C.138 initial content

CppCoreGuidelines.md

@@ -6561,11 +6561,49 @@ This a relatively rare use because implementation can often be organized into a
 
 ##### Reason
 
-???
+Without a using declaration, member functions in the derived class hide the entire inherited overload sets.
 
-##### Example
+##### Example, bad
 
-    ???
+    #include <iostream>
+    class B {
+    public:
+        virtual int f(int i) { std::cout << "f(int): "; return i; }
+        virtual double f(double d) { std::cout << "f(double): "; return d; }
+    };
+    class D: public B {
+    public:
+        int f(int i) override { std::cout << "f(int): "; return i+1; }
+    };
+    int main()
+    {
+        D d;
+        std::cout << d.f(2) << '\n';   // prints "f(int): 3"
+        std::cout << d.f(2.3) << '\n'; // prints "f(int): 3"
+    }
+
+##### Example, good
+
+    class D: public B {
+    public:
+        int f(int i) override { std::cout << "f(int): "; return i+1; }
+        using B::f; // exposes f(double)
+    };
+
+##### Note
+
+This issue affects both virtual and non-virtual member functions
+
+For variadic bases, C++17 introduced a variadic form of the using-declaration,
+
+    template <class... Ts>
+    struct Overloader : Ts... {
+        using Ts::operator()...; // exposes operator() from every base
+    };
+
+##### Enforcement
+
+Diagnose name hiding
 
 ### <a name="Rh-final"></a>C.139: Use `final` sparingly