Binding Virtual Functions in Parent Ctor

Question

Given this code:

class foo
{
public:
    foo() : _myFunc( bind( &foo::testCall, this ) ){}
    virtual void testCall(){ cout << "foo" << endl; }
    void call(){ _myFunc(); }
private:
    function< void() > _myFunc;
};

class bar: public foo
{
public:
    virtual void testCall(){ cout << "bar" << endl; }
};

void main()
{
    bar test;
    test.call();
}

Why does it print "bar". I read this issue and would have thought that "foo" would have been printed.

What will really bake you noodle is the output when invoking `call()` from `foo`'s constructor in addition to your invoke from `test.call()` in `main()`. — WhozCraig, Jul 17 '14 at 12:48
@πάνταῥεῖ, Given the proper includes and using directive, and changing the return type, it [compiles and prints "bar"](http://coliru.stacked-crooked.com/a/46ef39dfc458b02d) with GCC 4.9. With Clang and libc++, it doesn't compile. Your example did not include ``. — chris, Jul 17 '14 at 12:48
@ πάνταῥεῖ and @ chris this compiles fine in VS2012 — Jonathan Mee, Jul 17 '14 at 12:50

score 1 · Accepted Answer · edited May 23 '17 at 12:13

1

You are not calling the virtual function in the constructor you are binding to a member variable, and latter calling that variable (in this case dynamic dispatch is used).

The calling code should be: test.call();

More info Boost::Bind and virtual function overloads: why do they work?

edited May 23 '17 at 12:13

Community

1
1

answered Jul 17 '14 at 12:42

NetVipeC

4,402
1
17
19

So I get what you are saying but in `foo`'s ctor I would have expected a hard coded address not one that would thunk. – Jonathan Mee Jul 17 '14 at 12:47
Link added with more explaining about the behavior. – NetVipeC Jul 17 '14 at 12:49
@JonathanMee its the address of a virtual member function, which is typically a base+offset into the virtual method table of the instance. By the time the object is fully constructed the member function address at that offset is `bar`'s – WhozCraig Jul 17 '14 at 12:50

Binding Virtual Functions in Parent Ctor

1 Answers1