Span

A Span is a nonowning view to a sequence of contiguous elements.

It can replace the traditional pair of pointer and size arguments passed as array definitions in function calls.

Construction

Span objects can be constructed from a reference to a C++ array, a pointer to the sequence viewed and its size or the range of the sequence viewed:

const uint8_t str[] = "Hello mbed!";

Span<const uint8_t> span_from_array(str);
Span<const uint8_t> span_from_ptr_and_size(str, sizeof(str));
Span<const uint8_t> span_from_range(str, str + sizeof(str));

Operations

You can copy and assign Span objects like regular value types with the help of the copy constructor or the copy assignment (=) operator.

const uint8_t str[] = "Hello mbed!";

Span<uint8_t> str_span(hello_mbed);
Span<uint8_t> copy_constructed_span(str_span);
Span<uint8_t> copy_assigned_span;

copy_assigned_span = str_span;

You can retrieve elements of the object with the subscript ([]) operator. You can access the pointer to the first element of the sequence viewed with data(). The function size() returns the number of elements in the sequence, and empty() informs whether there is any element in the sequence.

void process_unit(uint8_t);

void process(const Span<uint8_t> &data)
{
    if (data.empty()) {
        // nothing to process
        return;
    }

    for (ptrdiff_t i = 0; i < data.size(); ++i) {
        process_unit(data[i]);
    }
}

You can slice Span from the beginning of the sequence (first()), from the end of the sequence (last()) or from an arbitrary point in the sequence (subspan()).

const uint8_t str[] = "Hello mbed!";

Span<uint8_t> str_span(hello_mbed);

ptrdiff_t half_size = str_span.size() / 2;

Span<uint8_t> lower_half = str_span.first(half_size);
Span<uint8_t> upper_half = str_span.last(half_size);
Span<uint8_t> interquartile_range = str_span.subspan(/* offset */ half_size / 2, half_size);

Size encoding

You can encode the size of the sequence in the type itself or in the value of the instance with the help of the template parameter Extent:

• Span<uint8_t, 6>: Span over a sequence of 6 uint8_t.
• Span<uint8_t>: Span over an arbitrary long sequence of uint8_t.

When you encode the size in the type itself, the Span view is guaranteed to be a valid sequence (not empty() and not NULL) - unless Extent equals 0. The type system also prevents automatic conversion from Span of different sizes. Finally, a single pointer internally represents the Span object.

Span<uint8_t> long_span;

// illegal
Span<uint8_t, 6> span_mac_address;
Span<uint8_t, 6> from_long_span(long_span);

// legal
uint8_t mac_address[6] = { };
Span<uint8_t, 6> span_mac_address(mac_address);
long_span = span_mac_address;

When you encode the size of the sequence viewed in the Span value, Span instances can view an empty sequence. The function empty() helps client code decide whether Span is viewing valid content or not.

Span class reference

Span example

template<typename T>
Span<const T> split(Span<const T> &range, const T& separator) {
    const ptrdiff_t out_of_range = range.size();

    ptrdiff_t start;
    for (start = 0; start != out_of_range && range[start] == separator; ++start) { }

    ptrdiff_t last;
    for (last = start; last != out_of_range && range[last] != separator; ++last) { }

    Span<const T> result = range.subspan(start, last - start);
    range = range.subspan(last);
    return result;
}

Span<const char> buffer("Hello World! Hello mbed-os!");
while(buffer.empty() == false) {
    Span<const char> token = split(buffer, ' ');
    printf("token: %.*s\r\n", token.size(), token.data());
}

//------------------------------------------------------------------------------
// Equivalent C-like code

template<typename T>
void split(const T** in_ptr, ptrdiff_t* in_size, const T** token_ptr, ptrdiff_t* token_size, const T& separator) {
    const ptrdiff_t out_of_range = *in_size;

    ptrdiff_t start;
    for (start = 0; start != out_of_range && (*in_ptr)[start] == separator; ++start) { }

    ptrdiff_t last;
    for (last = start; last != out_of_range && (*in_ptr)[last] != separator; ++last) { }

    *token_ptr = *in_ptr + start;
    *token_size = last - start;

    *in_size = *in_size - last;
    *in_ptr = *in_ptr + last;
}

const char* buffer_ptr = str;
ptrdiff_t buffer_size = sizeof(str);
while (buffer_size) {
    const char* token_ptr = NULL;
    ptrdiff_t token_size = 0;
    split(&buffer_ptr, &buffer_size, &token_ptr, &token_size, ' ');
    printf("token: %.*s\r\n", token_size, token_ptr);
}