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: 
Nope, you're right. Unless there is some additional business in the constructor (or constructor of the base classes) that require that, but the chances are very low...

**Later edit**

In case of destructor, one of the most common mistakes I saw is that some people assume that clear method will also call delete for vectors of pointers (vector), which, of course, it is not the case
**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