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

From: (Stefan =?iso-8859-1?Q?Gro=DFe?= Pawig)
Sun, 29 Jul 2007 09:10:22 CST
  Hi Stephan!

Stephan Tolksdorf <> writes:

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.

[added below]

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

This is indeed a language feature. The standard defines several
restrictions on conversions for template non-type arguments (14.3.2).
Relevant for your example is 14.3.2/5, the second to last bullet:

- For a nontype template-parameter of type pointer to member function,
  no conversions apply. If the template-argument represents a set of
  overloaded member functions, the matching member function is
  selected from the set (13.4).

#include <iostream>

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

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

B does not declare doSomething(), so any reference or pointer to
B::doSomething actually refers to A::doSomething.

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;

int main() {
     BMethodPointer p = &B::doSomething; // this compiles

Here we have a "pointer to member" conversion according to 4.11/2.

     C c;
     D<&B::doSomething> d; // but this doesn't

No conversion because of 14.3.2/5.


You can get your code to work if you redeclare doSomething() in
class B (and make the constructor of class D public):
class A { // interface
     virtual void doSomething() = 0;

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

Kind regards,
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