Adding missing span deduction guides#891
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JordanMaples merged 7 commits intoMay 29, 2020
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CaseyCarter
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May 29, 2020
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| template <class Container> | ||
| span(Container&)->span<typename Container::value_type>; | ||
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| template <class Container> | ||
| span(const Container&)->span<const typename Container::value_type>; |
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If you only care about containers, this is fine. Non-containers sometimes have const-qualified elements even when they are themselves not const, which will break this. For example, in:
#include <gsl/span>
#include <string_view>
#include <type_traits>
using namespace std::literals;
int main() {
auto s = "Hello, World!"sv;
gsl::span x{s}; // Line 9
static_assert(std::is_same_v<decltype(x)::element_type, const char>);
}CTAD deduces span<char> on line 9, resulting in the compiler complaining that initialization from const char* is ill-formed.
I suggest instead:
Suggested change
| template <class Container> | |
| span(Container&)->span<typename Container::value_type>; | |
| template <class Container> | |
| span(const Container&)->span<const typename Container::value_type>; | |
| template <class Container, | |
| class Element = std::remove_pointer_t<decltype(std::declval<Container&>().data())>> | |
| span(Container&)->span<Element>; | |
| template <class Container, | |
| class Element = std::remove_pointer_t<decltype(std::declval<const Container&>().data())>> | |
| span(const Container&)->span<Element>; |
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Okay, my OCD made me test these, so I'll submit a PR (JordanMaples#6) with this change and the updated test.
Add string_view test case and modify deduction guides
…ro unused' errors
…vector may not intend for type deduction.
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I noticed while experimenting with std::span that some of the functionality wasn't 1:1 regarding CTAD.
The deduction of container types was missing. The following std::span example compiles without any problems, while the the gsl equivalent does not.
Ex: Current behavior
std::vector v{1,2,3,4}; std::span sp{v}; // no problem, compilesstd::vector v{1,2,3,4}; gsl::span sp{v}; // fails to compile. gsl::span<int>, however does compileBy examining the container's value_type in the deduction guide, we're able to deduce the internal type and construct the span. This more closely aligns gsl::span's behavior to std::span.