How to use bind correctly here? - c++

Question

I can not figure out the correct syntax to bind member functions.
If I have a function that takes a function with a single argument,
how do I pass an object to it?
In the following example, what would be the correct syntax of passing the function?
#include <iostream>
#include <functional>
void caller(std::function<void(int)> f)
{
f(42);
}
class foo
{
public:
void f(int x)
{
std::cout<<x<<std::endl;
}
};
int main()
{
foo obj;
caller(std::bind(&foo::f,obj));
//^Wrong
}
Error was:
a.cpp: In function ‘int main()’:
a.cpp:18:34: error: could not convert ‘std::bind(_Func&&, _BoundArgs&& ...) [with _Func = void (foo::*)(int); _BoundArgs = {foo&}; typename std::_Bind_helper<std::__or_<std::is_integral<typename std::decay<_Tp>::type>, std::is_enum<typename std::decay<_Tp>::type> >::value, _Func, _BoundArgs ...>::type = std::_Bind<std::_Mem_fn<void (foo::*)(int)>(foo)>]((* & obj))’ from ‘std::_Bind_helper<false, void (foo::*)(int), foo&>::type {aka std::_Bind<std::_Mem_fn<void (foo::*)(int)>(foo)>}’ to ‘std::function<void(int)>’
caller(std::bind(&foo::f,obj));

Answer 1

Placeholders create a "space" for the actual arguments to be bound later:
int main()
{
foo obj;
caller(std::bind(&foo::f, &obj, std::placeholders::_1));
// ^ placeholder
// ^ address or even foo()
}
These placeholders are needed to correctly generate an appropriate signature for the std::bind result to be bound to the std::function<void(int)>.
You may also want to use the address of your object, or std::ref (so it won't be copied); this will vary on what you want the semantics to be.

Answer 2

There is an implicit first argument to member functions, which is the this point. You need to send it as a pointer; you also need a placeholder for the int argument. So:
caller(std::bind(&foo::f, &obj, std::placeholders::_1));
// ^ ^^^^^^^^^^^^^^^^^^^^^^^

Answer 3

You need to specify that the created function object takes a parameter using a placeholder:
std::bind(&foo::f,obj, std::placeholders::_1)