Add element accessors on Span. (#26366)

* Add element accessors on Span.

This matches std::span and avoids people having to write span.data()[index] and
whatnot.

* Remove the constexpr from element access, since it can't coexist with VerifyOrDie.

src/lib/support/Span.h

@@ -38,7 +38,8 @@ template <class T>
 class Span
 {
 public:
-    using pointer = T *;
+    using pointer   = T *;
+    using reference = T &;
 
     constexpr Span() : mDataBuf(nullptr), mDataLen(0) {}
     constexpr Span(pointer databuf, size_t datalen) : mDataBuf(databuf), mDataLen(datalen) {}
@@ -75,6 +76,18 @@ class Span
     constexpr pointer begin() const { return data(); }
     constexpr pointer end() const { return data() + size(); }
 
+    // Element accessors, matching the std::span API.
+    // VerifyOrDie cannot be used inside a constexpr function, because it uses
+    // "static" on some platforms (e.g. when CHIP_PW_TOKENIZER_LOGGING is true)
+    // and that's not allowed in constexpr functions.
+    reference operator[](size_t index) const
+    {
+        VerifyOrDie(index < size());
+        return data()[index];
+    }
+    reference front() const { return (*this)[0]; }
+    reference back() const { return (*this)[size() - 1]; }
+
     template <class U, typename = std::enable_if_t<std::is_same<std::remove_const_t<T>, std::remove_const_t<U>>::value>>
     bool data_equal(const Span<U> & other) const
     {
@@ -142,7 +155,8 @@ template <class T, size_t N>
 class FixedSpan
 {
 public:
-    using pointer = T *;
+    using pointer   = T *;
+    using reference = T &;
 
     constexpr FixedSpan() : mDataBuf(nullptr) {}
 
@@ -188,6 +202,18 @@ class FixedSpan
     constexpr pointer begin() const { return mDataBuf; }
     constexpr pointer end() const { return mDataBuf + N; }
 
+    // Element accessors, matching the std::span API.
+    // VerifyOrDie cannot be used inside a constexpr function, because it uses
+    // "static" on some platforms (e.g. when CHIP_PW_TOKENIZER_LOGGING is true)
+    // and that's not allowed in constexpr functions.
+    reference operator[](size_t index) const
+    {
+        VerifyOrDie(index < size());
+        return data()[index];
+    }
+    reference front() const { return (*this)[0]; }
+    reference back() const { return (*this)[size() - 1]; }
+
     // Allow data_equal for spans that are over the same type up to const-ness.
     template <class U, typename = std::enable_if_t<std::is_same<std::remove_const_t<T>, std::remove_const_t<U>>::value>>
     bool data_equal(const FixedSpan<U, N> & other) const

src/lib/support/tests/TestSpan.cpp

@@ -56,6 +56,11 @@ static void TestByteSpan(nlTestSuite * inSuite, void * inContext)
     NL_TEST_ASSERT(inSuite, s2.data_equal(s2));
     NL_TEST_ASSERT(inSuite, !s2.data_equal(s1));
     NL_TEST_ASSERT(inSuite, IsSpanUsable(s2) == true);
+    NL_TEST_ASSERT(inSuite, s2.front() == 1);
+    NL_TEST_ASSERT(inSuite, s2.back() == 3);
+    NL_TEST_ASSERT(inSuite, s2[0] == 1);
+    NL_TEST_ASSERT(inSuite, s2[1] == 2);
+    NL_TEST_ASSERT(inSuite, s2[2] == 3);
 
     ByteSpan s3 = s2;
     NL_TEST_ASSERT(inSuite, s3.data() == arr);
@@ -175,6 +180,11 @@ static void TestFixedByteSpan(nlTestSuite * inSuite, void * inContext)
     NL_TEST_ASSERT(inSuite, s2.data()[2] == 3);
     NL_TEST_ASSERT(inSuite, !s2.empty());
     NL_TEST_ASSERT(inSuite, s2.data_equal(s2));
+    NL_TEST_ASSERT(inSuite, s2.front() == 1);
+    NL_TEST_ASSERT(inSuite, s2.back() == 3);
+    NL_TEST_ASSERT(inSuite, s2[0] == 1);
+    NL_TEST_ASSERT(inSuite, s2[1] == 2);
+    NL_TEST_ASSERT(inSuite, s2[2] == 3);
 
     FixedByteSpan<3> s3 = s2;
     NL_TEST_ASSERT(inSuite, s3.data() == arr);