While reading about C++17 library features, I found public methods that simply call private pure virtual methods. For example:

std::pmr::memory_resource::allocate

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void* allocate( std::size_t bytes, std::size_t alignment = alignof(std::max_align_t) );

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Allocates storage with a size of at least bytes bytes, aligned to the specified alignment.

Equivalent to return do_allocate(bytes, alignment);

std::pmr::memory_resource::do_allocate

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virtual void* do_allocate( std::size_t bytes, std::size_t alignment ) = 0;

and

std::basic_streambuf<CharT,Traits>::pubsync, std::basic_streambuf<CharT,Traits>::sync

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(1) int pubsync();

protected:

(2) virtual int sync();

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1. Calls sync() of the most derived class

why such pattern is used?

I read What is the point of a private pure virtual function? and some articles about NVI idiom, but nothing explains the above cases that a method does nothing but calls private method and has no overloaded methods.