How to directly bind a member function to an std::function in Visual Studio 11?

Question

I can easily bind member functions to a std::function by wrapping them with a lambda expression with capture clause.

class Class
{
    Class()
    {
        Register([=](int n){ Function(n); });
    }

    void Register(std::function<void(int)> Callback)
    {

    }

    void Function(int Number)
    {

    }
};

But I want to bind them directly, something like the following.

// ...
Register(&Class::Function);
// ...

I think according to the C++11 standard, this should be supported. However, in Visual Studio 11 I get these compiler errors.

error C2440: 'newline' : cannot convert from 'int' to 'Class *'

error C2647: '.*' : cannot dereference a 'void (__thiscall Class::* )(int)' on a 'int'

Answer 1

I think according to the C++11 standard, this should be supported

Not really, because a non-static member function has an implicit first parameter of type (cv-qualified) YourType*, so in this case it does not match void(int). Hence the need for std::bind:

Register(std::bind(&Class::Function, PointerToSomeInstanceOfClass, _1));

For example

Class c;
using namespace std::placeholders; // for _1, _2 etc.
c.Register(std::bind(&Class::Function, &c, _1));

Edit You mention that this is to be called with the same Class instance. In that case, you can use a simple non-member function:

void foo(int n)
{
  theClassInstance.Function(n);
}

then

Class c;
c.Register(foo);

Answer 2

According to Stephan T. Lavavej - "Avoid using bind(), ..., use lambdas". https://www.youtube.com/watch?v=zt7ThwVfap0&t=32m20s

In this case:

Class()
{
    Register([this](int n){ Function(n); });
}

Answer 3

You can use std::bind:

using namespace std::placeholders;  // For _1 in the bind call

// ...

Register(std::bind(&Class::Function, this, _1));

Answer 4

In C++ 17, you can use:

Register([=](auto && ...args){ return Function(args...); });

which is sweet especially if the argument list is longer long. Of course the member function's argument list must then be compatible with the std::function's ones.

Answer 5

With std::function and std::bind, you can treat different class member function the same.

#include <iostream>
#include <functional>
#include <vector>
using namespace std;
using namespace std::placeholders;

class Foo
{
public:
    void foo(const string &msg)
    {
        cout << msg << '\n';
    }
};

class Bar
{
public:
    void bar(const string &msg, const string &suffix)
    {
        cout << msg << suffix << '\n';
    }
};

int main(int argc, char **argv)
{
    Foo foo;
    Bar bar;

    vector<function<void (const string &msg)>> collection;
    collection.push_back(bind(&Foo::foo, &foo, _1));
    collection.push_back(bind(&Bar::bar, &bar, _1, "bar"));

    for (auto f : collection) {
        f("foo");
    }

    return 0;
}

Answer 6

As others have already stated, you can use std::bind. The syntax is pretty ugly though. As such, I offer these macros:

#define bindToThis( className, funcName ) \
    std::bind( &className::funcName, this, std::placeholders::_1 )

#define bindToThat( className, funcName, objPtr ) \
    std::bind( &className::funcName, objPtr, std::placeholders::_1 )

Here's an example implementation of the macro, after first typedefing the std::function to define the required signature (which makes that easier to read/use as well):

typedef std::function<void(const int)> MyIntHandler;

class MyClass 
{
public:
    MyClass( const MyIntHandler handler );
};

...
SomeOtherClass::SomeOtherClass() 
    : myObjPtr_( new MyClass( 
        MyIntHandler( bindToThis( SomeOtherClass, handleSomeInt ) ) )
{}

void SomeOtherClass::handleSomeInt( const int i )
{
    // Do something...
}