Is There a Indirection Functor?

问题

I'm looking for a unary functor which will dereference it's argument and return the result. Of course I can write one, it just seemed like something should already exist.

So given the code:

const auto vals = { 0, 1, 2, 3 };
vector<const int*> test(size(vals), nullptr);

iota(begin(test), end(test), data(vals));

transform(cbegin(test), cend(test), ostream_iterator<int>(cout, " "), [](const auto& i){ return *i; });

Live Example

I was hoping that there was a functor that I could use instead of the lambda. Does such a thing exist, or do I need to just use the lambda?

回答1:

Assuming that by "functor" you mean "function object" or "callable object", there doesn't seem to be what you desire in the Standard Library.

It is trivial to implement it yourself:

struct deferencer
{
    template <typename T>
    decltype(auto) operator()(T&& x) const
        noexcept(noexcept(*x))
    { 
        return *x; 
    }
};

---

Note that your lambda doesn't do what you expect, as its implicit return type is -> auto, which makes a copy. One possible correct lambda is:

[](const auto& i) -> decltype(auto) { return *i; }

If you don't specify an explicit trailing return type for a lambda, the implicit one will be auto which is always a copy. It doesn't matter if operator* returns a reference, since the lambda returns a copy (i.e. the reference returned by operator* is then copied by the lambda's return statement).

struct A
{
    A() = default;
    A(const A&) { puts("copy ctor\n"); }
};

int main()
{
    []{ return *(new A); }(); // prints "copy ctor"
}

wandbox example

来源：https://stackoverflow.com/questions/41487287/is-there-a-indirection-functor