Fix empty string-view construction and assignment

include/boost/container/string.hpp

@@ -893,7 +893,7 @@ class basic_string
       : base_t()
    {
       this->priv_short_terminate();
-      this->assign(s, n);  // assign checks for s != 0
+      this->assign(s, n);  // assign checks for s != 0 when n != 0
    }
 
    //! <b>Effects</b>: Constructs a basic_string taking the allocator as parameter,
@@ -902,7 +902,7 @@ class basic_string
       : base_t(a)
    {
       this->priv_short_terminate();
-      this->assign(s, n);  // assign checks for s != 0
+      this->assign(s, n);  // assign checks for s != 0 when n != 0
    }
 
    //! <b>Effects</b>: Constructs a basic_string with a default-constructed allocator,
@@ -1710,6 +1710,7 @@ class basic_string
    }
 
    //! <b>Requires</b>: s points to an array of at least n elements of CharT.
+   //!   If n is zero, s may be null.
    //!
    //! <b>Effects</b>: Replaces the string controlled by *this with a string of
    //! length n whose elements are a copy of those pointed to by s.
@@ -1719,7 +1720,11 @@ class basic_string
    //! <b>Returns</b>: *this
    basic_string& assign(const CharT* s, size_type n)
    {
-      BOOST_ASSERT(s != 0);
+      BOOST_ASSERT(s != 0 || n == 0);
+      if(n == 0){
+         this->clear();
+         return *this;
+      }
       return this->assign(s, s + difference_type(n));
    }
 

test/string_view_compat_test.cpp

@@ -172,6 +172,49 @@ void assign_test()
    BOOST_TEST(sv == s);
 }
 
+template<class StringViewType>
+void empty_assign_test()
+{
+   typedef typename StringViewType::value_type char_type;
+   typedef boost::container::basic_string<char_type> string_type;
+   const char_type empty[] = { char_type(0) };
+   const char_type value = char_type('y');
+   const StringViewType views[] = { StringViewType(), StringViewType(empty) };
+   const std::size_t sizes[] = { 0u, 7u, 128u };
+   for(std::size_t i = 0; i != sizeof(views)/sizeof(views[0]); ++i){
+      const StringViewType sv = views[i];
+      string_type constructed(sv);
+      BOOST_TEST(constructed.empty());
+      BOOST_TEST(constructed.c_str()[0] == char_type(0));
+      string_type constructed_with_allocator(sv, constructed.get_allocator());
+      BOOST_TEST(constructed_with_allocator.empty());
+      BOOST_TEST(constructed_with_allocator.c_str()[0] == char_type(0));
+
+      for(std::size_t j = 0; j != sizeof(sizes)/sizeof(sizes[0]); ++j){
+         string_type s(sizes[j], char_type('x'));
+         const std::size_t capacity = s.capacity();
+         BOOST_TEST(&s.assign(sv) == &s);
+         BOOST_TEST(s.empty());
+         BOOST_TEST(s.c_str()[0] == char_type(0));
+         BOOST_TEST(s.capacity() == capacity);
+
+         s.assign(sizes[j], char_type('x'));
+         BOOST_TEST(&(s = sv) == &s);
+         BOOST_TEST(s.empty());
+         BOOST_TEST(s.c_str()[0] == char_type(0));
+
+         s.assign(sizes[j], char_type('x'));
+         BOOST_TEST(&s.assign(sv.data(), sv.size()) == &s);
+         BOOST_TEST(s.empty());
+         BOOST_TEST(s.c_str()[0] == char_type(0));
+
+         s.assign(StringViewType(&value, 1u));
+         BOOST_TEST(s.size() == 1u);
+         BOOST_TEST(s[0] == value);
+      }
+   }
+}
+
 template<class StringViewType>
 void plus_equal_test()
 {
@@ -310,6 +353,7 @@ void test_all()
    constructor_test<StringViewType>();
    assignment_test<StringViewType>();
    assign_test<StringViewType>();
+   empty_assign_test<StringViewType>();
    plus_equal_test<StringViewType>();
    append_test<StringViewType>();
    insert_test<StringViewType>();
@@ -327,10 +371,11 @@ void test_all()
 int main()
 {
    test_all<boost::string_view>();
+   empty_assign_test<boost::wstring_view>();
    #ifdef BOOST_CONTAINER_TEST_HAS_STD_STRING_VIEW
    test_all<std::string_view>();
+   empty_assign_test<std::wstring_view>();
    #endif
 
    return boost::report_errors();
 }
-