Forcing auto to be a reference type in a range for

问题:

Suppose I have foo which is a populated std::vector<double>.

I need to operate on the elements of this vector. I'm motivated to write

for (auto it : foo){
   /*ToDo - Operate on 'it'*/
}

But it appears that this will not write back to foo since it is a value type: a deep copy of the vector element has been taken.

Can I give some guidance to auto to make it a reference type? Then I could operate directly on it.

I suspect I'm missing some trivial syntax.

回答1:

A minimal auto reference

The loop can be declared as follows:

for (auto& it : foo) {
   //    ^ the additional & is needed
   /*ToDo - Operate on 'it'*/
}

This will allow it to be a reference to each element in foo.

There is some debate as to the "canonical form" of these loops, but the auto& should do the trick in this case.

General auto reference

In a more general sense (outside the specifics of the container), the following loop works (and may well be preferable).

for (auto&& it : container) {
   //    ^ && used here
}

The auto&& allows for bindings to lvalues and rvalues. When used in a generic or general (e.g. template situation) this form may strike the desired balance (i.e. references, copies, prvalue/xvalue returns (e.g. proxy objects) etc.).

Favour the general auto&&, but if you have to be specific about the form, then use a more specific variation (e.g. auto, auto const& etc.).

Why is auto&& better?

As noted in other answers here and the comments. Why is auto&& better? Simply it will do what you think it should in most cases, see this proposal and its update.

As always, Scott Meyers' blog about this also makes for a good read.

回答2:

I would use auto&&:

for (auto&& it : foo) {
    // bla
}

The reason is spelt out in N3994 "Range-Based For-Loops: The Next Generation (Revision 1)" that it would better work with proxy objects (such as those coming from std::vector<bool>).

In fact, that proposal for C++1z (supported already by Clang 3.5 SVN in -std=c++1z mode) proposes the syntax:

// c++1z only
for (it : foo) { 
    // bla
}

as a short-hand for for (auto&& it : foo).

Update: the N3994 proposal was not voted into the C++17 working paper.

回答3:

You may use:

for (auto&& it : foo){

}

auto && is prefered to auto & to manage proxy iterator as for std::vector<bool>.

回答4:

In many ways this confusion is arising from the convention that's grown up over the years to bind a * or a & to the type as opposed to the variable.

For example int* a is really int *a; i.e. a is a pointer to a value of type int.

The same applies to references: in the case of int& a, a is a reference to a value of type int.

So what you really want to do is write for (auto &it : foo) so it is a reference to the type inferred by auto. Then you can use it to manipulate the underlying vector elements. More often than not this will be written as

for (auto& it : foo)

Moving forward, you might want to use an r-value reference: for (auto&& it : foo) which is probably the best general form.