title: std::shared_ptr preallocation memory
body: <p>I want to preallocate memory both for control block and <code>value_type</code> for <code>shared_ptr</code> in one heap request (like <code>std::make_shared</code>), but do not construct immediately any object in it. And when I actually need to construct object use placement <code>new</code>. Is it possible? Neither <code>std::make_shared</code> or <code>std::allocate_shared</code> seem to solve my problem.</p>