I think when the type is very well-known amongst the co-programmers who work (or would work) in your project, then auto can be used, such as in the following code:
Or, more generally,
But when the type is not very well-known and infrequently used. then I think auto seems to reduce readability, such as here:
While in the former case, the usage of auto seems very good and doesn't reduce readability, and therefore, can be used extensively, but in the latter case, it reduces readabilty and hence shouldn't be used.
Another place where auto can be used is when you use new. such as here:
Here it is very good, as it reduces the use of keyboard, without reducing the readability, as anyone can know the type of pObject just by looking at the code.
Sometime, the type is so irrelevant that the knowledge of the type is not even needed, such as in expression template ; in fact, practically it is impossible to write the type (correctly), in such cases auto is a relief
for programmers. I've written expression template library which can be used as:
Now compare the above code with the following equivalent code which doesn't use auto :
As you can see, in such cases auto makes your life exponentially easier. The expressions used above are very simple; think about the type of some more complex expressions:
The type of such expressions would be even more huge and ugly, but thanks to auto. we now can let the compiler infer the type of the expressions.
So the bottomline is: the keyword auto might increase or decrease clarity and readability of your code, depending on the context. If the context makes it clear what type it is, or at least how it should be used (in case of standard container iterator) or the knowledge of the actual type is not even needed (such as in expression templates), then auto should be used. and if the context doesn't make it clear and isn't very common (such as the second case above), then it should better be avoided .