    int arr[] = {1, 2, 3, 4, 5};
    std::span<int> mySpan(arr);  // Lightweight, no data copy
    std::vector<int> myVector(arr, arr + 5);  // Allocates and copies data

    printSpan(mySpan);
    printVector(myVector);

    return 0;
}

View on godbolt.org

Explanation: In this example, std::span avoids creating a copy of the data, whereas std::vector allocates heap memory and copies the array elements into it. This difference makes std::span more efficient for non-modifying operations on existing data.​

Zero-Cost Abstraction in Practice

The term “zero-cost abstraction” refers to the idea that std::span provides a high-level interface without incurring additional runtime costs compared to manual pointer manipulation. This efficiency is achieved through:

1. Efficient Copying: Passing a std::span by value involves copying a pointer and a size, which is as fast as copying two integers.
2. Compatibility with Iterators: Iterating over a std::span has the same performance as iterating over a raw array or a pointer range.

Example: Iteration Efficiency

#include <span>
#include <iostream>

void processSpan(std::span<int> sp) {
    for (int& value : sp) {
        value *= 2;  // Modify in-place
    }
}

int main() {
    int data[] = {1, 2, 3, 4, 5};

    std::span<int> mySpan(data);
    processSpan(mySpan);

    for (int value : data) {
        std::cout << value << " ";  // Output: 2 4 6 8 10
    }

    return 0;
}

View on godbolt.org

Explanation: Iteration over std::span involves no additional overhead compared to directly iterating over the array. Since std::span references the original data, changes made through the span are immediately reflected in the source​.

When to Use std::span for Performance Gains

Ideal Scenarios:

• Function Parameters:
  • When you need to pass data to functions without copying or reallocating it.
  • Works seamlessly with arrays, std::vector, or std::array.
• Lightweight Views for Large Datasets:
  • Provides an efficient way to process specific portions of large datasets (e.g., with first(), last(), or subspan()).
• Performance-Critical Loops:
  • Allows efficient access to contiguous memory in high-frequency loops or real-time applications.

Notable Advantages:

• Avoiding Data Copies: Reduces memory overhead by reusing existing data.
• Minimal Overhead: Equivalent in cost to raw pointers, but much safer and more readable.
• Cache-Friendly: Maintains the same cache-friendly characteristics as the underlying data source.

Summary of Performance Benefits

By leveraging the lightweight and efficient design of std::span, you can achieve significant performance benefits in scenarios that require working with contiguous data. In the next section, we’ll explore the limitations of std::span and the trade-offs to consider when deciding whether it’s the right tool for your application.

Limitations of std::span

While std::span offers many benefits as a lightweight and flexible view over contiguous data, it also comes with specific limitations that developers must understand to use it effectively. These constraints stem from its non-owning nature and reliance on contiguous memory, which make it unsuitable for certain scenarios. In this section, we’ll explore the key limitations of std::span and provide guidance on when to consider alternative approaches.

std::span Requires Contiguous Memory

std::span is designed to work only with contiguous memory, such as arrays, std::vector, or std::array. It cannot represent data stored in non-contiguous containers like std::list or std::map, nor can it work with dynamically resized or fragmented memory structures.

Example: Non-Supported Containers

#include <list>
#include <span>
#include <iostream>

int main() {
    std::list<int> myList = {1, 2, 3, 4, 5};

    // This will not work, as std::span requires contiguous memory
    // std::span<int> invalidSpan(myList.begin(), myList.size()); // Error

    return 0;
}

Solution: Use std::span only with containers or data types that guarantee contiguous memory. If working with non-contiguous containers, consider alternatives like iterators or range views.

std::span Does Not Own Data

Since std::span is a non-owning view, it does not manage the lifetime of the underlying data. This means the developer is responsible for ensuring that the data remains valid while the span is in use. If the data is deallocated or goes out of scope, the span becomes invalid and may lead to undefined behavior.

Example: Lifetime Issues

std::span<int> danglingSpan;

void unsafeFunction() {
    int tempData[] = {1, 2, 3};
    danglingSpan = tempData;  // tempData will be destroyed at the end of this function
}

int main() {
    unsafeFunction();
    // Accessing danglingSpan here is undefined behavior
    return 0;
}

Solution: Ensure that the data referenced by a span outlives the span itself. Use heap-allocated or global data if necessary, and avoid assigning spans to temporary or local variables that may go out of scope.

Limited Compatibility with Dynamically Resized Container

std::span represents a fixed range of data and does not automatically adapt to changes in the size of the underlying container. For example, if you create a span from a std::vector and then modify the vector’s size, the span does not update to reflect the change.

Example: Mismatched Size After Resizing

#include <vector>
#include <span>
#include <iostream>

int main() {
    std::vector<int> vec = {1, 2, 3};
    std::span<int> mySpan(vec);

    // Resizing the vector
    vec.push_back(4);  // mySpan is now inconsistent with vec

    // Accessing beyond mySpan's original size is undefined behavior
    std::cout << mySpan[3] << "\n";  // Undefined behavior
    return 0;
}

Solution: Avoid resizing containers that are referenced by a span. If resizing is required, recreate the span after modifying the container.

Scope and Lifetime Considerations

The temporary nature of spans means they are best suited for short-term operations within limited scopes, such as within a function. Using spans in long-lived objects or across asynchronous operations can lead to unintended behavior if the underlying data is modified or destroyed.

Example: Using std::span Safely Within a Scope

#include <span>
#include <iostream>

void processSpan(std::span<int> sp) {
    for (int value : sp) {
        std::cout << value << " ";
    }
    std::cout << "\n";
}

int main() {
    int data[] = {1, 2, 3, 4, 5};
    processSpan(data);  // Safe: span's lifetime is within the scope of processSpan

    return 0;
}

Best Practice: Limit the use of spans to short-lived operations within well-defined scopes. For longer-lived data handling, consider using containers with ownership semantics, such as std::vector or std::shared_ptr.

Summary of Limitations

Understanding these limitations ensures that you use std::span effectively while avoiding pitfalls. By complementing std::span with other tools and techniques, you can design robust solutions that balance performance, safety, and flexibility. In the next section, we’ll summarize best practices for using std::span and provide actionable tips for integrating it into your production code.

std::span Best Practices Recap

std::span is a versatile and efficient tool for accessing contiguous data safely and effectively. However, to fully leverage its benefits while avoiding potential pitfalls, it’s important to adhere to best practices. This section summarizes key takeaways and practical guidelines for integrating std::span into your production code.

Use std::span for Readable and Flexible Function Interfaces

By using std::span in function parameters, you can simplify your APIs while ensuring type safety and clarity. This approach works seamlessly with raw arrays, std::vector, and std::array, reducing the need for multiple overloads or explicit pointer manipulation.

Example: Simplified Function Interface

#include <span>
#include <vector>
#include <iostream>

void processData(std::span<const int> data) {
    for (int value : data) {
        std::cout << value << " ";
    }
    std::cout << "\n";
}

int main() {
    int arr[] = {1, 2, 3};
    std::vector<int> vec = {4, 5, 6};

    processData(arr);  // Works with raw arrays
    processData(vec);  // Works with std::vector

    return 0;
}

View on godbolt.org

Best Practice: Use std::span<const T> for read-only views and std::span<T> for mutable views to communicate function intent clearly.

Ensure the Lifetime of Underlying Data

Because std::span does not manage memory, the data it references must remain valid for the span’s lifetime. Avoid using spans with temporary or short-lived data.

Example: Valid Data Lifetime

void safeUsage() {
    int permanentData[] = {10, 20, 30};
    std::span<int> safeSpan(permanentData);

    for (int value : safeSpan) {
        std::cout << value << " ";
    }
}

Best Practice: Use spans only with data that has a well-defined lifetime, such as arrays, std::vector, or heap-allocated memory.

Limit Span Usage to Contiguous Data

std::span requires contiguous memory, making it unsuitable for non-contiguous containers like std::list or fragmented memory. Use it exclusively with arrays, std::vector, or std::array.

Example: Correct Use with Contiguous Memory

#include <vector>
#include <span>

void processSpan(std::span<int> sp) {
    for (int& value : sp) {
        value *= 2;
    }
}

int main() {
    std::vector<int> vec = {1, 2, 3};
    processSpan(vec);  // Works with std::vector

    return 0;
}

Best Practice: Avoid attempting to use std::span with non-contiguous containers. Instead, rely on iterators or range-based loops for such cases.

Use Spans for Short-Lived, Local Operations

Spans are best suited for temporary operations within a well-defined scope, such as passing data to functions or performing local computations. For long-term storage or complex ownership scenarios, use containers like std::vector or smart pointers.

Example: Local Scope for Spans

#include <span>
#include <iostream>

void computeSum(std::span<const int> sp) {
    int sum = 0;
    for (int value : sp) {
        sum += value;
    }
    std::cout << "Sum: " << sum << "\n";
}

int main() {
    int data[] = {5, 10, 15};
    computeSum(data);  // Safe local operation

    return 0;
}

Best Practice: Limit span usage to temporary operations. Avoid retaining spans across asynchronous tasks or long-term object lifetimes.

Embrace Bounds-Checked Methods for Safety

While std::span provides fast, unchecked access with operator[], its bounds-checked methods like at() are safer for situations involving user input or uncertain data sizes.

at() for std::span is a C++26 feature that has already been implemented in some compilers as of this writing (2024).

Example: Safe Access with at()

#include <span>
#include <iostream>

void accessElement(std::span<int> sp, std::size_t index) {
    if (index < sp.size()) {
        std::cout << "Element at " << index << ": " << sp.at(index) << "\n";
    } else {
        std::cout << "Index out of bounds\n";
    }
}

int main() {
    int data[] = {1, 2, 3};
    std::span<int> span(data);

    accessElement(span, 1);  // Safe access
    accessElement(span, 5);  // Out of bounds

    return 0;
}

View on godbolt.org

Best Practice: Use at() or perform manual bounds-checking for safer access in applications where indices may be unpredictable.

Final Takeaways

By following these best practices, you can maximize the benefits of std::span while avoiding its pitfalls. This approach ensures your code remains efficient, safe, and maintainable.

In this series, we’ve explored how to use std::span effectively, from basic concepts to advanced manipulation, const-correctness, and performance considerations. With these tools, you’re well-equipped to make std::span an integral part of your modern C++ projects.

Check out all of the articles in this 5-part series:

• Exploring C++ std::span – Part 1: An Introduction To the Non-Owning View
• Exploring C++ std::span – Part 2: Accessing Elements and Iterating
• Exploring C++ std::span – Part 3: Subviews and Slices
• Exploring C++ std::span – Part 4: Const-Correctness and Type-Safety
• Exploring C++ std::span – Part 5: Performance and Practicality

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