Re: Pointer to virtual method in child class as template parameter

From:

Barry Ding <dhb2000@gmail.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Thu, 19 Jul 2007 09:56:56 CST

Message-ID:

<1184836742.270670.217100@x40g2000prg.googlegroups.com>

On 7 17 , 11 12 , Stephan Tolksdorf <andor...@gmx.de> wrote:

Hi all,

Could maybe somebody explain why all the compilers (Comeau, GCC, MSVC)
I tested printed error messages similar to

"error: argument of type "void (A::*)()" is incompatible with template
parameter of type "BMethodPointer"
D<&B::doSomething> d;
   ^
"

when compiling the following code snippet.

--
#include <iostream>

class A { // interface
public:
     virtual void doSomething() = 0;

};

class B : public A { }; // extended interface

class C : public B { // implementation
     virtual void doSomething() {
         std::cout << "test";
     }

};

typedef void (B::* BMethodPointer)();

template <BMethodPointer p>
class D {
     D() { // constructor that calls p on a C instance
         C c;
         (c.*p)();
     }

};

int main() {
     BMethodPointer p = &B::doSomething; // this compiles
     C c;
     (c.*p)();
     D<&B::doSomething> d; // but this doesn't}

--

None of the compilers had a problem when the last code line was
commented out.

Is this a language "feature" or a surprisingly common bug?

But the code works on MSVC6.0
By the way, D ctor should be public

change B to the following, then the code works on ICC 9.1 and MSVC 8.0
class B : public A {
public:
    virtual void doSomething() = 0;
}; // extended interface

My humble explanation is that non-type template parameter does not
allow implicit type conversion

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