c++ - Using any buffer to store any data, with placement new and memcpy -

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It's a while that I'm reading documents about alignment, aliasing, padding and placement new, but still not me Be sure to solve this problem - this is the first time that I am facing a memory issue at this level, so I do not believe.

The problem is: I have a buffer, an array of data that uses some type typeT , such as 

  typeT buff [N];   
 Inside that buffer, I have to store some other data, type  typeU  at the beginning of  buff . Let the element of the  of  of , which is equal to or equal to  M  element of type   is.  
 EDIT: This problem depends on the data type, but my question is normal, and hold for POD and non-POD. But you can assume that non-POD is just POD type aggregation, there is no dynamic memory to copy, and all the data is within the bytes of the  sizeof (typeX)  structure. That's it,  Do not worry about the deep / shallow transcription .  
 I am more interested in this matter where  typeT  and  typeU  are different alignments  
  Question 1 : (The most important question) is that  always  is safe to use the new placement to store my data, considering this fact on  buffer  Can I access those data using TyU type  always , like in the following code?  
  typeU * allocData = new (buff) type u [k]; AllocData [0] = foo;   
  Question 2 : This  always  copy any data from  typeU *  inside a buffer Using  memcpy  using  typeT * , I can always access the data using the following code in  typeU *  Am I    Typu Promptata [k]; Mammakpi (Bref, Preddata, Saiff (Typu) * K); // Is it safe to do? TypeU * accessData = reinterpret_cast & lt; Typeus * & gt; (Buff); Access data [1] = foo;  Question 3 : (at least the important question) Is it true that it is not always safe for only the cast pointer and using those people Write data? Like  
  typeU * castData = (typeU *) buff; CastData [2] = Something;   
 Even if I always use the first field of memory (first m elements) using the  typeU  and use the second code (element of M from N) While doing  TypeT ?    
 I hope it is clear what I mean ... I searched a lot, but I am still confused.  
 Also, please note that I am talking about a C ++ 98 or 03, not 11, with the minimum subset of STL, there is no boost and not always on x86 - But I can use memcpy and placement new, as I said.   
 
 
 provided that the size of  typeT  and  typeU  And the alignment requirements are the same, you can not use the allocated transmission for the  typeT  to hold the value of uninitialized  typeU .  
 If  typeT  and / or  typeU  are constructors or destructors, you will need to make sure that they are called properly and in correct order. Static_assert (typeof) == size (typeU) static_assert (alignof (typeT) == alignof (typeU) ty PET T [10]; & amp;; [0] - & Gt; ~ typeT (); // Delete the placement to type t [0] Type * u = p = & amp; t [0]; New (P) type u (); // composition type u [t] [0] Type UU & amp; = * P; u.doStuff (); p- & gt; ~ typeU (); // Placement removed on TYPE [0] new (& amp; t [0]) Type T (); // on resume type T [0]

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