BLI: Add "first" method to MutableSpan and Vector

This is convenient because having a uniform interface is nice, and
because of the similarity to "last".

Differential Revision: https://developer.blender.org/D11076

2 files changed, 27 insertions, 2 deletions

diff --git a/source/blender/blenlib/BLI_span.hh b/source/blender/blenlib/BLI_span.hh
index c32ba0826df..c3876d4eaf8 100644
--- a/source/blender/blenlib/BLI_span.hh
+++ b/source/blender/blenlib/BLI_span.hh
@@ -662,6 +662,16 @@ template<typename T> class MutableSpan {
   }
 
   /**
+   * Return a reference to the first element in the array. This invokes undefined behavior when the
+   * array is empty.
+   */
+  constexpr T &first() const
+  {
+    BLI_assert(size_ > 0);
+    return data_[0];
+  }
+
+  /**
    * Returns a reference to the last element. This invokes undefined behavior when the array is
    * empty.
    */
diff --git a/source/blender/blenlib/BLI_vector.hh b/source/blender/blenlib/BLI_vector.hh
index d08a5c65c52..3ffd5309a04 100644
--- a/source/blender/blenlib/BLI_vector.hh
+++ b/source/blender/blenlib/BLI_vector.hh
@@ -444,7 +444,7 @@ class Vector {
     this->append_as(std::move(value));
   }
   /* This is similar to `std::vector::emplace_back`. */
-  template<typename... ForwardValue> void append_as(ForwardValue &&... value)
+  template<typename... ForwardValue> void append_as(ForwardValue &&...value)
   {
     this->ensure_space_for_one();
     this->append_unchecked_as(std::forward<ForwardValue>(value)...);
@@ -486,7 +486,7 @@ class Vector {
   {
     this->append_unchecked_as(std::move(value));
   }
-  template<typename... ForwardT> void append_unchecked_as(ForwardT &&... value)
+  template<typename... ForwardT> void append_unchecked_as(ForwardT &&...value)
   {
     BLI_assert(end_ < capacity_end_);
     new (end_) T(std::forward<ForwardT>(value)...);
@@ -669,6 +669,21 @@ class Vector {
   }
 
   /**
+   * Return a reference to the first element in the vector.
+   * This invokes undefined behavior when the vector is empty.
+   */
+  const T &first() const
+  {
+    BLI_assert(this->size() > 0);
+    return *begin_;
+  }
+  T &first()
+  {
+    BLI_assert(this->size() > 0);
+    return *begin_;
+  }
+
+  /**
    * Return how many values are currently stored in the vector.
    */
   int64_t size() const