CODEWITHSUNDEEP
c++
Louis Strous
Louis Strous

Reputation: 1074

Does inheriting class-specific memory (de)allocation methods make the class non-POD?

If a C++ POD class inherits a class that does nothing except provide class-specific memory (de)allocation methods, then does that make the POD class non-POD?

For example, with

struct Myalloc
{
   static void* operator new(size_t size);
   static void operator delete(void* p);
   // ... complete the set of operators
};

struct X : Myalloc {
  int y;
  double z;
};

is X a POD type? It is a POD type if it doesn't inherit Myalloc.

Also, do I need to define a virtual destructor for Myalloc? I expect not, because Myalloc provides no data members and only static methods.

Do the answers depend on which C++ standard (e.g., C++03, C++11, C++17, ...) is used? Our code needs to compile on multiple operating systems so we must write to the lowest C++ standard supported between them, which is C++03 with parts of C++11.

The reason for these questions is that I (unfortunately) need to make our own classes/structs use our own memory allocation routines, and it's a bad idea to globally replace the default memory allocation routines. Our code base has ancient roots and some of it still uses realloc to extend the size of a buffer holding a sequence of POD values. If that POD type turns non-POD when I make it inherit Myalloc, then I expect I'll need to explicitly construct the non-POD values, rather than just allocating memory for them. This would mean a full redesign of those ancient parts of the code, rather than just replacing realloc with our own version. I cannot afford the considerable additional time for a full redesign right now.

Upvotes: 4

Views: 44

Answers (1)

NathanOliver
NathanOliver

Reputation: 180945

Your derived class will not be a POD type. A POD type is defined in C++03 as

9(4): "A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-define copy operator and no user-defined destructor. Similarly a POD-union is an aggregate union that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-define copy operator and no user-defined destructor.

Since it is defined as a aggregate if we look at how that is defined we have

8.5.1(1): "An aggregate is an array or class (clause 9) with no user-declared constructors (12.1), no private or protected non-static data members (clause 11), no base classes (clause 10) and no virtual functions (10.3)."

and we can see that it explicitly disallows having a base class. This means you cannot have POd type that is derived from some other type, not matter if that type is POD or not. C++11 does not change this as aggregates still cannot have base classes.

Upvotes: 3

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