Add basic_static_string::available()

This returns max_size() - size(), i.e. the number of characters that can
still be inserted before the string reaches its capacity. Intended as a
shorthand for capacity-capped writes in combination with the
pos/count-taking overloads of append, assign, and insert:

  str.append(sv, 0, str.available());

Addresses the ergonomic complaint in issue #81 without expanding the
overload set.

Closes issue #81.

include/boost/static_string/static_string.hpp

@@ -2314,6 +2314,23 @@ public:
     return N;
   }
 
+  /** Return the number of additional characters that can be stored.
+
+      Returns `max_size() - size()`, i.e. the number of characters
+      that can still be inserted before the string reaches its
+      capacity.
+
+      @par Complexity
+
+      Constant.
+   */
+  BOOST_STATIC_STRING_CPP11_CONSTEXPR
+  size_type
+  available() const noexcept
+  {
+    return max_size() - size();
+  }
+
   /** Increase the capacity.
 
       This function has no effect.

test/static_string.cpp

@@ -989,6 +989,29 @@ testCapacity()
     BOOST_TEST(static_string<3>{"abc"}.max_size() == 3);
     BOOST_TEST(static_string<5>{"abc"}.max_size() == 5);
 
+    // available()
+    BOOST_TEST(static_string<0>{}.available() == 0);
+    BOOST_TEST(static_string<1>{}.available() == 1);
+    BOOST_TEST(static_string<1>{"a"}.available() == 0);
+    BOOST_TEST(static_string<3>{"abc"}.available() == 0);
+    BOOST_TEST(static_string<5>{"abc"}.available() == 2);
+    BOOST_TEST(static_string<5>{}.available() == 5);
+    {
+        static_string<8> s("hi");
+        BOOST_TEST(s.available() == 6);
+        s.append(" there");
+        BOOST_TEST(s.available() == 0);
+        s.clear();
+        BOOST_TEST(s.available() == 8);
+    }
+    // Intended use with the pos/count-taking overloads.
+    {
+        static_string<5> s("ab");
+        s.append("xyzwv", 0, s.available());
+        BOOST_TEST(s == "abxyz");
+        BOOST_TEST(s.available() == 0);
+    }
+
     // reserve(std::size_t n)
     static_string<3>{}.reserve(0);
     static_string<3>{}.reserve(1);