Fix visitor to be able to return a reference (issue #39, thanks @ligfx)

martinmoene · Apr 11, 2020 · ab1ffb3 · ab1ffb3
1 parent a0a1be0
commit ab1ffb3
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Showing 3 changed files with 27 additions and 4 deletions.
diff --git a/include/nonstd/variant.hpp b/include/nonstd/variant.hpp
@@ -1919,7 +1919,8 @@ struct VisitorApplicatorImpl<R, TX<VT> >
     template< typename Visitor, typename T >
     static R apply(Visitor const&, T)
     {
-        return R();
+        // prevent default construction of a const reference, see issue #39:
+        std::terminate();
     }
 };
 
@@ -2106,7 +2107,8 @@ struct VisitorApplicator
             case 14: return apply_visitor<14>(v, arg);
             case 15: return apply_visitor<15>(v, arg);
 
-            default: return R();
+            // prevent default construction of a const reference, see issue #39:
+            default: std::terminate();
         }
     }
 

diff --git a/template/variant.hpp b/template/variant.hpp
@@ -1660,7 +1660,8 @@ struct VisitorApplicatorImpl<R, TX<VT> >
     template< typename Visitor, typename T >
     static R apply(Visitor const&, T)
     {
-        return R();
+        // prevent default construction of a const reference, see issue #39:
+        std::terminate();
     }
 };
 
@@ -1734,7 +1735,8 @@ struct VisitorApplicator
             {% for n in range(NumParams) -%}
             case {{n}}: return apply_visitor<{{n}}>(v, arg);
             {% endfor %}
-            default: return R();
+            // prevent default construction of a const reference, see issue #39:
+            default: std::terminate();
         }
     }
 

diff --git a/test/variant.t.cpp b/test/variant.t.cpp
@@ -1604,4 +1604,23 @@ CASE("max_index(): should not cause erroneous bad_variant_access in get()" "[.is
     }
 }
 
+namespace issue_39 {
+
+    struct tag_t {};
+
+    struct myvisitor
+    {
+        const std::string& operator()(const int&) const { throw std::exception(); }
+        const std::string& operator()(const tag_t&) const { throw std::exception(); }
+        const std::string& operator()(const std::string& s) const { return s; }
+    };
+}
+
+CASE("visitor: Visitors can't return references, but they can with std::variant" "[.issue-39]")
+{
+    using namespace issue_39;
+    nonstd::variant<int, tag_t, std::string> v("hello");
+    nonstd::visit( myvisitor(), v );
+}
+
 // end of file