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: 
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.
**Of course one has to call clear() or resize(0) or the equivalent say (std::\_Destroy\_range(...) in the destructor before deallocating.**

Deallocation is done via allocator::deallocate **which does NOT run any destructors**. It just frees the memory.

clear() is equivalent to resize(0) which runs destructors on the first size() things in the allocated buffer

NOT just allocated pointers, file handles, held mutexes, all other recoverable resources held by the object. The destructors MUST be run. Before instantiation the template doesn't know that the destructor is trivial. If the destructor is trivial, THEN its gets optimized out AFTER instantiation