CODEWITHSUNDEEP
c++inheritancepublicprivate-membersmemory-layout
PkDrew
PkDrew

Reputation: 588

Privacy property of inherited members affects overall size of class?

Running this code below, the sizeof base class and derived class are both 32.

  #include "iostream"
  #include "string"

  using namespace std;

  class base{
  //public:
      int a;
      double b;
      void * c;
      char d;
  };

  class c1 :  base{
  //public:
      int a;
  };

  int main() {
      cout << sizeof(base) << endl;
      cout << sizeof(c1) << endl;
      return 0;
  }

However, if you undo the comment of keyword public:, sizeof derived class becomes 40.

I am totally lost here, why member variable being private or not affect the overall size?

Reproducible in GCC.

Upvotes: 2

Views: 131

Answers (1)

Linda
Linda

Reputation: 61

If you were using Clang, you may add -Xclang -fdump-record-layouts to get a look at actual layout compiler makes for you.

If you make member variables in base class public, you get the layout shown as below:

*** Dumping AST Record Layout
         0 | class base
         0 |   int a
         8 |   double b
        16 |   void * c
        24 |   char d
           | [sizeof=32, dsize=32, align=8,
           |  nvsize=32, nvalign=8]

As you can see the dsize and nvsize both have already taken 32 bytes. There is no room for merge of int a from derived class, namely c1 in your code.

The justification of compiler comes down to this ABI specification:

If C is a POD, but not a POD for the purpose of layout, set dsize(C) = nvsize(C) = sizeof(C).

A funny enough fact is, by adding a dummy constructor in your base class, sizeof(base) will be 32, because you effectively made your base a non-POD one, compiler is free to apply all sort of optimizations now.

Upvotes: 1

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