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| -rw-r--r-- | doc/developer-notes.md | 13 | ||||
| -rw-r--r-- | src/span.h | 56 |
2 files changed, 69 insertions, 0 deletions
diff --git a/doc/developer-notes.md b/doc/developer-notes.md index b33b3ad18a..bd3daa3202 100644 --- a/doc/developer-notes.md +++ b/doc/developer-notes.md @@ -620,6 +620,19 @@ class A - *Rationale*: Easier to understand what is happening, thus easier to spot mistakes, even for those that are not language lawyers. +- Use `Span` as function argument when it can operate on any range-like container. + + - *Rationale*: Compared to `Foo(const vector<int>&)` this avoids the need for a (potentially expensive) + conversion to vector if the caller happens to have the input stored in another type of container. + However, be aware of the pitfalls documented in [span.h](../src/span.h). + +```cpp +void Foo(Span<const int> data); + +std::vector<int> vec{1,2,3}; +Foo(vec); +``` + - Prefer `enum class` (scoped enumerations) over `enum` (traditional enumerations) where possible. - *Rationale*: Scoped enumerations avoid two potential pitfalls/problems with traditional C++ enumerations: implicit conversions to `int`, and name clashes due to enumerators being exported to the surrounding scope. diff --git a/src/span.h b/src/span.h index 4931507719..841f1eadf7 100644 --- a/src/span.h +++ b/src/span.h @@ -21,6 +21,62 @@ /** A Span is an object that can refer to a contiguous sequence of objects. * * It implements a subset of C++20's std::span. + * + * Things to be aware of when writing code that deals with Spans: + * + * - Similar to references themselves, Spans are subject to reference lifetime + * issues. The user is responsible for making sure the objects pointed to by + * a Span live as long as the Span is used. For example: + * + * std::vector<int> vec{1,2,3,4}; + * Span<int> sp(vec); + * vec.push_back(5); + * printf("%i\n", sp.front()); // UB! + * + * may exhibit undefined behavior, as increasing the size of a vector may + * invalidate references. + * + * - One particular pitfall is that Spans can be constructed from temporaries, + * but this is unsafe when the Span is stored in a variable, outliving the + * temporary. For example, this will compile, but exhibits undefined behavior: + * + * Span<const int> sp(std::vector<int>{1, 2, 3}); + * printf("%i\n", sp.front()); // UB! + * + * The lifetime of the vector ends when the statement it is created in ends. + * Thus the Span is left with a dangling reference, and using it is undefined. + * + * - Due to Span's automatic creation from range-like objects (arrays, and data + * types that expose a data() and size() member function), functions that + * accept a Span as input parameter can be called with any compatible + * range-like object. For example, this works: +* + * void Foo(Span<const int> arg); + * + * Foo(std::vector<int>{1, 2, 3}); // Works + * + * This is very useful in cases where a function truly does not care about the + * container, and only about having exactly a range of elements. However it + * may also be surprising to see automatic conversions in this case. + * + * When a function accepts a Span with a mutable element type, it will not + * accept temporaries; only variables or other references. For example: + * + * void FooMut(Span<int> arg); + * + * FooMut(std::vector<int>{1, 2, 3}); // Does not compile + * std::vector<int> baz{1, 2, 3}; + * FooMut(baz); // Works + * + * This is similar to how functions that take (non-const) lvalue references + * as input cannot accept temporaries. This does not work either: + * + * void FooVec(std::vector<int>& arg); + * FooVec(std::vector<int>{1, 2, 3}); // Does not compile + * + * The idea is that if a function accepts a mutable reference, a meaningful + * result will be present in that variable after the call. Passing a temporary + * is useless in that context. */ template<typename C> class Span |