C++クラスオブジェクトの強制型変換

クラス継承構造:

class AClass {
public:
  virtual ~AClass() = default;

  int func_a() {
    printf("func_a in AClass this %p
", this);
    return 0;
  }

  virtual int func_b() = 0;

private:
  int inta = 0;
};

class DClass {
public:
  virtual ~DClass() = default;

  int func_d() {
    printf("func_d in DClass this %p
", this);
    return 0;
  }

private:
  int intd = 0;
};

class BClass : public AClass {
public:
  virtual ~BClass() = default;

  virtual int func_a() {
    printf("func_a in BClass
");
    return 0;
  }

private:
  int intb = 0;
};

class CClass : public BClass, public DClass {
public:
  virtual ~CClass() = default;

  int func_a() override {
    printf("func_a in CClass this %p
", this);
    AClass::func_a();
    func_d();
    return 0;
  }

  int func_b() override {
    printf("func_b in CClass
");
    return 0;
  }
private:
  int intb = 0;
};

テストコードは次のとおりです.

void test_func() {
  CClass c;
  c.func_a();
  CClass *pc = &c;
  BClass *pb = &c;
  AClass *pa = &c;
  DClass *pd = &c;

  BClass *pb_static_cast = static_cast(pc);
  AClass *pa_static_cast = static_cast(pc);
  DClass *pd_static_cast = static_cast(pc);

  BClass *pb_reinterpret_cast = reinterpret_cast(pc);
  AClass *pa_reinterpret_cast =  reinterpret_cast(pc);
  DClass *pd_reinterpret_cast =  reinterpret_cast(pc);

  printf("pc %p, pa %p, pa_static_cast %p, pa_reinterpret_cast %p
",
        pc, pa, pa_static_cast, pa_reinterpret_cast);

  printf("pc %p, pb %p, pb_static_cast %p, pb_reinterpret_cast %p
",
        pc, pb, pb_static_cast, pb_reinterpret_cast);

  printf("pc %p, pd %p, pd_static_cast %p, pd_reinterpret_cast %p
",
        pc, pd, pd_static_cast, pd_reinterpret_cast);
}

上記のコードがコンパイルされると、コンパイラはDClass *pd_reinterpret_cast = reinterpret_cast(pc);行に対して次のwarningを報告します.

test.cpp:85:34: warning: 'reinterpret_cast' from class 'CClass *' to its base at non-zero offset 'DClass *' behaves differently from 'static_cast' [-Wreinterpret-base-class]
  DClass *pd_reinterpret_cast =  reinterpret_cast(pc);
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test.cpp:85:34: note: use 'static_cast' to adjust the pointer correctly while upcasting
  DClass *pd_reinterpret_cast =  reinterpret_cast(pc);
                                 ^~~~~~~~~~~~~~~~
                                 static_cast
1 warning generated.

上のコード実行時には、端末に次のような出力があります.

func_a in CClass this 0x7fff859533c0
func_a in AClass this 0x7fff859533c0
func_d in DClass this 0x7fff859533d0
pc 0x7fff859533c0, pa 0x7fff859533c0, pa_static_cast 0x7fff859533c0, pa_reinterpret_cast 0x7fff859533c0
pc 0x7fff859533c0, pb 0x7fff859533c0, pb_static_cast 0x7fff859533c0, pb_reinterpret_cast 0x7fff859533c0
pc 0x7fff859533c0, pd 0x7fff859533d0, pd_static_cast 0x7fff859533d0, pd_reinterpret_cast 0x7fff859533c0

注意多重継承の場合、サブクラスポインタ強制タイプをベースクラスポインタに変換した場合のstatic_castとreinterpret_castの動作の違い.