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Implement Span::first and Span::last from C++20 (#274)

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This implements `first` and `last` methods on `Span` that mimics
ones in `std::span`.
This commit is contained in:
Girts 2019-03-08 12:05:39 -08:00 • committed by Derek Mauro
parent 38b704384c
commit c1cecb25a9
2 changed files with 68 additions and 0 deletions
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34
absl/types/span.h
View file

@ -485,6 +485,40 @@ class Span {
: (base_internal::ThrowStdOutOfRange("pos > size()"), Span());
}
// Span::first()
//
// Returns a `Span` containing first `len` elements. Parameter `len` is of
// type `size_type` and thus non-negative. `len` value must be <= size().
//
// Examples:
//
// std::vector<int> vec = {10, 11, 12, 13};
// absl::MakeSpan(vec).first(1); // {10}
// absl::MakeSpan(vec).first(3); // {10, 11, 12}
// absl::MakeSpan(vec).first(5); // throws std::out_of_range
constexpr Span first(size_type len) const {
return (len <= size())
? Span(data(), len)
: (base_internal::ThrowStdOutOfRange("len > size()"), Span());
}
// Span::last()
//
// Returns a `Span` containing last `len` elements. Parameter `len` is of
// type `size_type` and thus non-negative. `len` value must be <= size().
//
// Examples:
//
// std::vector<int> vec = {10, 11, 12, 13};
// absl::MakeSpan(vec).last(1); // {13}
// absl::MakeSpan(vec).last(3); // {11, 12, 13}
// absl::MakeSpan(vec).last(5); // throws std::out_of_range
constexpr Span last(size_type len) const {
return (len <= size())
? Span(data() + size() - len, len)
: (base_internal::ThrowStdOutOfRange("len > size()"), Span());
}
// Support for absl::Hash.
template <typename H>
friend H AbslHashValue(H h, Span v) {

34
absl/types/span_test.cc
View file

@ -295,6 +295,38 @@ TEST(IntSpan, Subspan) {
#endif
}
TEST(IntSpan, First) {
std::vector<int> empty;
EXPECT_THAT(absl::MakeSpan(empty).first(0), SpanIs(empty));
auto ramp = MakeRamp(10);
EXPECT_THAT(absl::MakeSpan(ramp).first(0), SpanIs(ramp.data(), 0));
EXPECT_THAT(absl::MakeSpan(ramp).first(10), SpanIs(ramp));
EXPECT_THAT(absl::MakeSpan(ramp).first(3), SpanIs(ramp.data(), 3));
#ifdef ABSL_HAVE_EXCEPTIONS
EXPECT_THROW(absl::MakeSpan(ramp).first(11), std::out_of_range);
#else
EXPECT_DEATH_IF_SUPPORTED(absl::MakeSpan(ramp).first(11), "");
#endif
}
TEST(IntSpan, Last) {
std::vector<int> empty;
EXPECT_THAT(absl::MakeSpan(empty).last(0), SpanIs(empty));
auto ramp = MakeRamp(10);
EXPECT_THAT(absl::MakeSpan(ramp).last(0), SpanIs(ramp.data() + 10, 0));
EXPECT_THAT(absl::MakeSpan(ramp).last(10), SpanIs(ramp));
EXPECT_THAT(absl::MakeSpan(ramp).last(3), SpanIs(ramp.data() + 7, 3));
#ifdef ABSL_HAVE_EXCEPTIONS
EXPECT_THROW(absl::MakeSpan(ramp).last(11), std::out_of_range);
#else
EXPECT_DEATH_IF_SUPPORTED(absl::MakeSpan(ramp).last(11), "");
#endif
}
TEST(IntSpan, MakeSpanPtrLength) {
std::vector<int> empty;
auto s_empty = absl::MakeSpan(empty.data(), empty.size());
@ -769,6 +801,8 @@ TEST(ConstIntSpan, ConstexprTest) {
ABSL_TEST_CONSTEXPR(span.begin());
ABSL_TEST_CONSTEXPR(span.cbegin());
ABSL_TEST_CONSTEXPR(span.subspan(0, 0));
ABSL_TEST_CONSTEXPR(span.first(1));
ABSL_TEST_CONSTEXPR(span.last(1));
ABSL_TEST_CONSTEXPR(span[0]);
}