std::vector::clear() in constructor and destructor

Question

I encounter many times with code where std::vector::clear() of class member of type std::vector is called in constructor and destructor.

I don't see why it's required:

1. constructor - the class member of type std::vector is empty by default, so no need to call clear().
2. destructor - the class member of type std::vector will be destroyed as part of standard destruction of object contnaining it. As part of vector destruction all value objects containied in it will be destroyed (if it heap allocated pointers to memory, they should be deleted "manually"), so again no need to call clear().

Do I miss something?

Answer 1

No, you're not missing anything. I suspect this is (harmless) voodoo programming, sort of like setting a pointer to null after freeing it, or randomly calling repaint/revalidate in GUI code. The programmer remembers that it helped with some sort of bug in the past and now adds it unnecessarily "just in case". Who knows, maybe it'll help. Voodoo.

Answer 2

From the sound of things, the people who wrote that code were the ones who missed something. The only time it would make sense to call clear() in a ctor or dtor would be in the middle of some other code. For example, a ctor might read in some data, process it, then read in more data. In such a case, it's probably faster to use a single container for the data as you read it in, and clear it each time, than to create a new container every iteration.