Is there any way to enforce certain size(of) class C by adding appropriate space/padding after its attributes?

Your best option would probably be to use a union. Roughly speaking, with some details missing:

class C 
{    
    int foo;
};

union CAlloc
{
    C instance;
    std::byte enforced_size[DESIRED_SIZE];
};

Then use CAlloc::instance where needed.

Update: as @Mgetz pointed out in the comments, std::byte is a C++17 addition, you may want/need to use char if you're using an older standard.

C++11 has introduced new keyword: alignas I think this is something more handy from your point of view.

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