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 function objects. However, when used with member functions, it can raise questions.

First Argument Referencing

The first argument to std::bind, when it involves a member function, is always a reference (&). This is not due to std::bind's implementation but rather the nature of member functions. Member functions require an object they can operate upon. In the case of std::bind, this object is passed in via the reference.

Second Argument as Object Reference

The second argument to std::bind when dealing with member functions specifies the object to be used as the context for the function call. Since member functions are always bound to a particular class, it is essential to provide an instance of that class.

Internally, std::bind detects the presence of a member function and automatically handles the creation of a callable object that incorporates the object specified in the second argument. This callable object takes the object's reference or pointer as its first argument and the function pointer as its second.

Example

Consider the following example:

<code class="cpp">struct Foo {
    void print_sum(int n1, int n2) { std::cout << n1 + n2 << '\n'; }
    int data = 10;
};

Foo foo;

auto f = std::bind(&Foo::print_sum, &foo, 95, _1);
f(5);</code>

In this example, the second argument to std::bind is &foo, which provides the reference to the foo object. The callable object created by std::bind takes &foo as its first argument and &Foo::print_sum as its second.

Understanding these concepts helps clarify how std::bind works with member functions and enables you to effectively utilize this feature in your C code.

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