boost::bind to class member with null object reference - boost-bind

Question

Take the following example
class foo {
public:
foo() {
cout << "foo has been constructed" << endl;
}
~foo() {};
void DoSomething( int i ) {
cout << "integer = " << i << endl;
}
};
int main() {
auto b = boost::bind( &foo::DoSomething,(foo*)0,_1);
b( 250 );
}
It compiles fine( this doesn't suprise me). But when I call b(), it runs fine. How is this the case? I expected, because I wasn't creating an instance of foo that calling DoSomething would case a run time problem.
Can someone explain where the instance of foo is being created? Since when I run it, I do not see the construction message printed.

Answer 1

foo does not get created at all.
boost-bind takes a pointer to the function. It does not care if it is C style function, a method or class function.
When you give an object instance to boost bind it just uses this object as first parameter. Since you do not require any of the instance members within DoSomething, you do not see any effect of the NULL instance.
Try and modify your example:
class foo {
public:
std:string* test_ptr;
foo() {
test_ptr=new std::string("Test");
cout << "foo has been constructed" << endl;
}
~foo() {};
void DoSomething( int i ) {
cout << "integer = " << i << endl;
cout << this->test_ptr << endl;
this->*test_ptr = "Test2"; // this is really messy with your example
}
static void DoSomething2 ( foo* sameAsThis_ptr, int i) {
this->*test_ptr = "Test3"; // compile error
// same behavior as DoSomething1
cout << "integer = " << i << endl;
cout << sameAsThis_ptr->test_ptr << endl;
sameAsThis_ptr->*test_ptr = "Test4"; // this is really messy with your example
}
};
int main() {
auto b = boost::bind( &foo::DoSomething,(foo*)0,_1);
b( 250 );
auto c = boost::bind ( &foo::DoSomething2, (foo*)NULL; _1);
c( 300 );
}
If you want to learn more you can take a closer look at "function pointers". Boost bind is just a very sophisticated way to make use of function pointers.
It wrap function pointers to an object, making functions to objects ("object functional programming")