0

我试图更深入地理解为什么以下程序的输出是这样的（见下文）。同时，我试图了解它与程序的汇编版本的关系。主要是，汇编代码中的类型在哪里，是什么导致一个比另一个大？使子类大于测试类的虚拟表在哪里？

代码

#include <iostream>

class parent {
  int glove;
public:
  parent() {}
  virtual ~parent() {}
};

class child : public parent {
  int ball;
public:
  child() {}
  ~child() {}
};

class test {
  int test1;
  int test2;

public:
  test() {}
};

int main() {
  std::cout << "Size of child is " << sizeof(child) << std::endl;
  std::cout << "Size of test is " << sizeof(test) << std::endl;
}

输出

Size of child is 12
Size of test is 8

部件 

.file   "test.cpp"
    .local  _ZStL8__ioinit
    .comm   _ZStL8__ioinit,1,1
    .section    .rodata
.LC0:
    .string "Size of child is "
.LC1:
    .string "Size of test is "
    .text
.globl main
    .type   main, @function
main:
.LFB974:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    andl    $-16, %esp
    subl    $16, %esp
    movl    $.LC0, 4(%esp)
    movl    $_ZSt4cout, (%esp)
    call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
    movl    $12, 4(%esp)
    movl    %eax, (%esp)
    call    _ZNSolsEj
    movl    $_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, 4(%esp)
    movl    %eax, (%esp)
    call    _ZNSolsEPFRSoS_E
    movl    $.LC1, 4(%esp)
    movl    $_ZSt4cout, (%esp)
    call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
    movl    $8, 4(%esp)
    movl    %eax, (%esp)
    call    _ZNSolsEj
    movl    $_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, 4(%esp)
    movl    %eax, (%esp)
    call    _ZNSolsEPFRSoS_E
    movl    $0, %eax
    leave
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    ret
    .cfi_endproc
.LFE974:
    .size   main, .-main
    .type   _Z41__static_initialization_and_destruction_0ii, @function
_Z41__static_initialization_and_destruction_0ii:
.LFB984:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    subl    $24, %esp
    cmpl    $1, 8(%ebp)
    jne .L5
    cmpl    $65535, 12(%ebp)
    jne .L5
    movl    $_ZStL8__ioinit, (%esp)
    call    _ZNSt8ios_base4InitC1Ev
    movl    $_ZNSt8ios_base4InitD1Ev, %eax
    movl    $__dso_handle, 8(%esp)
    movl    $_ZStL8__ioinit, 4(%esp)
    movl    %eax, (%esp)
    call    __cxa_atexit
.L5:
    leave
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    ret
    .cfi_endproc
.LFE984:
    .size   _Z41__static_initialization_and_destruction_0ii, .-_Z41__static_initialization_and_destruction_0ii
    .type   _GLOBAL__I_main, @function
_GLOBAL__I_main:
.LFB985:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    .cfi_offset 5, -8
    movl    %esp, %ebp
    .cfi_def_cfa_register 5
    subl    $24, %esp
    movl    $65535, 4(%esp)
    movl    $1, (%esp)
    call    _Z41__static_initialization_and_destruction_0ii
    leave
    .cfi_restore 5
    .cfi_def_cfa 4, 4
    ret
    .cfi_endproc
.LFE985:
    .size   _GLOBAL__I_main, .-_GLOBAL__I_main
    .section    .ctors,"aw",@progbits
    .align 4
    .long   _GLOBAL__I_main
    .weakref    _ZL20__gthrw_pthread_oncePiPFvvE,pthread_once
    .weakref    _ZL27__gthrw_pthread_getspecificj,pthread_getspecific
    .weakref    _ZL27__gthrw_pthread_setspecificjPKv,pthread_setspecific
    .weakref    _ZL22__gthrw_pthread_createPmPK14pthread_attr_tPFPvS3_ES3_,pthread_create
    .weakref    _ZL20__gthrw_pthread_joinmPPv,pthread_join
    .weakref    _ZL21__gthrw_pthread_equalmm,pthread_equal
    .weakref    _ZL20__gthrw_pthread_selfv,pthread_self
    .weakref    _ZL22__gthrw_pthread_detachm,pthread_detach
    .weakref    _ZL22__gthrw_pthread_cancelm,pthread_cancel
    .weakref    _ZL19__gthrw_sched_yieldv,sched_yield
    .weakref    _ZL26__gthrw_pthread_mutex_lockP15pthread_mutex_t,pthread_mutex_lock
    .weakref    _ZL29__gthrw_pthread_mutex_trylockP15pthread_mutex_t,pthread_mutex_trylock
    .weakref    _ZL31__gthrw_pthread_mutex_timedlockP15pthread_mutex_tPK8timespec,pthread_mutex_timedlock
    .weakref    _ZL28__gthrw_pthread_mutex_unlockP15pthread_mutex_t,pthread_mutex_unlock
    .weakref    _ZL26__gthrw_pthread_mutex_initP15pthread_mutex_tPK19pthread_mutexattr_t,pthread_mutex_init
    .weakref    _ZL29__gthrw_pthread_mutex_destroyP15pthread_mutex_t,pthread_mutex_destroy
    .weakref    _ZL30__gthrw_pthread_cond_broadcastP14pthread_cond_t,pthread_cond_broadcast
    .weakref    _ZL27__gthrw_pthread_cond_signalP14pthread_cond_t,pthread_cond_signal
    .weakref    _ZL25__gthrw_pthread_cond_waitP14pthread_cond_tP15pthread_mutex_t,pthread_cond_wait
    .weakref    _ZL30__gthrw_pthread_cond_timedwaitP14pthread_cond_tP15pthread_mutex_tPK8timespec,pthread_cond_timedwait
    .weakref    _ZL28__gthrw_pthread_cond_destroyP14pthread_cond_t,pthread_cond_destroy
    .weakref    _ZL26__gthrw_pthread_key_createPjPFvPvE,pthread_key_create
    .weakref    _ZL26__gthrw_pthread_key_deletej,pthread_key_delete
    .weakref    _ZL30__gthrw_pthread_mutexattr_initP19pthread_mutexattr_t,pthread_mutexattr_init
    .weakref    _ZL33__gthrw_pthread_mutexattr_settypeP19pthread_mutexattr_ti,pthread_mutexattr_settype
    .weakref    _ZL33__gthrw_pthread_mutexattr_destroyP19pthread_mutexattr_t,pthread_mutexattr_destroy
    .ident  "GCC: (GNU) 4.4.7 20120313 (Red Hat 4.4.7-3)"
    .section    .note.GNU-stack,"",@progbits
4

2 回答 2

2

它只是将指向虚拟表的指针添加到您的子类中，该子类恰好在您的特定平台上是 32 位的。

于 2013-05-16T18:28:19.710 回答
2

虚拟方法调度完全是一个实现问题（即特定于编译器）。该标准没有说明应该如何实现它，它可能是使用 VTables、黑魔法或谁知道还有什么。

话虽如此，我们只能猜测会发生什么......

是什么导致一个比另一个大？使子类大于测试类的虚拟表在哪里？

假设您的编译器实际上使用 VTable，实现它们的常用方法是当且仅当类具有虚拟方法（与它们是普通方法还是类的析构函数无关）时，才向每个对象添加一个指向 VTable 的隐藏指针。所以你的parent课程实际上看起来像这样：

class parent {
  VTable* hidden_vtable_ptr; // this is likely to be why child is bigger than test
  int glove;
public:
  parent() {}
  virtual ~parent() {}
};

请注意，无论您使用虚拟析构函数还是仅使用虚拟方法，都会发生这种情况：

struct test_pod {
  int t[2];
};
struct test_dtor {
  int t[2];
  virtual ~test_dtor() {}
};
struct test_method {
  int t[2];
  virtual void foo() {}
};

int main() {
  std::cout << sizeof(test_pod) << std::endl;    // 8
  std::cout << sizeof(test_dtor) << std::endl;   // 16
  std::cout << sizeof(test_method) << std::endl; // 16
  return 0;
}
于 2013-05-16T18:32:34.193 回答