Const 重载解析

TAG:

书上和网上在很多地方都对const 的重载做了一些解释，但感觉都不是很详细。还有很多同学在不同的地方发问关于const 重载的问题，这里我又重新看了一下，做了一个简单的分析也可能有不对的地方，欢迎讨论。
所谓重载，是指允许存在多个同名函数，而这些函数的参数表不同，即函数名相同但函数的签名不同。重载并不是面向对象编程的特有属性，这是因为重载是在编译阶段实现的，编译器根据函数不同的参数表，对同名函数的名称做修饰，然后这些同名函数就成了不同的函数（这一点稍后有例子）。了

Const 参数重载解析：

关于const 重载几乎在所有c++的书中者提到过但大部分只是一句话，例如在《C++ primer》一书中这样描述：“可基于函数的引用形参是指向 const 对象还是指向非 const 对象，实现函数重载。将引用形参定义为 const 来重载函数是合法的，因为编译器可以根据实参是否为 const 确定调用哪一个函数。”
但是这一段描述并没有给出引用、指针和值传递前加const的实质区别是什么。在用非const的指针，引用和值均可转化为const的。这一点没有太多可说明的东东。

对于函数值传递的情况，因为参数传递是通过复制实参创建一个临时变量传递进函数的，函数内只能改变临时变量，但无法改变实参。则这个时候无论加不加 const对实参不会产生任何影响。但是在引用或指针传递函数调用中，因为传进去的是一个引用或指针，这样函数内部可以改变引用或指针所指向的变量，这时 const 才是实实在在地保护了实参所指向的变量。因为在编译阶段编译器对调用函数的选择是根据实参进行的，所以，只有引用传递和指针传递可以用是否加const来重载。
下面给出一个例子可能就更明白了：

#include<iostream> 
  
  class A{ 
  public: 
     A(); 
     int foo(int *test); 
     int foo(const int *test); 
 }; 
 A::A(){ 
 } 
  int A::foo(int *test){ 
     std::cout << *test << " A::foo(int *test)" <<std::endl; 
     return 1; 
 } 
  int A::foo(const int *test){ 
     std::cout << *test << " A::foo(const int *test)" <<std::endl; 
     return 1; 
 } 
  int main() 
 { 
     const int b =5; 
     int c = 3; 
     A a; 
     a.foo(&b); 
     a.foo(&c); 
     return 1; 
 }

输出：

A::foo(const int *test)
A::foo(int *test)

那么编译器又是怎样工作的，通过g++ -S选项将汇编代码生成出来，通过AT&T汇编代码可以看出一些端倪来（之所以用AT&T汇编是因为VS生成的中间代码实在是让人头晕）：

.file    "overload.cpp"
     .section    .ctors,"aw",@progbits
     .align 4
     .long    _GLOBAL__I__ZN1AC2Ev
     .text
     .align 2
.globl _ZN1AC2Ev
     .type    _ZN1AC2Ev, @function
_ZN1AC2Ev:
.LFB1399:
     pushl    %ebp
.LCFI0:
     movl    %esp, %ebp
.LCFI1:
     popl    %ebp
     ret
.LFE1399:
     .size    _ZN1AC2Ev, .-_ZN1AC2Ev
.globl __gxx_personality_v0
     .align 2
.globl _ZN1AC1Ev
     .type    _ZN1AC1Ev, @function
_ZN1AC1Ev:
.LFB1400:
     pushl    %ebp
.LCFI2:
     movl    %esp, %ebp
.LCFI3:
     popl    %ebp
     ret
.LFE1400:
     .size    _ZN1AC1Ev, .-_ZN1AC1Ev
     .align 2
     .type    _Z41__static_initialization_and_destruction_0ii, @function
_Z41__static_initialization_and_destruction_0ii:
.LFB1411:
     pushl    %ebp
.LCFI4:
     movl    %esp, %ebp
.LCFI5:
     subl    $24, %esp
.LCFI6:
     movl    %eax, -4(%ebp)
     movl    %edx, -8(%ebp)
     cmpl    $1, -4(%ebp)
     jne    .L9
     cmpl    $65535, -8(%ebp)
     jne    .L9
     movl    $_ZSt8__ioinit, (%esp)
     call    _ZNSt8ios_base4InitC1Ev
     movl    $__dso_handle, 8(%esp)
     movl    $0, 4(%esp)
     movl    $__tcf_0, (%esp)
     call    __cxa_atexit
.L9:
     leave
     ret
.LFE1411:
     .size    _Z41__static_initialization_and_destruction_0ii, .-_Z41__static_initialization_and_destruction_0ii
     .align 2
     .type    _GLOBAL__I__ZN1AC2Ev, @function
_GLOBAL__I__ZN1AC2Ev:
.LFB1413:
     pushl    %ebp
.LCFI7:
     movl    %esp, %ebp
.LCFI8:
     subl    $8, %esp
.LCFI9:
     movl    $65535, %edx
     movl    $1, %eax
     call    _Z41__static_initialization_and_destruction_0ii
     leave
     ret
.LFE1413:
     .size    _GLOBAL__I__ZN1AC2Ev, .-_GLOBAL__I__ZN1AC2Ev
     .align 2
     .type    __tcf_0, @function
__tcf_0:
.LFB1412:
     pushl    %ebp
.LCFI10:
     movl    %esp, %ebp
.LCFI11:
     subl    $8, %esp
.LCFI12:
     movl    $_ZSt8__ioinit, (%esp)
     call    _ZNSt8ios_base4InitD1Ev
     leave
     ret
.LFE1412:
     .size    __tcf_0, .-__tcf_0
     .section    .rodata
.LC0:
     .string    " A::foo(const int *test)"
     .text
     .align 2
.globl _ZN1A3fooEPKi
     .type    _ZN1A3fooEPKi, @function
_ZN1A3fooEPKi:
.LFB1402:
     pushl    %ebp
.LCFI13:
     movl    %esp, %ebp
.LCFI14:
     subl    $8, %esp
.LCFI15:
     movl    12(%ebp), %eax
     movl    (%eax), %eax
     movl    %eax, 4(%esp)
     movl    $_ZSt4cout, (%esp)
     call    _ZNSolsEi
     movl    $.LC0, 4(%esp)
     movl    %eax, (%esp)
     call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
     movl    $_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, 4(%esp)
     movl    %eax, (%esp)
     call    _ZNSolsEPFRSoS_E
     movl    $1, %eax
     leave
     ret
.LFE1402:
     .size    _ZN1A3fooEPKi, .-_ZN1A3fooEPKi
     .section    .rodata
.LC1:
     .string    " A::foo(int *test)"
     .text
     .align 2
.globl _ZN1A3fooEPi
     .type    _ZN1A3fooEPi, @function
_ZN1A3fooEPi:
.LFB1401:
     pushl    %ebp
.LCFI16:
     movl    %esp, %ebp
.LCFI17:
     subl    $8, %esp
.LCFI18:
     movl    12(%ebp), %eax
     movl    (%eax), %eax
     movl    %eax, 4(%esp)
     movl    $_ZSt4cout, (%esp)
     call    _ZNSolsEi
     movl    $.LC1, 4(%esp)
     movl    %eax, (%esp)
     call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
     movl    $_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, 4(%esp)
     movl    %eax, (%esp)
     call    _ZNSolsEPFRSoS_E
     movl    $1, %eax
     leave
     ret
.LFE1401:
     .size    _ZN1A3fooEPi, .-_ZN1A3fooEPi
     .align 2
.globl main
     .type    main, @function
main:
.LFB1403:
     leal    4(%esp), %ecx
.LCFI19:
     andl    $-16, %esp
     pushl    -4(%ecx)
.LCFI20:
     pushl    %ebp
.LCFI21:
     movl    %esp, %ebp
.LCFI22:
     pushl    %ecx
.LCFI23:
     subl    $36, %esp
.LCFI24:
     movl    $5, -8(%ebp)
     movl    $3, -12(%ebp)
     leal    -13(%ebp), %eax
     movl    %eax, (%esp)
     call    _ZN1AC1Ev
     leal    -8(%ebp), %eax
     movl    %eax, 4(%esp)
     leal    -13(%ebp), %eax
     movl    %eax, (%esp)
     call    _ZN1A3fooEPKi
     leal    -12(%ebp), %eax
     movl    %eax, 4(%esp)
     leal    -13(%ebp), %eax
     movl    %eax, (%esp)
     call    _ZN1A3fooEPi
     movl    $1, %eax
     addl    $36, %esp
     popl    %ecx
     popl    %ebp
     leal    -4(%ecx), %esp
     ret
.LFE1403:
     .size    main, .-main
     .local    _ZSt8__ioinit
     .comm    _ZSt8__ioinit,1,1
     .weakref    _Z20__gthrw_pthread_oncePiPFvvE,pthread_once
     .weakref    _Z27__gthrw_pthread_getspecificj,pthread_getspecific
     .weakref    _Z27__gthrw_pthread_setspecificjPKv,pthread_setspecific
     .weakref    _Z22__gthrw_pthread_createPmPK14pthread_attr_tPFPvS3_ES3_,pthread_create
     .weakref    _Z22__gthrw_pthread_cancelm,pthread_cancel
     .weakref    _Z26__gthrw_pthread_mutex_lockP15pthread_mutex_t,pthread_mutex_lock
     .weakref    _Z29__gthrw_pthread_mutex_trylockP15pthread_mutex_t,pthread_mutex_trylock
     .weakref    _Z28__gthrw_pthread_mutex_unlockP15pthread_mutex_t,pthread_mutex_unlock
     .weakref    _Z26__gthrw_pthread_mutex_initP15pthread_mutex_tPK19pthread_mutexattr_t,pthread_mutex_init
     .weakref    _Z26__gthrw_pthread_key_createPjPFvPvE,pthread_key_create
     .weakref    _Z26__gthrw_pthread_key_deletej,pthread_key_delete
     .weakref    _Z30__gthrw_pthread_mutexattr_initP19pthread_mutexattr_t,pthread_mutexattr_init
     .weakref    _Z33__gthrw_pthread_mutexattr_settypeP19pthread_mutexattr_ti,pthread_mutexattr_settype
     .weakref    _Z33__gthrw_pthread_mutexattr_destroyP19pthread_mutexattr_t,pthread_mutexattr_destroy
     .section    .eh_frame,"a",@progbits
.Lframe1:
     .long    .LECIE1-.LSCIE1
.LSCIE1:
     .long    0x0
     .byte    0x1
     .string    "zP"
     .uleb128 0x1
     .sleb128 -4
     .byte    0x8
     .uleb128 0x5
     .byte    0x0
     .long    __gxx_personality_v0
     .byte    0xc
     .uleb128 0x4
     .uleb128 0x4
     .byte    0x88
     .uleb128 0x1
     .align 4
.LECIE1:
.LSFDE5:
     .long    .LEFDE5-.LASFDE5
.LASFDE5:
     .long    .LASFDE5-.Lframe1
     .long    .LFB1411
     .long    .LFE1411-.LFB1411
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI4-.LFB1411
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI5-.LCFI4
     .byte    0xd
     .uleb128 0x5
     .align 4
.LEFDE5:
.LSFDE7:
     .long    .LEFDE7-.LASFDE7
.LASFDE7:
     .long    .LASFDE7-.Lframe1
     .long    .LFB1413
     .long    .LFE1413-.LFB1413
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI7-.LFB1413
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI8-.LCFI7
     .byte    0xd
     .uleb128 0x5
     .align 4
.LEFDE7:
.LSFDE9:
     .long    .LEFDE9-.LASFDE9
.LASFDE9:
     .long    .LASFDE9-.Lframe1
     .long    .LFB1412
     .long    .LFE1412-.LFB1412
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI10-.LFB1412
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI11-.LCFI10
     .byte    0xd
     .uleb128 0x5
     .align 4
.LEFDE9:
.LSFDE11:
     .long    .LEFDE11-.LASFDE11
.LASFDE11:
     .long    .LASFDE11-.Lframe1
     .long    .LFB1402
     .long    .LFE1402-.LFB1402
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI13-.LFB1402
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI14-.LCFI13
     .byte    0xd
     .uleb128 0x5
     .align 4
.LEFDE11:
.LSFDE13:
     .long    .LEFDE13-.LASFDE13
.LASFDE13:
     .long    .LASFDE13-.Lframe1
     .long    .LFB1401
     .long    .LFE1401-.LFB1401
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI16-.LFB1401
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI17-.LCFI16
     .byte    0xd
     .uleb128 0x5
     .align 4
.LEFDE13:
.LSFDE15:
     .long    .LEFDE15-.LASFDE15
.LASFDE15:
     .long    .LASFDE15-.Lframe1
     .long    .LFB1403
     .long    .LFE1403-.LFB1403
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI19-.LFB1403
     .byte    0xc
     .uleb128 0x1
     .uleb128 0x0
     .byte    0x9
     .uleb128 0x4
     .uleb128 0x1
     .byte    0x4
     .long    .LCFI20-.LCFI19
     .byte    0xc
     .uleb128 0x4
     .uleb128 0x4
     .byte    0x4
     .long    .LCFI21-.LCFI20
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI22-.LCFI21
     .byte    0xd
     .uleb128 0x5
     .byte    0x4
     .long    .LCFI23-.LCFI22
     .byte    0x84
     .uleb128 0x3
     .align 4
.LEFDE15:
     .ident    "GCC: (GNU) 4.1.2 20070925 (Red Hat 4.1.2-33)"
     .section    .note.GNU-stack,"",@progbits

如上面的代码函数：

int foo(int *test);和int foo(const int *test);

分别被编译器生成名为：_ZN1A3fooEPKi和_ZN1A3fooEPi（这两个名字会因为编译器的不同而不同，名字只是一个区分的符号而已不用深究，只用知道重载的函数经过编译器的处理函数名字已经发生了变化。所以对于后面的汇编和链接工作就不存在重载的问题了。）这里也同时说明对重载来说在编译阶段已经完成。

对于a.foo(&b);因为变量b有const修饰所以就调用了int foo(const int *test);对于a.foo(&c);调用int foo(int *test);因为这个是精确匹配的。但是如果没有定义int foo(const int *test);则在代码24行会出现编译错误。反过来如果没有定义函数：int foo(int *test)；如下：

#include<iostream> 
  
  class A{ 
  public: 
     A(); 
  //    int foo(int *test); 
      int foo(const int *test); 
 }; 
 A::A(){ 
 } 
  /*int A::foo(int *test){ 
     std::cout << *test << " A::foo(int *test)" <<std::endl; 
     return 1; 
 } 
 */ 
 int A::foo(const int *test){ 
     std::cout << *test << " A::foo(const int *test)" <<std::endl; 
     return 1; 
 } 
 int main() 
 { 
     const int b =5; 
     int c = 3; 
     A a; 
     a.foo(&b); 
     a.foo(&c); 
     return 1; 
 }

则输出结果为：

5 A::foo(const int *test)
3 A::foo(const int *test)

原因c++ primer上讲的很清楚：“We can use a nonconst object to initializer either a const or nonconst reference. However, initializing a const reference to a nonconst object requires a conversion, whereas initializing a nonconst parameter is an exact match.”

const 成员函数重载的解析：

const 成员函数重载的解析和const参数重载解析的原理可以说是一样的。之所以这样说是因为const成员函数的解析可被看做是对函数this参数用const来修饰的过程。例如下面代码：

#include<iostream> 
  
 class A{ 
 public: 
     A(); 
     int foo(int *test); //可看做：int foo(A *this,int *test); 
     int foo(int *test) const;//可看做：int foo(const A *this,int *test); 
 }; 
 A::A(){ 
 } 
 int A::foo(int *test){ 
     std::cout << *test << "foo" <<std::endl; 
     return 1; 
 } 
 int A::foo(int *test) const { 
     std::cout << *test << "foo const" <<std::endl; 
     return 1; 
 } 
 int main() 
 { 
     int b = 5; 
     const A a; 
     a.foo(&b); 
     return 1; 
 }

生成汇编为：

.file    "overload1.cpp"
     .section    .ctors,"aw",@progbits
     .align 4
     .long    _GLOBAL__I__ZN1AC2Ev
     .text
     .align 2
.globl _ZN1AC2Ev
     .type    _ZN1AC2Ev, @function
_ZN1AC2Ev:
.LFB1399:
     pushl    %ebp
.LCFI0:
     movl    %esp, %ebp
.LCFI1:
     popl    %ebp
     ret
.LFE1399:
     .size    _ZN1AC2Ev, .-_ZN1AC2Ev
.globl __gxx_personality_v0
     .align 2
.globl _ZN1AC1Ev
     .type    _ZN1AC1Ev, @function
_ZN1AC1Ev:
.LFB1400:
     pushl    %ebp
.LCFI2:
     movl    %esp, %ebp
.LCFI3:
     popl    %ebp
     ret
.LFE1400:
     .size    _ZN1AC1Ev, .-_ZN1AC1Ev
     .align 2
     .type    _Z41__static_initialization_and_destruction_0ii, @function
_Z41__static_initialization_and_destruction_0ii:
.LFB1411:
     pushl    %ebp
.LCFI4:
     movl    %esp, %ebp
.LCFI5:
     subl    $24, %esp
.LCFI6:
     movl    %eax, -4(%ebp)
     movl    %edx, -8(%ebp)
     cmpl    $1, -4(%ebp)
     jne    .L9
     cmpl    $65535, -8(%ebp)
     jne    .L9
     movl    $_ZSt8__ioinit, (%esp)
     call    _ZNSt8ios_base4InitC1Ev
     movl    $__dso_handle, 8(%esp)
     movl    $0, 4(%esp)
     movl    $__tcf_0, (%esp)
     call    __cxa_atexit
.L9:
     leave
     ret
.LFE1411:
     .size    _Z41__static_initialization_and_destruction_0ii, .-_Z41__static_initialization_and_destruction_0ii
     .align 2
     .type    _GLOBAL__I__ZN1AC2Ev, @function
_GLOBAL__I__ZN1AC2Ev:
.LFB1413:
     pushl    %ebp
.LCFI7:
     movl    %esp, %ebp
.LCFI8:
     subl    $8, %esp
.LCFI9:
     movl    $65535, %edx
     movl    $1, %eax
     call    _Z41__static_initialization_and_destruction_0ii
     leave
     ret
.LFE1413:
     .size    _GLOBAL__I__ZN1AC2Ev, .-_GLOBAL__I__ZN1AC2Ev
     .align 2
     .type    __tcf_0, @function
__tcf_0:
.LFB1412:
     pushl    %ebp
.LCFI10:
     movl    %esp, %ebp
.LCFI11:
     subl    $8, %esp
.LCFI12:
     movl    $_ZSt8__ioinit, (%esp)
     call    _ZNSt8ios_base4InitD1Ev
     leave
     ret
.LFE1412:
     .size    __tcf_0, .-__tcf_0
     .section    .rodata
.LC0:
     .string    "foo const"
     .text
     .align 2
.globl _ZNK1A3fooEPi
     .type    _ZNK1A3fooEPi, @function
_ZNK1A3fooEPi:
.LFB1402:
     pushl    %ebp
.LCFI13:
     movl    %esp, %ebp
.LCFI14:
     subl    $8, %esp
.LCFI15:
     movl    12(%ebp), %eax
     movl    (%eax), %eax
     movl    %eax, 4(%esp)
     movl    $_ZSt4cout, (%esp)
     call    _ZNSolsEi
     movl    $.LC0, 4(%esp)
     movl    %eax, (%esp)
     call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
     movl    $_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, 4(%esp)
     movl    %eax, (%esp)
     call    _ZNSolsEPFRSoS_E
     movl    $1, %eax
     leave
     ret
.LFE1402:
     .size    _ZNK1A3fooEPi, .-_ZNK1A3fooEPi
     .align 2
.globl main
     .type    main, @function
main:
.LFB1403:
     leal    4(%esp), %ecx
.LCFI16:
     andl    $-16, %esp
     pushl    -4(%ecx)
.LCFI17:
     pushl    %ebp
.LCFI18:
     movl    %esp, %ebp
.LCFI19:
     pushl    %ecx
.LCFI20:
     subl    $36, %esp
.LCFI21:
     movl    $5, -8(%ebp)
     leal    -9(%ebp), %eax
     movl    %eax, (%esp)
     call    _ZN1AC1Ev
     leal    -8(%ebp), %eax
     movl    %eax, 4(%esp)
     leal    -9(%ebp), %eax
     movl    %eax, (%esp)
     call    _ZNK1A3fooEPi
     movl    $1, %eax
     addl    $36, %esp
     popl    %ecx
     popl    %ebp
     leal    -4(%ecx), %esp
     ret
.LFE1403:
     .size    main, .-main
     .section    .rodata
.LC1:
     .string    "foo"
     .text
     .align 2
.globl _ZN1A3fooEPi
     .type    _ZN1A3fooEPi, @function
_ZN1A3fooEPi:
.LFB1401:
     pushl    %ebp
.LCFI22:
     movl    %esp, %ebp
.LCFI23:
     subl    $8, %esp
.LCFI24:
     movl    12(%ebp), %eax
     movl    (%eax), %eax
     movl    %eax, 4(%esp)
     movl    $_ZSt4cout, (%esp)
     call    _ZNSolsEi
     movl    $.LC1, 4(%esp)
     movl    %eax, (%esp)
     call    _ZStlsISt11char_traitsIcEERSt13basic_ostreamIcT_ES5_PKc
     movl    $_ZSt4endlIcSt11char_traitsIcEERSt13basic_ostreamIT_T0_ES6_, 4(%esp)
     movl    %eax, (%esp)
     call    _ZNSolsEPFRSoS_E
     movl    $1, %eax
     leave
     ret
.LFE1401:
     .size    _ZN1A3fooEPi, .-_ZN1A3fooEPi
     .local    _ZSt8__ioinit
     .comm    _ZSt8__ioinit,1,1
     .weakref    _Z20__gthrw_pthread_oncePiPFvvE,pthread_once
     .weakref    _Z27__gthrw_pthread_getspecificj,pthread_getspecific
     .weakref    _Z27__gthrw_pthread_setspecificjPKv,pthread_setspecific
     .weakref    _Z22__gthrw_pthread_createPmPK14pthread_attr_tPFPvS3_ES3_,pthread_create
     .weakref    _Z22__gthrw_pthread_cancelm,pthread_cancel
     .weakref    _Z26__gthrw_pthread_mutex_lockP15pthread_mutex_t,pthread_mutex_lock
     .weakref    _Z29__gthrw_pthread_mutex_trylockP15pthread_mutex_t,pthread_mutex_trylock
     .weakref    _Z28__gthrw_pthread_mutex_unlockP15pthread_mutex_t,pthread_mutex_unlock
     .weakref    _Z26__gthrw_pthread_mutex_initP15pthread_mutex_tPK19pthread_mutexattr_t,pthread_mutex_init
     .weakref    _Z26__gthrw_pthread_key_createPjPFvPvE,pthread_key_create
     .weakref    _Z26__gthrw_pthread_key_deletej,pthread_key_delete
     .weakref    _Z30__gthrw_pthread_mutexattr_initP19pthread_mutexattr_t,pthread_mutexattr_init
     .weakref    _Z33__gthrw_pthread_mutexattr_settypeP19pthread_mutexattr_ti,pthread_mutexattr_settype
     .weakref    _Z33__gthrw_pthread_mutexattr_destroyP19pthread_mutexattr_t,pthread_mutexattr_destroy
     .section    .eh_frame,"a",@progbits
.Lframe1:
     .long    .LECIE1-.LSCIE1
.LSCIE1:
     .long    0x0
     .byte    0x1
     .string    "zP"
     .uleb128 0x1
     .sleb128 -4
     .byte    0x8
     .uleb128 0x5
     .byte    0x0
     .long    __gxx_personality_v0
     .byte    0xc
     .uleb128 0x4
     .uleb128 0x4
     .byte    0x88
     .uleb128 0x1
     .align 4
.LECIE1:
.LSFDE5:
     .long    .LEFDE5-.LASFDE5
.LASFDE5:
     .long    .LASFDE5-.Lframe1
     .long    .LFB1411
     .long    .LFE1411-.LFB1411
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI4-.LFB1411
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI5-.LCFI4
     .byte    0xd
     .uleb128 0x5
     .align 4
.LEFDE5:
.LSFDE7:
     .long    .LEFDE7-.LASFDE7
.LASFDE7:
     .long    .LASFDE7-.Lframe1
     .long    .LFB1413
     .long    .LFE1413-.LFB1413
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI7-.LFB1413
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI8-.LCFI7
     .byte    0xd
     .uleb128 0x5
     .align 4
.LEFDE7:
.LSFDE9:
     .long    .LEFDE9-.LASFDE9
.LASFDE9:
     .long    .LASFDE9-.Lframe1
     .long    .LFB1412
     .long    .LFE1412-.LFB1412
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI10-.LFB1412
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI11-.LCFI10
     .byte    0xd
     .uleb128 0x5
     .align 4
.LEFDE9:
.LSFDE11:
     .long    .LEFDE11-.LASFDE11
.LASFDE11:
     .long    .LASFDE11-.Lframe1
     .long    .LFB1402
     .long    .LFE1402-.LFB1402
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI13-.LFB1402
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI14-.LCFI13
     .byte    0xd
     .uleb128 0x5
     .align 4
.LEFDE11:
.LSFDE13:
     .long    .LEFDE13-.LASFDE13
.LASFDE13:
     .long    .LASFDE13-.Lframe1
     .long    .LFB1403
     .long    .LFE1403-.LFB1403
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI16-.LFB1403
     .byte    0xc
     .uleb128 0x1
     .uleb128 0x0
     .byte    0x9
     .uleb128 0x4
     .uleb128 0x1
     .byte    0x4
     .long    .LCFI17-.LCFI16
     .byte    0xc
     .uleb128 0x4
     .uleb128 0x4
     .byte    0x4
     .long    .LCFI18-.LCFI17
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI19-.LCFI18
     .byte    0xd
     .uleb128 0x5
     .byte    0x4
     .long    .LCFI20-.LCFI19
     .byte    0x84
     .uleb128 0x3
     .align 4
.LEFDE13:
.LSFDE15:
     .long    .LEFDE15-.LASFDE15
.LASFDE15:
     .long    .LASFDE15-.Lframe1
     .long    .LFB1401
     .long    .LFE1401-.LFB1401
     .uleb128 0x0
     .byte    0x4
     .long    .LCFI22-.LFB1401
     .byte    0xe
     .uleb128 0x8
     .byte    0x85
     .uleb128 0x2
     .byte    0x4
     .long    .LCFI23-.LCFI22
     .byte    0xd
     .uleb128 0x5
     .align 4
.LEFDE15:
     .ident    "GCC: (GNU) 4.1.2 20070925 (Red Hat 4.1.2-33)"
     .section    .note.GNU-stack,"",@progbits

上面可以看到编译阶段的调用也是通过对重载函数的别名来实现的。

总结：

1.const重载主要是通过能否对传入的参数进行修改为判断的。

2.const参数重载和const函数重载机制都是一样的，因为对于const 函数重载可看做是对隐含的指针this的参数重载。

3.重载是在编译阶段已经完成，对于汇编和链接来说透明的。

(qingquan)

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