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Merged
ssam18 merged 3 commits into from
May 1, 2026
Merged

Add remove_prefix and remove_suffix (P3729) #8

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fb53441
Add remove_prefix and remove_suffix (P3729)
ssam18 Apr 26, 2026
6928940
Merge branch 'main' into remove-prefix-suffix-p3729
ssam18 May 1, 2026
cd59340
Use C++20 requires clauses for remove_prefix/remove_suffix
ssam18 May 1, 2026
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13 changes: 13 additions & 0 deletions include/beman/span/span.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -229,6 +229,19 @@ class span {
return {data_ + offset, count == dynamic_extent ? size() - offset : count};
}

template <std::size_t E = Extent, std::enable_if_t<E == dynamic_extent, int> = 0>
constexpr void remove_prefix(size_type n) noexcept {
assert(n <= size());
data_ += n;
size_ -= n;
}

template <std::size_t E = Extent, std::enable_if_t<E == dynamic_extent, int> = 0>
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constexpr void remove_suffix(size_type n) noexcept {
assert(n <= size());
size_ -= n;
}

// 26.7.3.4 Observers [span.obs]

[[nodiscard]] constexpr size_type size() const noexcept { return size_; }
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95 changes: 95 additions & 0 deletions tests/beman/span/span.test.cpp
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Expand Up @@ -8,6 +8,8 @@
#include <array>
#include <cstddef>
#include <numeric>
#include <type_traits>
#include <utility>
#include <vector>

namespace bsp = beman::span;
Expand Down Expand Up @@ -366,6 +368,99 @@ TEST(SpanSubviews, subspan_dynamic_to_end) {
EXPECT_EQ(sub[1], 4);
}

// ---------------------------------------------------------------------------
// remove_prefix / remove_suffix (P3729)
// ---------------------------------------------------------------------------

TEST(SpanModifiers, remove_prefix_zero_is_noop) {
int arr[] = {1, 2, 3, 4, 5};
bsp::span<int> s(arr);
s.remove_prefix(0);
EXPECT_EQ(s.size(), 5u);
EXPECT_EQ(s.data(), arr);
}

TEST(SpanModifiers, remove_prefix_partial) {
int arr[] = {1, 2, 3, 4, 5};
bsp::span<int> s(arr);
s.remove_prefix(2);
EXPECT_EQ(s.size(), 3u);
EXPECT_EQ(s.data(), arr + 2);
EXPECT_EQ(s[0], 3);
EXPECT_EQ(s[2], 5);
}

TEST(SpanModifiers, remove_prefix_full_empties_span) {
int arr[] = {1, 2, 3};
bsp::span<int> s(arr);
s.remove_prefix(s.size());
EXPECT_EQ(s.size(), 0u);
EXPECT_TRUE(s.empty());
}

TEST(SpanModifiers, remove_suffix_zero_is_noop) {
int arr[] = {1, 2, 3, 4, 5};
bsp::span<int> s(arr);
s.remove_suffix(0);
EXPECT_EQ(s.size(), 5u);
EXPECT_EQ(s.data(), arr);
}

TEST(SpanModifiers, remove_suffix_partial) {
int arr[] = {1, 2, 3, 4, 5};
bsp::span<int> s(arr);
s.remove_suffix(2);
EXPECT_EQ(s.size(), 3u);
EXPECT_EQ(s.data(), arr);
EXPECT_EQ(s[0], 1);
EXPECT_EQ(s[2], 3);
}

TEST(SpanModifiers, remove_suffix_full_empties_span) {
int arr[] = {1, 2, 3};
bsp::span<int> s(arr);
s.remove_suffix(s.size());
EXPECT_EQ(s.size(), 0u);
EXPECT_TRUE(s.empty());
}

TEST(SpanModifiers, remove_prefix_and_suffix_combined) {
int arr[] = {10, 20, 30, 40, 50, 60};
bsp::span<int> s(arr);
s.remove_prefix(1);
s.remove_suffix(2);
EXPECT_EQ(s.size(), 3u);
EXPECT_EQ(s[0], 20);
EXPECT_EQ(s[2], 40);
}

namespace modifiers_detail {
template <class T, class = void>
struct has_remove_prefix : std::false_type {};

template <class T>
struct has_remove_prefix<
T,
std::void_t<decltype(std::declval<T&>().remove_prefix(std::size_t{0}))>>
: std::true_type {};

template <class T, class = void>
struct has_remove_suffix : std::false_type {};

template <class T>
struct has_remove_suffix<
T,
std::void_t<decltype(std::declval<T&>().remove_suffix(std::size_t{0}))>>
: std::true_type {};
} // namespace modifiers_detail

TEST(SpanModifiers, fixed_extent_has_no_modifiers) {
static_assert(modifiers_detail::has_remove_prefix<bsp::span<int>>::value);
static_assert(modifiers_detail::has_remove_suffix<bsp::span<int>>::value);
static_assert(!modifiers_detail::has_remove_prefix<bsp::span<int, 3>>::value);
static_assert(!modifiers_detail::has_remove_suffix<bsp::span<int, 3>>::value);
}

// ---------------------------------------------------------------------------
// as_bytes / as_writable_bytes
// ---------------------------------------------------------------------------
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