C++: 编译器何时构建或扩展默认构造函数 Default Constructor

[导读] 《深度探索C++对象模型》一书第二章关于编译器构建或扩展构造函数的结尾指出，对于我们C++新手有两个常见的误解：1，任何没有定义default constructor的class，编译

《深度探索C++对象模型》一书第二章关于编译器构建或扩展构造函数的结尾指出，对于我们C++新手有两个常见的误解：

1，任何没有定义default constructor的class，编译器都会自动替程序员合成一个出来；

2，编译器合成出来的default constructor会默认对class的数据成员进行初始化。

然而，事实上并非如此，但是有很多地方我们确实见到过这种说法呀，到底什么时候编译器会为我们做这个事情呢？书中总结了四种情况下，编译器会为了需要合成或者扩展构造函数：

1，class中有data member是某class的object，并且此object所属的class有default constructor；

2，class继承自有default constructor的base class；

3，class中定义了virtual function或者继承有virtual function，编译器需要为此类object初始化virtual function table；

4，class以virtual方式继承base class。

第二种情况是最简单的，如果class没有定义某class类型的object data member，同时没有virtual function和virtual base class，则如果此class没有定义默认构造函数，有两种情况发生：1，如果定义有其他带参数构造函数，则创建对象时必须传入参数，而不管其继承的base class是否有默认构造函数，否则会报找不到构造函数的错误；2，如果没有定义任何构造函数，并且其继承的base class有默认构造函数，则编译器会为其合成默认构造函数，但是不会对其基本数据类型的数据成员初始化。

class BaseA{
       
          
};
class ChildB:public BaseA{
public:
};

int main()
{
    ChildB child;
    return 0;
}

class BaseA{
public:
BaseA() {}
};
class ChildB:public BaseA{
public:
};

int main()
{
    ChildB child;
    return 0;
}

编译结果：

##################################

.file "testdef.cpp"

.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6




movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main


.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits .file"testinh.cpp"
.section .text._ZN5BaseAC2Ev,"axG",@progbits,_ZN5BaseAC5Ev,comdat
.align 2
.weak _ZN5BaseAC2Ev
.type _ZN5BaseAC2Ev, @function
_ZN5BaseAC2Ev:
.LFB1:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -8(%rbp)
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE1:
.size _ZN5BaseAC2Ev, .-_ZN5BaseAC2Ev
.section .text._ZN6ChildBC2Ev,"axG",@progbits,_ZN6ChildBC5Ev,comdat
.align 2
.weak _ZN6ChildBC2Ev
.type _ZN6ChildBC2Ev, @function
_ZN6ChildBC2Ev:
.LFB5:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
movq %rdi, -8(%rbp)
movq -8(%rbp), %rax
movq %rax, %rdi
call _ZN5BaseAC2Ev
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE5:
.size _ZN6ChildBC2Ev, .-_ZN6ChildBC2Ev
.text
.globl main
.type main, @function
main:
.LFB3:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
leaq -1(%rbp), %rax
movq %rax, %rdi
call _ZN6ChildBC1Ev
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE3:
.size main, .-main
.weak _ZN5BaseAC1Ev
.set _ZN5BaseAC1Ev,_ZN5BaseAC2Ev
.weak _ZN6ChildBC1Ev
.set _ZN6ChildBC1Ev,_ZN6ChildBC2Ev
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits

由此对比，Child Class的默认构造函数由编译器自动构造出来了。

针对第一种情况，如果class定义了某个object成员，此object所属类有默认构造函数，则：

1，如果class中无构造函数，则编译器将自动合成默认构造函数，并初始化object成员

class BaseA{
public:
BaseA() {}
};

class ChildB{
public:
           
BaseA basea;
};


int main()
{
ChildB child;
return 0;
}

class BaseA{
public:
BaseA() {}
};

class ChildB{
public:
ChildB() {}
BaseA basea;
};


int main()
{
ChildB child;
return 0;
}

编译结果：

.file "testdef.cpp"
.section .text._ZN5BaseAC2Ev,"axG",@progbits,_ZN5BaseAC5Ev,comdat
.align 2
.weak _ZN5BaseAC2Ev
.type _ZN5BaseAC2Ev, @function
_ZN5BaseAC2Ev:
.LFB1:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -8(%rbp)
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE1:
.size _ZN5BaseAC2Ev, .-_ZN5BaseAC2Ev
.section .text._ZN6ChildBC2Ev,"axG",@progbits,_ZN6ChildBC5Ev,comdat
.align 2
.weak _ZN6ChildBC2Ev
.type _ZN6ChildBC2Ev, @function
_ZN6ChildBC2Ev:
.LFB5:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
movq %rdi, -8(%rbp)
movq -8(%rbp), %rax
movq %rax, %rdi
call _ZN5BaseAC1Ev
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE5:
.size _ZN6ChildBC2Ev, .-_ZN6ChildBC2Ev
.text
.globl main
.type main, @function
main:
.LFB3:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
leaq -1(%rbp), %rax
movq %rax, %rdi
call _ZN6ChildBC1Ev
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE3:
.size main, .-main
.weak _ZN5BaseAC1Ev
.set _ZN5BaseAC1Ev,_ZN5BaseAC2Ev
.weak _ZN6ChildBC1Ev
.set _ZN6ChildBC1Ev,_ZN6ChildBC2Ev
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits .file "testinh.cpp"
.section .text._ZN5BaseAC2Ev,"axG",@progbits,_ZN5BaseAC5Ev,comdat
.align 2
.weak _ZN5BaseAC2Ev
.type _ZN5BaseAC2Ev, @function
_ZN5BaseAC2Ev:
.LFB1:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -8(%rbp)
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE1:
.size _ZN5BaseAC2Ev, .-_ZN5BaseAC2Ev
.section .text._ZN6ChildBC2Ev,"axG",@progbits,_ZN6ChildBC5Ev,comdat
.align 2
.weak _ZN6ChildBC2Ev
.type _ZN6ChildBC2Ev, @function
_ZN6ChildBC2Ev:
.LFB4:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
movq %rdi, -8(%rbp)
movq -8(%rbp), %rax
movq %rax, %rdi
call _ZN5BaseAC1Ev
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE4:
.size _ZN6ChildBC2Ev, .-_ZN6ChildBC2Ev
.text
.globl main
.type main, @function
main:
.LFB6:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
leaq -1(%rbp), %rax
movq %rax, %rdi
call _ZN6ChildBC1Ev
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE6:
.size main, .-main
.weak _ZN5BaseAC1Ev
.set _ZN5BaseAC1Ev,_ZN5BaseAC2Ev
.weak _ZN6ChildBC1Ev
.set _ZN6ChildBC1Ev,_ZN6ChildBC2Ev
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits

2，若class中提供了任意构造函数，但是未对object成员初始化，则编译器将在构造函数中插入object成员的初始化操作，即扩展已有的构造函数

class BaseA{
public:
BaseA() {}
};

class ChildB{
public:
ChildB() {}
BaseA basea;
};


int main()
{
ChildB child;
return 0;
}

class BaseA{
public:
BaseA() {}
};

class ChildB{
public:
ChildB():basea() {}
BaseA basea;
};


int main()
{
ChildB child;
return 0;
}

编译结果

.file "testinh.cpp"
.section .text._ZN5BaseAC2Ev,"axG",@progbits,_ZN5BaseAC5Ev,comdat
.align 2
.weak _ZN5BaseAC2Ev
.type _ZN5BaseAC2Ev, @function
_ZN5BaseAC2Ev:
.LFB1:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -8(%rbp)
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE1:
.size _ZN5BaseAC2Ev, .-_ZN5BaseAC2Ev
.section .text._ZN6ChildBC2Ev,"axG",@progbits,_ZN6ChildBC5Ev,comdat
.align 2
.weak _ZN6ChildBC2Ev
.type _ZN6ChildBC2Ev, @function
_ZN6ChildBC2Ev:
.LFB4:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
movq %rdi, -8(%rbp)
movq -8(%rbp), %rax
movq %rax, %rdi
call _ZN5BaseAC1Ev
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE4:
.size _ZN6ChildBC2Ev, .-_ZN6ChildBC2Ev
.text
.globl main
.type main, @function
main:
.LFB6:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
leaq -1(%rbp), %rax
movq %rax, %rdi
call _ZN6ChildBC1Ev
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE6:
.size main, .-main
.weak _ZN5BaseAC1Ev
.set _ZN5BaseAC1Ev,_ZN5BaseAC2Ev
.weak _ZN6ChildBC1Ev
.set _ZN6ChildBC1Ev,_ZN6ChildBC2Ev
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits .file "testdef.cpp"
.section .text._ZN5BaseAC2Ev,"axG",@progbits,_ZN5BaseAC5Ev,comdat
.align 2
.weak _ZN5BaseAC2Ev
.type _ZN5BaseAC2Ev, @function
_ZN5BaseAC2Ev:
.LFB1:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -8(%rbp)
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE1:
.size _ZN5BaseAC2Ev, .-_ZN5BaseAC2Ev
.section .text._ZN6ChildBC2Ev,"axG",@progbits,_ZN6ChildBC5Ev,comdat
.align 2
.weak _ZN6ChildBC2Ev
.type _ZN6ChildBC2Ev, @function
_ZN6ChildBC2Ev:
.LFB4:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
movq %rdi, -8(%rbp)
movq -8(%rbp), %rax
movq %rax, %rdi
call _ZN5BaseAC1Ev
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE4:
.size _ZN6ChildBC2Ev, .-_ZN6ChildBC2Ev
.text
.globl main
.type main, @function
main:
.LFB6:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
leaq -1(%rbp), %rax
movq %rax, %rdi
call _ZN6ChildBC1Ev
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE6:
.size main, .-main
.weak _ZN5BaseAC1Ev
.set _ZN5BaseAC1Ev,_ZN5BaseAC2Ev
.weak _ZN6ChildBC1Ev
.set _ZN6ChildBC1Ev,_ZN6ChildBC2Ev
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits 第三种情况，class中定义有virtual function或者继承链中有类定义了virtual function，编译器需要为class类型的对象创建vtl和vptr，因此编译器将自动合成默认构造函数

class ChildB{
public:
           
virtual void func() {}
};


int main()
{
ChildB child;
return 0;
}

class ChildB{
public:
ChildB() {}
virtual void func() {}
};


int main()
{
ChildB child;
return 0;
}

编译结果

.file "testdef.cpp"
.section .text._ZN6ChildB4funcEv,"axG",@progbits,_ZN6ChildB4funcEv,comdat
.align 2
.weak _ZN6ChildB4funcEv
.type _ZN6ChildB4funcEv, @function
_ZN6ChildB4funcEv:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -8(%rbp)

popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size _ZN6ChildB4funcEv, .-_ZN6ChildB4funcEv
.section .text._ZN6ChildBC2Ev,"axG",@progbits,_ZN6ChildBC5Ev,comdat
.align 2
.weak _ZN6ChildBC2Ev
.type _ZN6ChildBC2Ev, @function
_ZN6ChildBC2Ev:
.LFB3:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -8(%rbp)
movq -8(%rbp), %rax
movq $_ZTV6ChildB+16, (%rax)
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE3:
.size _ZN6ChildBC2Ev, .-_ZN6ChildBC2Ev
.text
.globl main
.type main, @function
main:
.LFB1:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
leaq -16(%rbp), %rax
movq %rax, %rdi
call _ZN6ChildBC1Ev
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE1:
.size main, .-main
.weak _ZTV6ChildB
.section .rodata._ZTV6ChildB,"aG",@progbits,_ZTV6ChildB,comdat
.align 16
.type _ZTV6ChildB, @object
.size _ZTV6ChildB, 24
_ZTV6ChildB:
.quad 0
.quad _ZTI6ChildB
.quad _ZN6ChildB4funcEv
.weak _ZTS6ChildB
.section .rodata._ZTS6ChildB,"aG",@progbits,_ZTS6ChildB,comdat
.type _ZTS6ChildB, @object
.size _ZTS6ChildB, 8
_ZTS6ChildB:
.string "6ChildB"
.weak _ZTI6ChildB
.section .rodata._ZTI6ChildB,"aG",@progbits,_ZTI6ChildB,comdat
.align 16
.type _ZTI6ChildB, @object
.size _ZTI6ChildB, 16
_ZTI6ChildB:
.quad _ZTVN10__cxxabiv117__class_type_infoE+16
.quad _ZTS6ChildB
.weak _ZN6ChildBC1Ev
.set _ZN6ChildBC1Ev,_ZN6ChildBC2Ev
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits .file "testinh.cpp"
.section .text._ZN6ChildBC2Ev,"axG",@progbits,_ZN6ChildBC5Ev,comdat
.align 2
.weak _ZN6ChildBC2Ev
.type _ZN6ChildBC2Ev, @function
_ZN6ChildBC2Ev:
.LFB1:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -8(%rbp)
movq -8(%rbp), %rax
movq $_ZTV6ChildB+16, (%rax)
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE1:
.size _ZN6ChildBC2Ev, .-_ZN6ChildBC2Ev
.section .text._ZN6ChildB4funcEv,"axG",@progbits,_ZN6ChildB4funcEv,comdat
.align 2
.weak _ZN6ChildB4funcEv
.type _ZN6ChildB4funcEv, @function
_ZN6ChildB4funcEv:
.LFB3:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -8(%rbp)

popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE3:
.size _ZN6ChildB4funcEv, .-_ZN6ChildB4funcEv
.text
.globl main
.type main, @function
main:
.LFB4:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
leaq -16(%rbp), %rax
movq %rax, %rdi
call _ZN6ChildBC1Ev
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE4:
.size main, .-main
.weak _ZTV6ChildB
.section .rodata._ZTV6ChildB,"aG",@progbits,_ZTV6ChildB,comdat
.align 16
.type _ZTV6ChildB, @object
.size _ZTV6ChildB, 24
_ZTV6ChildB:
.quad 0
.quad _ZTI6ChildB
.quad _ZN6ChildB4funcEv
.weak _ZTS6ChildB
.section .rodata._ZTS6ChildB,"aG",@progbits,_ZTS6ChildB,comdat
.type _ZTS6ChildB, @object
.size _ZTS6ChildB, 8
_ZTS6ChildB:
.string "6ChildB"
.weak _ZTI6ChildB
.section .rodata._ZTI6ChildB,"aG",@progbits,_ZTI6ChildB,comdat
.align 16
.type _ZTI6ChildB, @object
.size _ZTI6ChildB, 16
_ZTI6ChildB:
.quad _ZTVN10__cxxabiv117__class_type_infoE+16
.quad _ZTS6ChildB
.weak _ZN6ChildBC1Ev
.set _ZN6ChildBC1Ev,_ZN6ChildBC2Ev
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits 第四种情况，class以virtual方式继承，或者其继承链中有virtual方式的继承，编译器为此class和base class均自动合成默认构造函数

class BaseA{
};
class ChildB:public BaseA{
};


int main()
{
ChildB child;
return 0;
}

class BaseA{
};
class ChildB: virtual public BaseA{
};


int main()
{
ChildB child;
return 0;
}

编译结果

#############################################
.file "testinh.cpp"





































.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6



movl $0, %eax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main






































.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits .file "testdef.cpp"
.section .text._ZN5BaseAC2Ev,"axG",@progbits,_ZN5BaseAC5Ev,comdat
.align 2
.weak _ZN5BaseAC2Ev
.type _ZN5BaseAC2Ev, @function
_ZN5BaseAC2Ev:
.LFB3:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -8(%rbp)
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE3:
.size _ZN5BaseAC2Ev, .-_ZN5BaseAC2Ev
.section .text._ZN6ChildBC1Ev,"axG",@progbits,_ZN6ChildBC1Ev,comdat
.align 2
.weak _ZN6ChildBC1Ev
.type _ZN6ChildBC1Ev, @function
_ZN6ChildBC1Ev:
.LFB6:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
movq %rdi, -8(%rbp)
movq -8(%rbp), %rax
movq %rax, %rdi
call _ZN5BaseAC2Ev
movl $_ZTV6ChildB+24, %edx
movq -8(%rbp), %rax
movq %rdx, (%rax)
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE6:
.size _ZN6ChildBC1Ev, .-_ZN6ChildBC1Ev
.text
.globl main
.type main, @function
main:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $16, %rsp
leaq -16(%rbp), %rax
movq %rax, %rdi
call _ZN6ChildBC1Ev
movl $0, %eax
leave
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size main, .-main
.weak _ZTV6ChildB
.section .rodata._ZTV6ChildB,"aG",@progbits,_ZTV6ChildB,comdat
.align 16
.type _ZTV6ChildB, @object
.size _ZTV6ChildB, 24
_ZTV6ChildB:
.quad 0
.quad 0
.quad _ZTI6ChildB
.weak _ZTT6ChildB
.section .rodata._ZTT6ChildB,"aG",@progbits,_ZTV6ChildB,comdat
.align 8
.type _ZTT6ChildB, @object
.size _ZTT6ChildB, 8
_ZTT6ChildB:
.quad _ZTV6ChildB+24
.weak _ZTS6ChildB
.section .rodata._ZTS6ChildB,"aG",@progbits,_ZTS6ChildB,comdat
.type _ZTS6ChildB, @object
.size _ZTS6ChildB, 8
_ZTS6ChildB:
.string "6ChildB"
.weak _ZTI6ChildB
.section .rodata._ZTI6ChildB,"aG",@progbits,_ZTI6ChildB,comdat
.align 32
.type _ZTI6ChildB, @object
.size _ZTI6ChildB, 40
_ZTI6ChildB:
.quad _ZTVN10__cxxabiv121__vmi_class_type_infoE+16
.quad _ZTS6ChildB
.long 0
.long 1
.quad _ZTI5BaseA
.quad -6141
.weak _ZTS5BaseA
.section .rodata._ZTS5BaseA,"aG",@progbits,_ZTS5BaseA,comdat
.type _ZTS5BaseA, @object
.size _ZTS5BaseA, 7
_ZTS5BaseA:
.string "5BaseA"
.weak _ZTI5BaseA
.section .rodata._ZTI5BaseA,"aG",@progbits,_ZTI5BaseA,comdat
.align 16
.type _ZTI5BaseA, @object
.size _ZTI5BaseA, 16
_ZTI5BaseA:
.quad _ZTVN10__cxxabiv117__class_type_infoE+16
.quad _ZTS5BaseA
.weak _ZN5BaseAC1Ev
.set _ZN5BaseAC1Ev,_ZN5BaseAC2Ev
.ident "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
.section .note.GNU-stack,"",@progbits

但是对于上述说的第一种误解，我是没有真正理解的，如果没有构造函数，编译器也不自动合成，对象是如何建立的？

#includeusing namespace std;

class NoDef{
public:
int k;
void print() { cout << "k is " << k << endl;}
};

int main()
{
NoDef def;
def.print();

def.k = 2;
def.print();

def.k = 199566;
def.print();

return 0;
}

由于上方的代码可以执行，那么是否说明def对象创建了？如果为NoDef添加了默认构造函数，从编译结果看，main函数中明显调用了构造函数。理解还是不到位，继续找资料学习。