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Using the Guidelines Support Library (GSL): A Tutorial and FAQ

by Herb Sutter

updated 2018-01-08

Overview: "Is this document a tutorial or a FAQ?"

It aims to be both:

a tutorial you can read in order, following a similar style as the introduction of K&R by building up examples of increasing complexity; and
a FAQ you can use as a reference, with each section showing the answer to a specific question.

Motivation: "Why would I use GSL, and where can I get it?"

First look at the C++ Core Guidelines; this is a support library for that document. Select a set of guidelines you want to adopt, then bring in the GSL as directed by those guidelines.

You can try out the examples in this document on all major compilers and platforms using this GSL reference implementation.

gsl::span: "What is gsl::span, and what is it for?"

gsl::span is a replacement for (pointer, length) pairs to refer to a sequence of contiguous objects. It can be thought of as a pointer to an array, but that knows its bounds.

For example, a span<int,7> refers to a sequence of seven contiguous integers.

A span does not own the elements it points to. It is not a container like an array or a vector, it is a view into the contents of such a container.

span parameters: "How should I choose between span and traditional (ptr, length) parameters?"

In new code, prefer the bounds-checkable span<T> instead of separate pointer and length parameters. In older code, adopt span where reasonable as you maintain the code.

A function that takes a pointer to an array and a separate length, such as:

// Error-prone: Process n contiguous ints starting at *p
void dangerous_process_ints(const int* p, size_t n);

is error-prone and difficult to use correctly:

int a[100];
dangerous_process_ints(a, 1000); // oops: buffer overflow

vector<int> v(200);
dangerous_process_ints(v.data(), 1000); // oops: buffer overflow

auto remainder = find(v.begin(), v.end(), some_value);
    // now call dangerous_process_ints() to fill the rest of the container from *remainder to the end
dangerous_process_ints(&*remainder, v.end() - remainder); // correct but convoluted

Instead, using span encapsulates the pointer and the length:

// BETTER: Read s.size() contiguous ints starting at s[0]
void process_ints(span<const int> s);

which makes process_ints easier to use correctly because it conveniently deduces from common types:

int a[100];
process_ints(a); // deduces correct length: 100 (constructs the span from a container)

vector<int> v(200);
process_ints(v); // deduces correct length: 200 (constructs the span from a container)

and conveniently supports modern C++ argument initialization when the calling code does have distinct pointer and length arguments:

auto remainder = find(v.begin(), v.end(), some_value);
    // now call process_ints() to fill the rest of the container from *remainder to the end
process_ints({remainder, v.end()}); // correct and clear (constructs the span from an iterator pair)

Things to remember

Prefer span instead of (pointer, length) pairs.

Pass a span like a pointer (i.e., by value for "in" parameters). Treat it like a pointer range.

span and const: "What's the difference between `span<const T>` and `const span<T>`?"

span<const T> means that the T objects are read-only. Prefer this by default, especially as a parameter, if you don't need to modify the Ts.

const span<T> means that the span itself can't be made to point at a different target.

const span<const T> means both.

Things to remember

Prefer a span<const T> by default to denote that the contents are read-only, unless you do need read-write access.

Iteration: "How do I iterate over a span?"

A span is an encapsulated range, and so can be visited using a range-based for loop.

Consider the implementation of a function like the process_ints that we saw in an earlier example. Visiting every object using a (pointer, length) pair requires an explicit index:

void dangerous_process_ints(int* p, size_t n) {
    for (auto i = 0; i < n; ++i) {
        p[i] = next_character();
    }
}

A span supports range-for -- note this is zero-overhead and does not need to perform any range check, because the range-for loop is known by construction not to exceed the range's bounds:

void process_ints(span<int> s) {
    for (auto& c : s) {
        c = next_character();
    }
}

A span also supports normal iteration using .begin() and .end().

Note that you cannot compare iterators from different spans, even if they refer to the same array.

An iterator is valid as long as the span that it is iterating over exists.

Element access: "How do I access a single element in a span?"

Use myspan[offset] to subscript, or equivalently use iter + offset wheren iter is a span<T>::iterator. Both are range-checked.

Sub-spans: "What if I need a subrange of a span?"

To refer to a sub-span, use first, last, or subspan.

void process_ints(span<widget> s) {
    if (s.length() > 10) {
        read_header(s.first(10));   // first 10 entries
        read_rest(s.subspan(10));   // remaining entries
        // ...
    }
}

In rarer cases, when you know the number of elements at compile time and want to enable constexpr use of span, you can pass the length of the sub-span as a template argument:

constexpr int process_ints(span<widget> s) {
    if (s.length() > 10) {
        read_header(s.first<10>());   // first 10 entries
        read_rest(s.subspan<10>());   // remaining entries
        // ...
    }
    return s.size();
}

span and STL: "How do I pass a span to an STL-style [begin,end) function?"

Use span::iterators. A span is iterable like any STL range.

To call an STL [begin,end)-style interface, use begin and end by default, or other valid iterators if you don't want to pass the whole range:

void f(span<widget> s) {
    // ... 
    auto found = find_if(s.begin(), s.end(), some_value);
    // ... 
}

If you are using a range-based algorithm such as from Range-V3, you can use a span as a range directly:

void f(span<widget> s) {
    // ... 
    auto found = find_if(s, some_value); 
    // ... 
}

Comparison: "When I compare `span<T>`s, do I compare the `T` values or the underlying pointers?"

Comparing two span<T>s compares the T values. To compare two spans for identity, to see if they're pointing to the same thing, use .data().

int a[] = { 1, 2, 3};
span<int> sa{a};

vector<int> v = { 1, 2, 3 };
span<int> sv{v};

assert(sa == sv); // sa and sv both point to contiguous ints with values 1, 2, 3
assert(sa.data() != sv.data()); // but sa and sv point to different memory areas

Things to remember

Comparing spans compares their contents, not whether they point to the same location.

Empty vs null: "Do I have to explicitly check whether a span is null?"

Usually not, because the thing you usually want to check for is that the span is not empty, which means its size is not zero. It's safe to test the size of a span even if it's null.

Remember that the following all have identical meaning for a span s:

!s.empty()
s.size() != 0
s.data() != nullptr && s.size() != 0 (the first condition is actually redundant)

The following is also functionally equivalent as it just tests whether there are zero elements:

s != nullptr (compares s against a null-constructed empty span)

For example:

void f(span<const int> s) {
    if (s != nullptr && s.size() > 0) { // bad: redundant, overkill
        // ...
    }

    if (s.size() > 0) { // good: not redundant
        // ...
    }

    if (!s.empty()) { // good: same as "s.size() > 0"
        // ...
    }
}

Things to remember

Usually you shouldn't check for a null span. For a span s, if you're comparing s != nullptr or s.data() != nullptr, check to make sure you shouldn't just be asking !s.empty().

as_bytes: "Why would I convert a span to `span<const byte>`?"

Because it's a type-safe way to get a read-only view of the objects' bytes.

Without span, to view the bytes of an object requires writing a brittle cast:

void serialize(char* p, int length); // bad: forgot const

void f(widget* p, int length) {
    // serialize one object's bytes (incl. padding)
    serialize(p, 1); // bad: copies just the first byte, forgot sizeof(widget)
}

With span the code is safer and cleaner:

void serialize(span<const byte>); // can't forget const, the first test call site won't compile

void f(span<widget> s) {
    // ...
    // serialize one object's bytes (incl. padding)
    serialize(as_bytes(s)); // ok
}

Also, span<T> lets you distinguish between .size() and .size_bytes(); make use of that distinction instead of multiplying by sizeof(T).

Things to remember

Prefer span<T>'s .size_bytes() instead of .size() * sizeof(T).

And a few `span`-related hints

These are not directly related to span but can often come up while using span.

Use byte everywhere you are handling memory (as opposed to characters or integers). That is, when accessing a chunk of raw memory, use gsl::span<std::byte>.
Use narrow() when you cannot afford to be surprised by a value change during conversion to a smaller range. This includes going between a signed span size or index and an unsigned today's-STL-container .size(), though the span constructors from containers nicely encapsulate many of these conversions.
Similarly, use narrow_cast() when you are sure you won’t be surprised by a value change during conversion to a smaller range

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