In which scenario do I use a particular STL container?

Question

This cheat sheet provides a pretty good summary of the different containers.

See the flowchart at the bottom as a guide on which to use in different usage scenarios:

_{Created by David Moore and licensed CC BY-SA 3.0}

Here is a flowchart inspired by David Moore's version (see above) that I created, which is up-to-date (mostly) with the new standard (C++11). This is only my personal take on it, it's not indisputable, but I figured it could be valuable to this discussion:

Simple answer: use std::vector for everything unless you have a real reason to do otherwise.

When you find a case where you're thinking, "Gee, std::vector doesn't work well here because of X", go on the basis of X.

Look at Effective STL by Scott Meyers. It's good at explaining how to use the STL.

If you want to store a determined/undetermined number of objects and you're never going to delete any, then a vector is what you want. It's the default replacement for a C array, and it works like one, but doesn't overflow. You can set its size beforehand as well with reserve().

If you want to store an undetermined number of objects, but you'll be adding them and deleting them, then you probably want a list...because you can delete an element without moving any following elements - unlike vector. It takes more memory than a vector, though, and you can't sequentially access an element.

If you want to take a bunch of elements and find only the unique values of those elements, reading them all into a set will do it, and it will sort them for you as well.

If you have a lot of key-value pairs, and you want to sort them by key, then a map is useful...but it will only hold one value per key. If you need more than one value per key, you could have a vector/list as your value in the map, or use a multimap.

It's not in the STL, but it is in the TR1 update to the STL: if you have a lot of key-value pairs that you're going to look up by key, and you don't care about their order, you might want to use a hash - which is tr1::unordered_map. I've used it with Visual C++ 7.1, where it was called stdext::hash_map. It has a lookup of O(1) instead of a lookup of O(log n) for map.

I redesigned the flowchart to have 3 properties:

1. I think STL containers are devided to 2 main classes. The basic containers and those leverages the basic containers to implement a policy.
2. At first the flowchart should divide the decision process to the main situations we should decide on and then elaborate on each case.
3. Some extended containers have the possibility of choosing different basic container as their inner container. The Flowchart should consider the situations in which each of the basic containers can be used.

The flowchart:

More info provided in this link.

An important point only briefly mentioned so far, is that if you require contiguous memory (like a C array gives), then you can only use vector, array, or string.

Use array if the size is known at compile time.

Use string if you only need to work with character types and need a string, not just a general-purpose container.

Use vector in all other cases (vector should be the default choice of container in most cases anyway).

With all three of these you can use the data() member function to get a pointer to the first element of the container.

It all depends on what you want to store and what you want to do with the container. Here are some (very non-exhaustive) examples for the container classes that I tend to use most:

vector: Compact layout with little or no memory overhead per contained object. Efficient to iterate over. Append, insert and erase can be expensive, particularly for complex objects. Cheap to find a contained object by index, e.g. myVector[10]. Use where you would have used an array in C. Good where you have a lot of simple objects (e.g. int). Don't forget to use reserve() before adding a lot of objects to the container.

list: Small memory overhead per contained object. Efficient to iterate over. Append, insert and erase are cheap. Use where you would have used a linked list in C.

set (and multiset): Significant memory overhead per contained object. Use where you need to find out quickly if that container contains a given object, or merge containers efficiently.

map (and multimap): Significant memory overhead per contained object. Use where you want to store key-value pairs and look up values by key quickly.

The flow chart on the cheat sheet suggested by zdan provides a more exhaustive guide.