When is it a good idea to use a database

I am doing an information retrieval project in c++. What are the advantages of using a database to store terms, as o开发者_如何转开发pposed to storing it in a data structure such as a vector? More generally when is it a good idea to use a database rather than a data structure?

1. (Shawn): Whenever you want to keep the data beyond the length of the instance of the program. (persistence across time)

2. (Michael Kjörling): Whenever you want many instances of your program, either in the same computer or in many computers, like in a network or the Net, access and manipulate (share) the same data. (persistence across network space)

3. Whenever you have very big amount of data that do not fit into memory.

4. Whenever you have very complex data structures and you prefer to not rewrite code to manipulate them, e.g search, update them, when the db programmers have already written such code and probably much faster than the code you (or I)'ll write.

Whenever you want to keep the data beyond the length of the instance of the program?

In addition to Shawn pointing out persistence: whenever you want multiple instances of the program to be able to easily share data?

In-memory data structures are great, but they are not a replacement for persistence.

It really depends on the scope. For example if you're going to have multiple applications accessing the data then a database is better because you won't have to worry about file locks, etc. Also, you'd use a database when you need to do things like joining other data, sorting, etc... unless you like to implement Quicksort.