Understanding bind - c++

Question

I have a bit of trouble understanding a std::bind call.
In the following example:
#include <functional>
#include <iostream>
#include <memory>
class Notifier
{
public:
Notifier(std::function<void(Notifier&)> on_notify)
:on_notify_(on_notify)
{ }
void notify()
{
if (on_notify_)
on_notify_(*this);
}
std::function<void(Notifier&)> on_notify_;
};
struct Manager
{
Manager()
{
n_ = std::make_unique<Notifier>(std::bind(&Manager::trigger, this));
}
void trigger()
{
std::cout << "notified" << std::endl;
}
std::unique_ptr<Notifier> n_;
};
int main()
{
Manager s;
s.n_->notify();
}
I don't understand how on_notify_(*this); calls back the functor with a Notifier& parameter, but the functor created by bind doesn't specify it.
The calls result correctly to the void notify() method, but I don't understand what exactly will be the functor created by bind to result in this.
If I were to write a lambda instead, I would need to specify the parameter, otherwise it would compile.
What kind of operation does bind here behind my back? :-)

Answer 1

std::bind basically ignores the invalid given argument according to this.
If some of the arguments that are supplied in the call to g() are not matched by any placeholders stored in g, the unused arguments are evaluated and discarded.
It might surprise you that when even more absurd arguments are provided, the binded functor can still successfully reach Manager::trigger() as follows:
#include <functional>
#include <iostream>
#include <memory>
// Some classes that have nothing to do with on_notify_
class AAA {};
class BBB {};
class Notifier
{
public:
Notifier(std::function<void(AAA&, BBB&)> on_notify)
:on_notify_(on_notify)
{ }
void notify()
{
if (on_notify_)
{
// Arguments not matching.
AAA a{};
BBB b{};
// Invoke with them.
on_notify_(a, b);
}
}
std::function<void(AAA&, BBB&)> on_notify_;
};
struct Manager
{
Manager()
{
n_ = std::make_unique<Notifier>(std::bind(&Manager::trigger, this));
}
void trigger()
{
std::cout << "it's also notified!" << std::endl;
}
std::unique_ptr<Notifier> n_;
};
int main()
{
Manager s;
s.n_->notify();
}
Live demo is here.