Re: what is wrong with this code?

From:

Walter Bright <newshound1@digitalmars.com>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 20 Jul 2009 19:01:15 CST

Message-ID:

<h42uf2$omn$1@news.eternal-september.org>

Andrei Alexandrescu wrote:

void f ( void ) {}

struct B
{
     void f ( void ) {}
     typedef void (B::*mfn_t)( void );
};

template <typename T>
void loop( T fn )
{
     fn();
};

int main(int argc, char* argv[])
{
     loop(::f); //loop works OK

     B::mfn_t mfn(&B::f);
     B* pobject = new B;
     ((pobject)->*(mfn)) (); //member function B::f on instance
*pobject can be accessed OK

     //******************************
     loop(((pobject)->*(mfn))); // comment out to get correct compilation
     //******************************

     delete pobject;
     return 0;
}

The result of operators .* and ->* does not have a type. It's an anomaly
and the only place in C++ where an expression does not have a type. All
you can do with the result of these operators is apply the function
operator call to it immediately.

And, indeed, the Digital Mars C++ compiler indicates this:

======================================
      loop(((pobject)->*(mfn))); // comment out to get correct compilation
                              ^
test.cpp(25) : Error: '(' expected
======================================

----------------
Walter Bright
Digital Mars
http://www.digitalmars.com
free C, C++, D programming language compilers and compiler source code

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