Why would I want a .* operator in C++?

Question

I recently found out that the .* operator (and the closely related ->* operator) exists in C++. (See this question.)

Seems neat at first, but why would I ever need such a thing? The two answers in the linked question provided contrived examples which would benefit from a direct function call.

Where a direct function call is inconvenient, a function object could be used instead, like the lambda functions that may be used in std::sort. This removes a level of indirection and hence would be more performant than using .*.

The linked question also mentioned a simplified version of this example:

struct A {
    int a;
    int b;
};

void set_member(A& obj, int A::* ptr, int val){
    obj.*ptr = val;
}

int main()
{
    A obj;
    set_member(obj, &A::b, 5);
    set_member(obj, &A::a, 7);
    // Both members of obj are now assigned
}

But it's pretty trivial (perhaps even better practice because it's cleaner and isn't unnecessarily constrained to members of A) to do this instead:

struct A {
    int a;
    int b;
};

void set_me(int& out, int val){
    out = val;
}

int main()
{
    A obj;
    set_me(obj.b, 5);
    set_me(obj.a, 7);
    // Both members of obj are now assigned
}

In conclusion, a pointer-to-member-function might be replaced by a function object, and a pointer-to-member-variable might be replaced by a direct reference of said variable or a function object. Doing so might also increase the efficiency of the code due to one less indirection.

This question only provides examples where my conclusion stands, so it does not answer my question.

Apart from interfacing legacy code which uses .* (in which there would be no choice at all), when, really, would I want to use .*?

Answer 1

You could create collections of pointers to members and iterate over them. E.g.:

struct UserStrings
{
    std::string first_name;
    std::string surname;
    std::string preferred_name;
    std::string address;
};

...

std::array<std::string UserStrings::*, 4> str_cols = { &UserStrings::first_name, &UserStrings::surname, &UserStrings::preferred_name, &UserStrings::address };
std::vector<UserStrings> users = GetUserStrings();

for (auto& user : users)
{
    for (auto& column : str_cols)
    {
        SanitizeForSQLQuery(user.*column);
    }
}

Answer 2

Your example is too trivial to be illustrative. Consider a bit more complicated one

struct A {
    int a;
    int b;
};

void set_n_members(A objs[], unsigned n, int A::* ptr, int val)
{
  for (unsigned i = 0; i < n; ++i)
     objs[i].*ptr = val;
}

int main()
{
    A objs[100];
    set_n_members(objs, 100, &A::b, 5);
    set_n_members(objs, 100, &A::a, 7);
}

How would you rewrite this without int A::* ptr and without inducing code bloat?

Answer 3

It is used to implement std::mem_fn, which is used to implement std::function.

The following code shows how the ->* operator works in a naive Function class implemention.

Similarly, you can implement a member invoker class using the .* operator and a class reference.

#include <iostream>

class A
{
public:
    void greet()
    {
        std::cout << "Hello world"<<std::endl;
    }
};

template<typename R, typename ...TArgs>
class Invoker 
{
public:
    virtual R apply(TArgs&& ...args) = 0;
};

template<typename C, typename R, typename ...TArgs>
class MemberInvoker :public Invoker<R, TArgs...>
{
protected:
    C*                          sender;
    R(C::*function)(TArgs ...args);

public:
    MemberInvoker(C* _sender, R(C::*_function)(TArgs ...args))
        :sender(_sender)
        , function(_function)
    {
    }

    virtual R apply(TArgs&& ...args) override
    {
        return (sender->*function)(std::forward<TArgs>(args)...);
    }
};

template<typename T>
class Func
{
};

template<typename R, typename ...TArgs>
class Func<R(TArgs...)>
{
public:
    Invoker<R,TArgs...>* invoker=nullptr;

    template<typename C>
    Func(C* sender, R(C::*function)(TArgs...))
    {
        invoker =new MemberInvoker<C, R, TArgs...>(sender, function);
    }

    R operator()(TArgs&& ...args)
    {
        return  invoker->apply(std::forward<TArgs>(args)...);
    }

    ~Func()
    {
        if (invoker)
        {
            delete invoker;
            invoker = nullptr;
        }
    }
};

int main()
{
    A a;
    Func<void()> greetFunc(&a, &A::greet);
    greetFunc();
    system("PAUSE");
}

Answer 4

Let's say you wanted to write a LINQ style library for C++ that could be used something like this:

struct Person
{
    std::string first_name;
    std::string last_name;
    std::string occupation;
    int age;
    int children;
};

std::vector<Person> people = loadPeople();
std::vector<std::string> result = from(people)
     .where(&Person::last_name == "Smith")
     .where(&Person::age > 30)
     .select("%s %s",&Person::first_name,&Person::last_name);
for(std::string person : result) { ... };

Under the covers, the where function accepts an expression tree containing a pointer to member (among other stuff) and is applied to each vector item looking for one that matches. The select statement accepts a format string and some pointer to members and does an sprintf style formatting of whichever vector items get through the where statements.

I have written something like this, and there are several others out there that do it slightly differently (Is there a LINQ library for C++?). Pointer-to-member allows the library user to specify whichever members of their struct that they want and the library doesn't need to know anything about what they might do.