Should I use the new C++11 'auto' feature, especially in loops?

Question

What are the pros/cons to using the auto keyword, especially in for loops?

for(std::vector<T>::iterator it = x.begin(); it != x.end(); i++)
{
   it->something();
}

for(std::map<T>::iterator it = x.begin(); it != x.end(); i++)
{
   it->second->something();
}

for(auto it = x.begin(); it != x.end(); i++)
{
   it->??
}

Seems like if you don't know whether you have an iterator for a map or a vector you wouldn't know whether to use first or second or just directly access properties of the object, no?

This reminds me of the C# debate on whether to use the keyword var. The impression I'm getting so far is that in the C++ world people are ready to adopt the auto keyword with less of a fight than var in the C# world. For me my first instinct is that I like to know the type of the variable so I can know what operations I can expect to perform on it.

Answer 1

In your example:

for(auto it = x.begin(); it != x.end(); i++)
{
  it->??
}

there has to be a declaration for x visible. Therefore the type of it should be obvious. If the type of x isn't obvious, then the method is too long, or the class is too large.

Answer 2

The motivations in C++ are more extreme, as types can become vastly more convoluted and complex than C# types due to metaprogramming and other things. auto is faster to write and read and more flexible/maintainable than an explicit type. I mean, do you want to start typing

boost::multi_map<NodeType, indexed_by<ordered_unique<identity<NodeType>>, hashed_non_unique<identity<NodeType>, custom_hasher>>::iterator_type<0> it

That's not even the full type. I missed off a couple template arguments.

Answer 3

YES, you should! auto does not erase the type; even if you "don't know" what x.begin() is, the compiler knows and will report an error if you try to use the type wrongly. Also, it is not unusual to emulate map with a vector<pair<Key,Value>>, so the code using auto will work for both dictionary representations.

Answer 4

Objection! Loaded question.

Can you explain to me why the third code has ?? in it, yet the first and second don’t? For fairness’ sake, your code must read as follows:

for(std::vector<T>::iterator it = x.begin(); it != x.end(); i++)
{
   it->???
}

for(std::map<T>::iterator it = x.begin(); it != x.end(); i++)
{
   it->second->???
}

There. Same problem even though you didn’t use auto.

And in all cases, the answer is the same: Context matters. You can’t meaningfully talk about a piece of code in isolation. Even if you hadn’t used templates but some concrete type, this would only have moved the problem somewhere else, since the reader of your code would have to know about the declaration of said type.

If use of auto in those situations render your code unreadable you should treat this as a warning sign that there’s something wrong with your code design. Of course, there are cases where low-level details matter (such as when dealing with bit operations or legacy API) in which cases an explicit type might aid readability. But in general – no.

Regarding var (since you explicitly mentioned it), there’s also a vast consensus in the C# community for using var. Arguments against its use are generally built on fallacies.

Answer 5

What are loops and how we can use them?

In general a loop is a sequence of statements which is specified once but which may be carried out several times in succession. The function of the loop is to repeat the calculation a given number of times until it reaches a certain value. The code inside the loop is obeyed a specified number of times, or once for each of a collection of items, or until some condition is met, or indefinitely. We can divide loops into: 1) Count-controlled loops, 2) Condition – controlled loops, 3) Collection - controlled loops, 4) General iteration, 5) Infinite loops, 6) Continuation with the next iteration, 7) Redo the current interation and 8) Restart loop

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