当我应该使用std :: map :: at来检索map元素

Question

I have read different articles on web and questions at stackoverflow , but for me it is not clear is there any exclusive case when it is better to use std::map::at to retrieve map element.

According to definition , std::map::at

Returns a reference to the mapped value of the element identified with key k.

If k does not match the key of any element in the container, the function throws an out_of_range exception.

For me only case when it is worth to use std::map::at when you 100% sure that element with particular key exist, otherwise you should consider exception handling.

1. Is there any case where std::map::at considered as most efficient and elegant way to do? In what cases you will recommend to use std::map::at ?
2. Am I right that it is better to use map::find() when there is a possibility to not have element with such a key? And map::find() it is faster and more elegant approach?

 if ( map.find("key") != map.end() ) { // found } else { // not found } 

ps

map::operator[] sometimes can be dangerous, because if an element doesn't exist then it will inserts it.

EDITED: links somehow related link 1 link 2 link 3 link 4 link 5 link 6

Answer 1

Contrary to most existing answers here, note that there are actually 4 methods related to finding an element in a map (ignoring lower_bound , upper_bound and equal_range , which are less precise):

• operator[] only exist in non-const version, as noted it will create the element if it does not exist
• at() , introduced in C++11, returns a reference to the element if it exists and throws an exception otherwise
• find() returns an iterator to the element if it exists or an iterator to map::end() if it does not
• count() returns the number of such elements, in a map , this is 0 or 1

Now that the semantics are clear, let us review when to use which:

• if you only wish to know whether an element is present in the map (or not), then use count() .
• if you wish to access the element, and it shall be in the map , then use at() .
• if you wish to access the element, and do not know whether it is in the map or not, then use find() ; do not forget to check that the resulting iterator is not equal to the result of end() .
• finally, if you wish to access the element if it exists or create it (and access it) if it does not, use operator[] ; if you do not wish to call the type default constructor to create it, then use either insert or emplace appropriately

Answer 2

std::map::at() throws an out_of_range exception if the element could not be found. This exception is a kind of logic_error exception which for me is a kind of synonym of assert() from the usage standpoint: it should be used to report errors in the internal logic of the program, like violation of logical preconditions or class invariants.

Also, you can use at() to access const maps.

So, for your questions:

1. I will recommend using at() instead of [] when accessing const maps and when element absence is a logic error.
2. Yes, it's better to use map::find() when you're not sure element is here: in this case it's not a logic error and so throwing and catching std::logic_error exception will not be very elegant way of programming, even if we don't think about performance.

Answer 3

As you noted, there are three different ways to access elements in a map: at() , operator[] and find() (there are also upper_bound , lower_bound and equal_range , but those are for more complicated circumstances where you might want to find a next/previous element etc.)

So, when should you use which one?

operator[] is basically "if it does not exist, create one with a default-constructed mapped element". That means it won't throw (except in the corner cases when the memory allocation throws or one of the key or value constructors throw), and you definitely get a reference to the element you looked for - either the existing one or the newly created.

at() throws if there is no element for that key. Since you should not use exceptions for normal program flow, using at() is saying "I am sure there is such an element." But with the added benefit that you get an exception (and not undefined behavior) if you are wrong. Don't use this if you are not positive that the element exists.

find() says "there may or may not be such an element, let's see..." and offers you the possibility to react to both cases differently. It therefore is the more general approach.

Answer 4

All 3 of find , operator[] and at are useful.

• find is good if you don't want to accidentally insert elements, but merely act if they exist.

• at is good if you expect that something should be on a map and you'd throw an exception if it wasn't anyway. It can also access const maps in a more concise matter than find (where you can't use op[] )

• op[] is good if you want to insert a default element, such as for the word counting program which puts an int 0 for every word encountered for the first time (with the idiom words[word]++; ).

Answer 5

This depends on what the requirements are for this function and how you are structuring the project. If you are supposed to return an object and you can't because it was not found then it leaves you with two options on how to handle that. You could through an exception or you could return some sort of sentinel that means nothing was found. If you want to throw an exception then use at() as the exception will be thrown for you. If you do not want to throw an exception then use find() so you do not have to deal with handling an exception just to return a sentinel object.

Answer 6

I think, it depends on your usecase. The return type of std::map::at() is an lvalue reference to the value of the found element, while std::map::find() returns an iterator. You might prefer

return myMap.at("asdf"s) + 42;

in expressions over the more elaborate

return myMap.find("asdf"s)->second + 42;

Whenever you use the result of std::map::at() in an expression, you expect the element to exist, and regard a missing element as an error. So an exception is a good choice to handle that.

Answer 7

I guess the difference is semantics.

std::map::at() looks like this on my machine:

mapped_type&
at(const key_type& __k)
{
    iterator __i = lower_bound(__k);
    if (__i == end() || key_comp()(__k, (*__i).first))
        __throw_out_of_range(__N("map::at"));
    return (*__i).second;
}

As you can see, it uses lower_bound , then checks for end() , compares keys, and throws the exception where needed.

find() looks like this:

iterator
find(const key_type& __x)
{ return _M_t.find(__x); }

where _M_t is a red-black tree that stores the actual data. Obviously, both function have the same (logarithmic) complexity. When you use find() + check for end() , you are doing almost the same thing that at does. I would say the semantic difference is:

• use at() when you need an element at a specific location, and you assume that it is there. In this case, the situation of the element missing from the desired place is exceptional, thus at() throws an exception.
• use find() when you need to find the element in the map. In this case the situation when the element is not present is normal. Also note that find() returns an iterator which you may use for purposes other than simply obtaining it's value.

Answer 8

map::at() returns a l-value reference, and when you return by reference, you can use all its available benefits such as method chaining.

example:

  map<int,typ> table;
  table[98]=a;
  table[99]=b;

  table.at(98)=table.at(99);

operator[] also returns the mapped value by reference, but it may insert a value if searched for key is not found, in which case container size increases by one.

This requires you to be extra cautious since you have to take care of iterator invalidation .

Am I right that it is better to use map::find() when there is a possibility to not have element with such a key? And map::find() it is faster and more elegant approach?

Yes, semantically it makes sense to use find() when you are not sure of the existence of element.Makes the code easier to understand even for a newbie.

As for the time efficiency, map is generally implemented as a RB-tree/some balanced binary search tree and hence, complexity is O(logN) for find().

C++ Spec:

T& operator[](const key_type& x);
Effects: If there is no key equivalent to x in the map, inserts value_type(x, T()) into the map. Requires: key_type shall be CopyInsertable and mapped_type shall be DefaultInsertable into *this. Returns: A reference to the mapped_type corresponding to x in *this. 4 Complexity: Logarithmic.

T& at(const key_type& x);
const T& at(const key_type& x) const; Returns: A reference to the mapped_type corresponding to x in *this. Throws: An exception object of type out_of_range if no such element present. Complexity: Logarithmic.