Re: Inheritance not letting signature to come through.

From:

James Kanze <james.kanze@gmail.com>

Newsgroups:

comp.lang.c++

Date:

25 May 2007 15:15:50 -0700

Message-ID:

<1180131350.403640.172990@q69g2000hsb.googlegroups.com>

On May 25, 7:29 pm, Alan Johnson <a...@yahoo.com> wrote:

Adrian Hawryluk wrote:

Hi, I was posed this question and I didn't know the answer. Anybody
here know why a member function of the same name but different signature
from that of a class it inherits from is not viable unless explicitly
made so using the 'using' keyword?

Here is an example:

  1 class C1
  2 {
  3 public:
  4 void M1(int i) {}
  5 };

  8 class C2: public C1
  9 {
10 public:
11 // using C1::M1; //< When commented, g++ and VC++ emit error
12 void M1(int i, int j) {}
13 };

15 int main()
16 {
17 C2 c;
18 c.M1(14); //< Error emitted here
19 c.M1(1, 2);
20 return 0;
21 }

g++ emits:
  18: error: no matching function for call to `C2::M1(int)'
  12: note: candidates are: void C2::M1(int, int)

Because the standard says so in 10.2.2:
"A member name f in one sub-object B hides a member name f in a
sub-object A if A is a base class sub-object of B. Any declarations
that are so hidden are eliminated from consideration."

Someone around here probably knows the actual justification for that
being in the standard.

One possible reason is so that adding functions to the base
class won't break the derived class. Consider something like:

    class Base
    {
    } ;

    class Derived : public Base
    {
    public:
        void f( int i ) { /* ... */ }
        void g( char c ) { /* ... */ ; f( c ) ; /* ... */ }
    } ;

What happens now if you add a function "void f( int )" to base?