Re: passing function object pointer to for_each argument

From:

Markus Moll <moll.markus@arcor.de>

Newsgroups:

comp.lang.c++.moderated

Date:

Mon, 1 Oct 2007 08:15:31 CST

Message-ID:

<46fbd3bb$0$16103$9b4e6d93@newsspool1.arcor-online.net>

Hi

hongseok.yoon@gmail.com schreef:
[class test_b derived from test_a and overriding _test]

int main()
{
    test_a* a = new test_a;
    test_a* b = new test_b;

    vector<int> v;
    v.push_back(1);

    for_each(v.begin(), v.end(), *a);
    for_each(v.begin(), v.end(), *b);

    delete a;
    delete b;

    return 0;
}

Result :
test_a::_test()
test_a::_test()
------------------------------------------------
Why the result is not...
test_a::_test()
test_b::_test()
???

tell me why and how can I fix it?

Slicing. The problem is that template argument deduction chooses
"test_a" as the type of the second parameter. So for_each will only ever
see a test_a object, which is a copy of the test_a sub-object of *b.

To avoid this, I see two options:

  1. explicitly instantiate for_each with a reference parameter:
     for_each<vector<int>::iterator, test_a&>(v.begin(), v.end(), *b);

But I'm not completely certain if this is guaranteed to work.

  2. use a combination of boost::function and boost::ref:
     for_each(v.begin(), v.end(),
       boost::function<void(int)>(boost::ref(*b)));

Markus

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