Add sanity check asserts to span when -DDEBUG

Author: sipa

src/span.h

@@ -10,6 +10,14 @@
 #include <algorithm>
 #include <assert.h>
 
+#ifdef DEBUG
+#define CONSTEXPR_IF_NOT_DEBUG
+#define ASSERT_IF_DEBUG(x) assert((x))
+#else
+#define CONSTEXPR_IF_NOT_DEBUG constexpr
+#define ASSERT_IF_DEBUG(x)
+#endif
+
 /** A Span is an object that can refer to a contiguous sequence of objects.
  *
  * It implements a subset of C++20's std::span.
@@ -37,7 +45,10 @@ class Span
      * which is hard to implement without std::address_of.
      */
     template <typename T, typename std::enable_if<std::is_convertible<T (*)[], C (*)[]>::value, int>::type = 0>
-    constexpr Span(T* begin, T* end) noexcept : m_data(begin), m_size(end - begin) {}
+    CONSTEXPR_IF_NOT_DEBUG Span(T* begin, T* end) noexcept : m_data(begin), m_size(end - begin)
+    {
+        ASSERT_IF_DEBUG(end >= begin);
+    }
 
     /** Implicit conversion of spans between compatible types.
      *
@@ -73,15 +84,42 @@ class Span
     constexpr C* data() const noexcept { return m_data; }
     constexpr C* begin() const noexcept { return m_data; }
     constexpr C* end() const noexcept { return m_data + m_size; }
-    constexpr C& front() const noexcept { return m_data[0]; }
-    constexpr C& back() const noexcept { return m_data[m_size - 1]; }
+    CONSTEXPR_IF_NOT_DEBUG C& front() const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > 0);
+        return m_data[0];
+    }
+    CONSTEXPR_IF_NOT_DEBUG C& back() const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > 0);
+        return m_data[m_size - 1];
+    }
     constexpr std::size_t size() const noexcept { return m_size; }
-    constexpr C& operator[](std::size_t pos) const noexcept { return m_data[pos]; }
-
-    constexpr Span<C> subspan(std::size_t offset) const noexcept { return Span<C>(m_data + offset, m_size - offset); }
-    constexpr Span<C> subspan(std::size_t offset, std::size_t count) const noexcept { return Span<C>(m_data + offset, count); }
-    constexpr Span<C> first(std::size_t count) const noexcept { return Span<C>(m_data, count); }
-    constexpr Span<C> last(std::size_t count) const noexcept { return Span<C>(m_data + m_size - count, count); }
+    CONSTEXPR_IF_NOT_DEBUG C& operator[](std::size_t pos) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() > pos);
+        return m_data[pos];
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= offset);
+        return Span<C>(m_data + offset, m_size - offset);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> subspan(std::size_t offset, std::size_t count) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= offset + count);
+        return Span<C>(m_data + offset, count);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> first(std::size_t count) const noexcept
+    {
+        ASSERT_IF_DEBUG(size() >= count);
+        return Span<C>(m_data, count);
+    }
+    CONSTEXPR_IF_NOT_DEBUG Span<C> last(std::size_t count) const noexcept
+    {
+         ASSERT_IF_DEBUG(size() >= count);
+         return Span<C>(m_data + m_size - count, count);
+    }
 
     friend constexpr bool operator==(const Span& a, const Span& b) noexcept { return a.size() == b.size() && std::equal(a.begin(), a.end(), b.begin()); }
     friend constexpr bool operator!=(const Span& a, const Span& b) noexcept { return !(a == b); }
@@ -106,7 +144,7 @@ template <typename T>
 T& SpanPopBack(Span<T>& span)
 {
     size_t size = span.size();
-    assert(size > 0);
+    ASSERT_IF_DEBUG(size > 0);
     T& back = span[size - 1];
     span = Span<T>(span.data(), size - 1);
     return back;