How does std::bind handle member functions in C ?

Understanding std::bind with Member Functions

std::bind is a powerful tool in C for creating callable objects. When working with member functions, it introduces some unique aspects that can be confusing. Let's explore how std::bind interprets and utilizes member functions.

The First Argument: Why a Pointer?

The first argument to std::bind is a callable object, which can be a function, lambda, or even a member function. When using a member function, it's essential to understand that the first argument is a pointer to that member function. This is because all non-static member functions are stored as pointers within an object.

To get a pointer to a member function, the & operator is used. For instance, in the following code:

Foo foo;
auto f = std::bind(&Foo::print_sum, &foo, 95, _1);

&Foo::print_sum returns a pointer to the print_sum member function within the class Foo.

The Second Argument: Referencing the Object

The second argument to std::bind when using a member function is a reference to the object that the member function belongs to. This is because member functions need access to the object they're associated with.

In the example above, f needs to be able to access the foo object. By providing the &foo reference as the second argument, std::bind ensures that any calls made to f will be bound to the foo object.

std::bind's Internal Handling

Internally, std::bind analyzes the first argument and determines if it's a pointer to a member function. If so, it converts it into a suitable callable object using std::mem_fn() or a similar mechanism. This callable object encapsulates both the pointer to the member function and the reference to the object.

An Example

To better illustrate, consider the following code:

struct Foo {
    void print(int n) { std::cout << n << '\n'; }
};

Foo foo;

std::bind(&Foo::print, &foo, 42)();

In this example, std::bind creates a callable object that combines the print member function and a reference to the foo object. When this callable object is invoked, it effectively calls foo.print(42).

Conclusion

Understanding how std::bind works with member functions is crucial for effectively using this technique. Remember that the first argument is a pointer to the member function and the second argument is a reference to the object. std::bind handles these arguments internally to create callable objects that provide a convenient way to bind member functions to specific objects.

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