Fix operator+, add tests

include/boost/static_string/static_string.hpp

@@ -4456,33 +4456,41 @@ operator+(
   CharT lhs,
   const basic_static_string<N, CharT, Traits>& rhs)
 {
-  return basic_static_string<N + 1, CharT, Traits>(rhs).insert(0, lhs);
+  // The cast to std::size_t is needed here since 0 is a null pointer constant
+  return basic_static_string<N + 1, CharT, Traits>(rhs).insert(
+    std::size_t(0), 1, lhs);
 }
 
+// Add a null-terminated character array to a string.
 template<
   std::size_t N, std::size_t M,
   typename CharT, typename Traits>
 BOOST_STATIC_STRING_CPP14_CONSTEXPR
 inline
-basic_static_string<N + M, CharT, Traits>
+basic_static_string<(N + M) - 1, CharT, Traits>
 operator+(
   const basic_static_string<N, CharT, Traits>& lhs,
   const CharT(&rhs)[M])
 {
-  return basic_static_string<N + M, CharT, Traits>(lhs).append(+rhs, M);
+  // Subtract 1 to account for the null terminator, as "hello" is a char[6]
+  return basic_static_string<(N + M) - 1, CharT, Traits>(lhs).append(+rhs, M - 1);
 }
 
+// Add a string to a null-terminated character array.
 template<
   std::size_t N, std::size_t M,
   typename CharT, typename Traits>
 BOOST_STATIC_STRING_CPP14_CONSTEXPR
 inline
-basic_static_string<N + M, CharT, Traits>
+basic_static_string<(N + M) - 1, CharT, Traits>
 operator+(
   const CharT(&lhs)[N],
   const basic_static_string<M, CharT, Traits>& rhs)
 {
-  return basic_static_string<N + M, CharT, Traits>(rhs).insert(0, +rhs, N);
+  // Subtract 1 to account for the null terminator, as "hello" is a char[6]
+  // The cast to std::size_t is needed here since 0 is a null pointer constant
+  return basic_static_string<(N + M) - 1, CharT, Traits>(rhs).insert(
+    std::size_t(0), +lhs, N - 1);
 }
 
 //------------------------------------------------------------------------------

test/static_string.cpp

@@ -7009,6 +7009,53 @@ testStream()
   BOOST_TEST(a.str() == b.subview());
 }
 
+void
+testOperatorPlus()
+{
+  static_string<10> s1 = "hello";
+  static_string<10> s2 = "world";
+
+  // operator+(static_string, static_string)
+  {
+    auto res = s1 + s2;
+    BOOST_TEST(res == "helloworld");
+    BOOST_TEST(res.capacity() == 20);
+    BOOST_TEST(res.size() == 10);
+  }
+
+  // operator+(static_string, CharT)
+  {
+    auto res = s1 + '!';
+    BOOST_TEST(res == "hello!");
+    BOOST_TEST(res.capacity() == 11);
+    BOOST_TEST(res.size() == 6);
+  }
+
+  // operator+(CharT, static_string)
+  {
+    auto res = '!' + s1;
+    BOOST_TEST(res == "!hello");
+    BOOST_TEST(res.capacity() == 11);
+    BOOST_TEST(res.size() == 6);
+  }
+
+  // operator+(static_string, CharT(&)[N])
+  {
+    auto res = s1 + "world";
+    BOOST_TEST(res == "helloworld");
+    BOOST_TEST(res.capacity() == 15);
+    BOOST_TEST(res.size() == 10);
+  }
+
+  // operator+(CharT(&)[N], static_string)
+  {
+    auto res = "hello" + s2;
+    BOOST_TEST(res == "helloworld");
+    BOOST_TEST(res.capacity() == 15);
+    BOOST_TEST(res.size() == 10);
+  }
+}
+
 int
 runTests()
 {
@@ -7047,6 +7094,7 @@ runTests()
   testHash();
   testEmpty();
   testStream();
+  testOperatorPlus();
 
   return report_errors();
 }