Why does GCC allocate "too much" stack space for local C struct?

I've written a code snippet that stores a C++ object inside a char array wrapped in a struct, with the intent of passing it to a function written in C:

class cpp_type
{
public:  
  // ...

private:
  int _state;
};

struct c_type
{ char buf[4]; };

struct c_type make_c_type()
{
  c_type tmp;
  new (tmp.buf) cpp_type();
  return tmp;
}

Then I was curious how the compiler translates make_c_type to x86_64 assembly code. GCC (with optimizations) produces:

0000000000000000 <_Z11make_c_typev>:
   0:   48 83 ec 18             sub    $0x18,%rsp
   4:   48 8d 7c 24 0c          lea    0xc(%rsp),%rdi
   9:   e8 00 00 00 00          call   e <_Z11make_c_typev+0xe>
   e:   8b 44 24 0c             mov    0xc(%rsp),%eax
  12:   48 83 c4 18             add    $0x18,%rsp
  16:   c3                      ret

I'm confused as to why the stack pointer is decremented by 24 bytes and rdi set to rsp + 12. The structure itself is only 4 bytes large, why doesn't GCC output e.g. sub $0x4,%rsp followed by mov %rsp,%rdi? Furthermore, shouldn't the stack always remain 16 byte aligned anyways?