Question

有这样的C ++代码：

#include <iostream>

class MojaKlasa{
public:
  int z;
  void fun(){ std::cout << this;};
};

int main()
{
  MojaKlasa a;
  a.z = 2;
  a.fun();
  return 0;
}

相应的汇编代码：

    .file   "prog50.cpp"
    .local  _ZStL8__ioinit
    .comm   _ZStL8__ioinit,1,1
    .section    .text._ZN9MojaKlasa3funEv,"axG",@progbits,_ZN9MojaKlasa3funEv,comdat
    .align 2
    .weak   _ZN9MojaKlasa3funEv
    .type   _ZN9MojaKlasa3funEv, @function
_ZN9MojaKlasa3funEv:
.LFB957:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5
    subl    $24, %esp
    movl    8(%ebp), %eax
    movl    %eax, 4(%esp)
    movl    $_ZSt4cout, (%esp)
    call    _ZNSolsEPKv
    leave
    ret
    .cfi_endproc
.LFE957:
    .size   _ZN9MojaKlasa3funEv, .-_ZN9MojaKlasa3funEv
    .text
.globl main
    .type   main, @function
main:
.LFB958:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5
    andl    $-16, %esp
    subl    $32, %esp
    movl    $2, 28(%esp)     # a.z = 2;
    leal    28(%esp), %eax
    movl    %eax, (%esp)
    call    _ZN9MojaKlasa3funEv   # a.fun();
    movl    $0, %eax
    leave
    ret
    .cfi_endproc
.LFE958:
    .size   main, .-main
    .type   _Z41__static_initialization_and_destruction_0ii, @function
_Z41__static_initialization_and_destruction_0ii:
.LFB960:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5
    subl    $24, %esp
    cmpl    $1, 8(%ebp)
    jne .L7
    cmpl    $65535, 12(%ebp)
    jne .L7
    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
.L7:
    leave
    ret
    .cfi_endproc
.LFE960:
    .size   _Z41__static_initialization_and_destruction_0ii, .-_Z41__static_initialization_and_destruction_0ii
    .type   _GLOBAL__I_main, @function
_GLOBAL__I_main:
.LFB961:
    .cfi_startproc
    .cfi_personality 0x0,__gxx_personality_v0
    pushl   %ebp
    .cfi_def_cfa_offset 8
    movl    %esp, %ebp
    .cfi_offset 5, -8
    .cfi_def_cfa_register 5
    subl    $24, %esp
    movl    $65535, 4(%esp)
    movl    $1, (%esp)
    call    _Z41__static_initialization_and_destruction_0ii
    leave
    ret
    .cfi_endproc
.LFE961:
    .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: (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5"
    .section    .note.GNU-stack,"",@progbits

似乎MojaKlasa类的函数 fun（）被翻译为普通常规函数。函数如何知道为特定对象调用它，例如 a.fun（） - ＆gt;为对象 a 调用 fun（）？
第二个问题是，MojaKlasa类的默认构造函数和析构函数在哪里？据说C ++会自动为类创建构造函数和析构函数，但似乎其中没有一个包含在汇编代码中。

Answer 1

函数如何知道为特定对象调用它，例如a.fun() -> call fun() for object a？

指向对象this的指针作为第一个隐藏参数隐式传递给成员函数。

class MojaKlasa的默认构造函数和析构函数在哪里？

C ++原则是：您只需支付使用的费用。
除非你的代码明确需要一个默认的无参数构造函数，否则编译器就不会生成它。

Answer 2

根本不查看汇编代码，您应该知道成员函数传递了一个隐藏参数。这称为this指针。因此，调用具有自己的this指针，并且每个函数都专门为该对象调用。

关于你的第二个问题，你可以确信生成了默认的构造函数和析构函数，虽然没有gcc编译器内部的专业知识，但实际上很难找到调用它的位置或方式。

我在Visual Studio 2010中尝试过此操作，但我也无法在dissasembly中找到此信息。

Answer 3

简而言之，成员函数有一个隐藏参数：

void Class::func(int i); // Becomes...
void func(Class const* this, int i);

当您调用该函数obj.func(7)时，它会被“转换”为func(&obj, 7)。这几乎就是它的主旨。

在C ++中，这是一个隐藏/隐式参数，但有些语言要求它是一个显式参数。例如，在Python中，你必须写这个（Python中的self相当于C ++中的this）：

class Class:
    def func(self, i):
        ...

成员函数如何知道为特定对象调用它？

3 个答案: