Add small_vector<T, N> class template

It has a (mostly) std::vector interface (it derives from it) and will store
up to N elements in the same storage as the vector instance itself.

4 files changed, 398 insertions, 1 deletions

diff --git a/butl/small-vector b/butl/small-vector
new file mode 100644
index 0000000..f09d91d
--- /dev/null
+++ b/butl/small-vector
@@ -0,0 +1,257 @@
+// file      : butl/small-vector -*- C++ -*-
+// copyright : Copyright (c) 2014-2016 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef BUTL_SMALL_VECTOR
+#define BUTL_SMALL_VECTOR
+
+#include <vector>
+#include <cassert>
+#include <cstddef> // size_t
+#include <utility> // more(), forward()
+
+namespace butl
+{
+  template <typename T, std::size_t N>
+  struct small_vector_buffer
+  {
+    // Size keeps track of the number of elements that are constructed in
+    // the buffer. Size equal N + 1 means the buffer is not allocated.
+    //
+    // Note that the names are decorated in order no to conflict with
+    // std::vector interface.
+    //
+    alignas (alignof (T)) char data_[sizeof (T) * N];
+    bool free_ = true;
+
+    // Note that the buffer should be constructed before std::vector and
+    // destroyed after (since std::vector's destructor will be destroying
+    // elements potentially residing in the buffer). This means that the
+    // buffer should be inherited from and before std::vector.
+    //
+    small_vector_buffer () = default;
+
+    small_vector_buffer (small_vector_buffer&&) = delete;
+    small_vector_buffer (const small_vector_buffer&) = delete;
+
+    small_vector_buffer& operator= (small_vector_buffer&&) = delete;
+    small_vector_buffer& operator= (const small_vector_buffer&) = delete;
+  };
+
+  template <typename T, std::size_t N>
+  class small_vector_allocator
+  {
+  public:
+    using buffer_type = small_vector_buffer<T, N>;
+
+    explicit
+    small_vector_allocator (buffer_type* b) noexcept: buf_ (b) {}
+
+    // Allocator interface.
+    //
+  public:
+    using value_type = T;
+
+    T*
+    allocate(std::size_t n)
+    {
+      if (n <= N)
+      {
+        assert (buf_->free_); // Why would we need another small buffer?
+        buf_->free_ = false;
+        return reinterpret_cast<T*> (buf_->data_);
+      }
+      else
+        return static_cast<T*> (::operator new (sizeof (T) * n));
+    }
+
+    void
+    deallocate (void* p, std::size_t) noexcept
+    {
+      if (p == buf_->data_)
+        buf_->free_ = true;
+      else
+        ::operator delete (p);
+    }
+
+    friend bool
+    operator== (small_vector_allocator x, small_vector_allocator y) noexcept
+    {
+      // We can use y to deallocate x's allocations if they use the same small
+      // buffer or neither uses its small buffer (which means all allocations,
+      // if any, have been from the shared heap). Of course this assumes no
+      // copy will be called to deallocate what has been allocated after said
+      // copy was made:
+      //
+      // A x;
+      // A y (x);
+      // p = x.allocate ();
+      // y.deallocate (p);  // Ouch.
+      //
+      return (x.buf_ == y.buf_) || (x.buf_->free_ && y.buf_->free_);
+    }
+
+    friend bool
+    operator!= (small_vector_allocator x, small_vector_allocator y) noexcept
+    {
+      return !(x == y);
+    }
+
+    // It might get instantiated but should not be called.
+    //
+    small_vector_allocator
+    select_on_container_copy_construction () const noexcept
+    {
+      return small_vector_allocator (nullptr);
+    }
+
+    // propagate_on_container_copy_assignment = false
+    // propagate_on_container_move_assignment = false
+    // propagate_on_container_swap = false
+
+    // Shouldn't be needed except to satisfy some static_assert's.
+    //
+    template <typename U>
+    struct rebind {using other = small_vector_allocator<U, N>;};
+
+  private:
+    buffer_type* buf_;
+  };
+
+  // Issues and limitations.
+  //
+  // - vector::reserve() may allocate more per the spec. But the three main
+  //   C++ runtimes (libstdc++, libc++, and msvc) all seem to do the right
+  //   thing.
+  //
+  // - What if in most cases the vector is empty? How can we avoid initial
+  //   reserve? Provide no_reserve flag or some such? Is it really worth it?
+  //
+  // - swap() is deleted (see notes below).
+  //
+  template <typename T, std::size_t N>
+  class small_vector: private small_vector_buffer<T, N>,
+                      public std::vector<T, small_vector_allocator<T, N>>
+  {
+  public:
+    using allocator_type = small_vector_allocator<T, N>;
+    using buffer_type = small_vector_buffer<T, N>;
+    using base_type = std::vector<T, small_vector_allocator<T, N>>;
+
+    small_vector ()
+      : base_type (allocator_type (this))
+    {
+      reserve ();
+    }
+
+    small_vector (std::initializer_list<T> v)
+      : base_type (allocator_type (this))
+    {
+      if (v.size () <= N)
+        reserve ();
+
+      static_cast<base_type&> (*this) = v;
+    }
+
+    template <typename I>
+    small_vector (I b, I e)
+      : base_type (allocator_type (this))
+    {
+      // While we could optimize this for random access iterators, N will
+      // usually be pretty small. Let's hope the compiler sees this and does
+      // some magic for us.
+      //
+      std::size_t n (0);
+      for (I i (b); i != e && n <= N; ++i) ++n;
+
+      if (n <= N)
+        reserve ();
+
+      this->assign (b, e);
+    }
+
+    explicit
+    small_vector (std::size_t n)
+      : base_type (allocator_type (this))
+    {
+      if (n <= N)
+        reserve ();
+
+      this->resize (n);
+    }
+
+    small_vector (std::size_t n, const T& x)
+      : base_type (allocator_type (this))
+    {
+      if (n <= N)
+        reserve ();
+
+      this->assign (n, x);
+    }
+
+    small_vector (const small_vector& v)
+      : buffer_type (), base_type (allocator_type (this))
+    {
+      if (v.size () <= N)
+        reserve ();
+
+      static_cast<base_type&> (*this) = v;
+    }
+
+    small_vector (small_vector&& v)
+      : base_type (allocator_type (this))
+    {
+      if (v.size () <= N)
+        reserve ();
+
+      static_cast<base_type&> (*this) = std::move (v);
+    }
+
+    small_vector&
+    operator= (const small_vector& v)
+    {
+      // Note: propagate_on_container_copy_assignment = false
+      //
+      static_cast<base_type&> (*this) = v;
+      return *this;
+    }
+
+    small_vector&
+    operator= (small_vector&& v)
+    {
+      // Note: propagate_on_container_move_assignment = false
+      //
+      static_cast<base_type&> (*this) = std::move (v);
+      return *this;
+    }
+
+    small_vector&
+    operator= (std::initializer_list<T> v)
+    {
+      static_cast<base_type&> (*this) = v;
+      return *this;
+    }
+
+    // Implementing swap() under small buffer optimization is not trivial, to
+    // say the least (think of swapping two such buffers of different sizes).
+    // One easy option would be to force both in to the heap.
+    //
+    void
+    swap (small_vector&) = delete;
+
+    void
+    reserve (std::size_t n = N)
+    {
+      base_type::reserve (n < N ? N : n);
+    }
+
+    void
+    shrink_to_fit ()
+    {
+      if (this->capacity () > N)
+        base_type::shrink_to_fit ();
+    }
+  };
+}
+
+#endif // BUTL_SMALL_VECTOR
diff --git a/tests/buildfile b/tests/buildfile
index 84a17aa..909432c 100644
--- a/tests/buildfile
+++ b/tests/buildfile
@@ -4,7 +4,7 @@
 
 d = base64/ cpfile/ dir-iterator/ fdstream/ link/ manifest-parser/ \
     manifest-serializer/ manifest-roundtrip/ pager/ path/ prefix-map/ \
-    process/ sha256/ strcase/ timestamp/ triplet/
+    process/ sha256/ small-vector/ strcase/ timestamp/ triplet/
 
 ./: $d
 include $d
diff --git a/tests/small-vector/buildfile b/tests/small-vector/buildfile
new file mode 100644
index 0000000..78d2d06
--- /dev/null
+++ b/tests/small-vector/buildfile
@@ -0,0 +1,8 @@
+# file      : tests/small-vector/buildfile
+# copyright : Copyright (c) 2014-2016 Code Synthesis Ltd
+# license   : MIT; see accompanying LICENSE file
+
+exe{driver}: cxx{driver} ../../butl/lib{butl}
+exe{driver}: test.arguments = $src_root
+
+include ../../butl/
diff --git a/tests/small-vector/driver.cxx b/tests/small-vector/driver.cxx
new file mode 100644
index 0000000..5b7cfa6
--- /dev/null
+++ b/tests/small-vector/driver.cxx
@@ -0,0 +1,132 @@
+// file      : tests/small-vector/driver.cxx -*- C++ -*-
+// copyright : Copyright (c) 2014-2016 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#include <string>
+#include <cassert>
+#include <iostream>
+
+#include <butl/small-vector>
+
+using namespace std;
+using namespace butl;
+
+// Return if v.data() points to somewhere inside v.
+//
+template <typename T, size_t N>
+inline bool
+small (const small_vector<T, N>& v)
+{
+  const void* d (v.data ());
+  return d >= &v && d < (&v + 1);
+}
+
+int
+main ()
+{
+  using vector = small_vector<string, 1>;
+
+  {
+    vector v;
+    assert (v.capacity () == 1 && small (v));
+
+    v.push_back ("abc");
+    assert (v[0] == "abc" && v.capacity () == 1 && small (v));
+
+    string* d (v.data ());              // Small buffer...
+
+    v.push_back ("xyz");
+    assert (v[0] == "abc" && v.data () != d && !small (v));
+
+    v.pop_back ();
+    v.shrink_to_fit ();
+    assert (v[0] == "abc" && v.data () == d);
+  }
+
+  // Allocator comparison.
+  //
+  {
+    vector v1, v2;
+    assert (v1.get_allocator () != v2.get_allocator ()); // stack/stack
+
+    v1.assign ({"abc", "xyz"});
+    assert (v1.get_allocator () != v2.get_allocator ()); // heap/stack
+
+    v2.assign ({"abc", "xyz"});
+    assert (v1.get_allocator () == v2.get_allocator ()); // heap/heap
+
+    v1.pop_back ();
+    v1.shrink_to_fit ();
+    assert (v1.get_allocator () != v2.get_allocator ()); // stack/heap
+
+    v2.pop_back ();
+    v2.shrink_to_fit ();
+    assert (v1.get_allocator () != v2.get_allocator ()); // stack/stack
+  }
+
+  // Copy constructor.
+  //
+  {
+    vector s1 ({"abc"}), s2 (s1);
+    assert (s1 == s2 && s2.capacity () == 1 && small (s2));
+
+    vector l1 ({"abc", "xyz"}), l2 (l1);
+    assert (l1 == l2 && !small (l2));
+  }
+
+  // Move constructor.
+  //
+  {
+    struct mstring: string // Move-only string.
+    {
+      mstring () = default;
+      explicit mstring (const char* s): string (s) {}
+
+      mstring (mstring&&) = default;
+      mstring& operator= (mstring&&) = default;
+
+      mstring (const mstring&) = delete;
+      mstring& operator= (const mstring&) = delete;
+    };
+
+    using vector = small_vector<mstring, 1>;
+
+    vector s1; s1.emplace_back ("abc");
+    vector s2 (move (s1));
+    assert (s2[0] == "abc" && s2.capacity () == 1 && small (s2));
+
+    vector l1; l1.emplace_back ("abc"); l1.emplace_back ("xyz");
+    vector l2 (move (l1));
+    assert (l2[0] == "abc" && l2[1] == "xyz" && !small (l2));
+  }
+
+  // Other constructors.
+  //
+
+  {
+    const char* sa[] = {"abc"};
+    const char* la[] = {"abc", "xyz"};
+
+    vector s (sa, sa + 1);
+    assert (s[0] == "abc" && s.capacity () == 1 && small (s));
+
+    vector l (la, la + 2);
+    assert (l[0] == "abc" && l[1] == "xyz" && !small (l));
+  }
+
+  {
+    vector s (1, "abc");
+    assert (s[0] == "abc" && s.capacity () == 1 && small (s));
+
+    vector l (2, "abc");
+    assert (l[0] == "abc" && l[1] == "abc" && !small (l));
+  }
+
+  {
+    vector s (1);
+    assert (s[0] == "" && s.capacity () == 1 && small (s));
+
+    vector l (2);
+    assert (l[0] == "" && l[1] == "" && !small (l));
+  }
+}