std::pointer_traits

#include <iostream>
#include <memory>

template<class Ptr>
struct BlockList
{
    // Predefine a memory block
    struct block;

    // Define a pointer to a memory block from the kind of pointer Ptr s
    // If Ptr is any kind of T*, block_ptr_t is block*
    // If Ptr is smart_ptr<T>, block_ptr_t is smart_ptr<block>
    using block_ptr_t = typename std::pointer_traits<Ptr>::template rebind<block>;

    struct block
    {
        std::size_t size{};
        block_ptr_t next_block{};
    };

    block_ptr_t free_blocks;
};

int main()
{
    [[maybe_unused]]
    BlockList<int*> bl1;
    // The type of bl1.free_blocks is BlockList<int*>:: block*

    BlockList<std::shared_ptr<char>> bl2;
    // The type of bl2.free_blocks is
    // std::shared_ptr<BlockList<std::shared_ptr<char>>::block>
    std::cout << bl2.free_blocks.use_count() << '\n';
}

Вывод:

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