Thread: sorting containers of structs

Originally posted by bigSteve
Q1
How do you sort containers of structs using the sort() algorithm ?
With primitive types its fairly easy, but with structs how can you use one of the fields as a sort key ?

It depends on the struct. By default, the sort() algorithm uses the operator< defined for your struct. If none exists, then you have to make one. It is generally pretty easy to do, and you can customize it to use whichever fields in whatever way you'd like. Another option is to pass in a sorting function that takes two const Whatever& arguments. It should return true if the first object is less than the second, and false if the first object is greater than or equal to the second. If your container is list, you should use its member function sort() instead of the one from the algorithm header.

Originally posted by bigSteve
Q2
When might you need to create a container of user defined objects ?

All the time. Most real applications create dozens or even hundreds of user-defined objects, and there are many reasons to put these into containers. In fact, in my experience there are more situations where a user-defined object is contained than a built-in type. Since in many cases an idea is wrapped up into a class with an appropriate interface, it is rare to just use raw datatypes except as members of another class.

Some examples are files or sockets. You might make classes to represent these and have containers of the ones that are currently open or active.

Originally posted by bigSteve
Q3
When might you need to create a container object on the heap rather than the stack ?

I can't think of any reason you'd need to create an STL container on the heap instead of the stack. Sometimes you do this when you are allocating large chunks of memory, but a container itself is very small. The data it holds is stored on the heap anyway, so even if you have a vector of 1000000 ints and you create it on the stack, only some pointers and some state information inside the vector end up on the stack and the 1000000 ints still end up on the heap.

It might be necessary to create it on the heap if you are creating the container in one scope but want it to stay around past the end of that scope.

Originally posted by bigSteve
Q4
Can auto_ptr be used to create a container on the heap ?

Yes, auto_ptr can be used to hold a container that has been allocated on the heap. However, as I said above, rarely do you need to allocate the STL container on the heap. When it comes to auto_ptrs and containers, the rule is never to have a container of auto_ptrs. It is ok to have an auto_ptr of a container.