Fix #247: add to_basic_string().

Add to_basic_string

gsl/string_span

@@ -486,6 +486,16 @@ inline std::wstring to_string(wstring_span<> view)
 
 #endif
 
+template <typename CharT,
+	  typename Traits = typename std::char_traits<CharT>,
+	  typename Allocator = std::allocator<CharT>,
+	  typename gCharT,
+	  std::ptrdiff_t Extent>
+std::basic_string<CharT, Traits, Allocator> to_basic_string(basic_string_span<gCharT, Extent> view)
+{
+  return {view.data(), static_cast<size_t>(view.length())};
+}
+  
 // zero-terminated string span, used to convert
 // zero-terminated spans to legacy strings
 template <typename CharT, std::ptrdiff_t Extent = dynamic_extent>

tests/string_span_tests.cpp

@@ -114,6 +114,18 @@ SUITE(string_span_tests)
         CHECK(s2.length() == 5);
     }
 
+    TEST(TestToBasicString)
+    {
+        auto s = gsl::to_basic_string<char,std::char_traits<char>,::std::allocator<char>>(cstring_span<>{});
+        CHECK(s.length() == 0);
+
+        char stack_string[] = "Hello";
+        cstring_span<> v = ensure_z(stack_string);
+        auto s2 = gsl::to_basic_string<char,std::char_traits<char>,::std::allocator<char>>(v);
+        CHECK(static_cast<cstring_span<>::index_type>(s2.length()) == v.length());
+        CHECK(s2.length() == 5);
+    }
+    
     TEST(EqualityAndImplicitConstructors)
     {
         {