MSVC
在编译std::list的时候,MSVC 2012会存储链表的长度。除此以外,它的实现方法和GCC基本一致。这就是说,内置函数.size()只需要从内存中读取一个值就可以返回函数结果,其速度相当快。不过,这也意味着每次添增/删除节点的时候都需要调整这个值。
MSVC的链存储结构也和GCC有所区别:
可见,GCC构造的虚节点在链尾,而MSVC构造的虚节点在链首。
使用MSVC 2012(启用/Fa2.asm/GS-/Ob1选项)编译上述程序,可得到:
指令清单51.31 MSVC 2012带参数/Fa2.asm/GS-/Ob1优化的程序
_l$ = -16 ; size = 8
_t1$ = -8 ; size = 8
_main PROC
sub esp, 16
push ebx
push esi
push edi
push 0
push 0
lea ecx, DWORD PTR _l$[esp+36]
mov DWORD PTR _l$[esp+40], 0
; allocate first "garbage" element
call ?_Buynode0@?$_List_alloc@$0A@U?$_List_base_types@Ua@@V? ↙
↘ $allocator@Ua@@@std@@@std@@@std@@QAEPAU?$_List_node@Ua@@PAX@2@PAU32@0@Z ; std:: ↙
↘ _List_alloc<0,std::_List_base_types<a,std::allocator<a> > >::_Buynode0
mov edi, DWORD PTR __imp__printf
mov ebx, eax
push OFFSET $SG40685 ; '* empty list:'
mov DWORD PTR _l$[esp+32], ebx
call edi ; printf
lea eax, DWORD PTR _l$[esp+32]
push eax
call ?dump_List_val@@YAXPAI@Z ; dump_List_val
mov esi, DWORD PTR [ebx]
add esp, 8
lea eax, DWORD PTR _t1$[esp+28]
push eax
push DWORD PTR [esi+4]
lea ecx, DWORD PTR _l$[esp+36]
push esi
mov DWORD PTR _t1$[esp+40], 1 ; data for a new node
mov DWORD PTR _t1$[esp+44], 2 ; data for a new node
; allocate new node
call ??$_Buynode@ABUa@@@?$_List_buy@Ua@@V?$allocator@Ua@@@std@@@std@@QAEPAU? ↙
↘ $_List_node@Ua@@PAX@1@PAU21@0ABUa@@@Z ; std::_List_buy<a,std::allocator<a> >::_Buynode<a ↙
↘ const &>
mov DWORD PTR [esi+4], eax
mov ecx, DWORD PTR [eax+4]
mov DWORD PTR _t1$[esp+28], 3 ; data for a new node
mov DWORD PTR [ecx], eax
mov esi, DWORD PTR [ebx]
lea eax, DWORD PTR _t1$[esp+28]
push eax
push DWORD PTR [esi+4]
lea ecx, DWORD PTR _l$[esp+36]
push esi
mov DWORD PTR _t1$[esp+44], 4 ; data for a new node
; allocate new node
call ??$_Buynode@ABUa@@@?$_List_buy@Ua@@V?$allocator@Ua@@@std@@@std@@QAEPAU? ↙
↘ $_List_node@Ua@@PAX@1@PAU21@0ABUa@@@Z ; std::_List_buy<a,std::allocator<a> >::_Buynode<a ↙
↘ const &>
mov DWORD PTR [esi+4], eax
mov ecx, DWORD PTR [eax+4]
mov DWORD PTR _t1$[esp+28], 5 ; data for a new node
mov DWORD PTR [ecx], eax
lea eax, DWORD PTR _t1$[esp+28]
push eax
push DWORD PTR [ebx+4]
lea ecx, DWORD PTR _l$[esp+36]
push ebx
mov DWORD PTR _t1$[esp+44], 6 ; data for a new node
; allocate new node
call ??$_Buynode@ABUa@@@?$_List_buy@Ua@@V?$allocator@Ua@@@std@@@std@@QAEPAU? ↙
↘ $_List_node@Ua@@PAX@1@PAU21@0ABUa@@@Z ; std::_List_buy<a,std::allocator<a> >::_Buynode<a ↙
↘ const &>
mov DWORD PTR [ebx+4], eax
mov ecx, DWORD PTR [eax+4]
push OFFSET $SG40689 ; '* 3-elements list:'
mov DWORD PTR _l$[esp+36], 3
mov DWORD PTR [ecx], eax
call edi ; printf
lea eax, DWORD PTR _l$[esp+32]
push eax
call ?dump_List_val@@YAXPAI@Z ; dump_List_val
push OFFSET $SG40831 ; 'node at .begin:'
call edi ; printf
push DWORD PTR [ebx] ; get next field of node l variable points to
call ?dump_List_node@@YAXPAUList_node@@@Z ; dump_List_node
push OFFSET $SG40835 ; 'node at .end:'
call edi ; printf
push ebx ; pointer to the node $l$ variable points to!
call ?dump_List_node@@YAXPAUList_node@@@Z ; dump_List_node
push OFFSET $SG40839 ; '* let''s count from the begin:'
call edi ; printf
mov esi, DWORD PTR [ebx] ; operator++: get ->next pointer
push DWORD PTR [esi+12]
push DWORD PTR [esi+8]
push OFFSET $SG40846 ; '1st element: %d %d'
call edi ; printf
mov esi, DWORD PTR [esi] ; operator++: get ->next pointer
push DWORD PTR [esi+12]
push DWORD PTR [esi+8]
push OFFSET $SG40848 ; '2nd element: %d %d'
call edi ; printf
mov esi, DWORD PTR [esi] ; operator++: get ->next pointer
push DWORD PTR [esi+12]
push DWORD PTR [esi+8]
push OFFSET $SG40850 ; '3rd element: %d %d'
call edi ; printf
mov eax, DWORD PTR [esi] ; operator++: get ->next pointer
add esp, 64
push DWORD PTR [eax+12]
push DWORD PTR [eax+8]
push OFFSET $SG40852 ; 'element at .end(): %d %d'
call edi ; printf
push OFFSET $SG40853 ; '* let''s count from the end:'
call edi ; printf
push DWORD PTR [ebx+12] ; use x and y fields from the node l variable points to
push DWORD PTR [ebx+8]
push OFFSET $SG40860 ; 'element at .end(): %d %d'
call edi ; printf
mov esi, DWORD PTR [ebx+4] ; operator--: get ->prev pointer
push DWORD PTR [esi+12]
push DWORD PTR [esi+8]
push OFFSET $SG40862 ; '3rd element: %d %d'
call edi ; printf
mov esi, DWORD PTR [esi+4] ; operator--: get ->prev pointer
push DWORD PTR [esi+12]
push DWORD PTR [esi+8]
push OFFSET $SG40864 ; '2nd element: %d %d'
call edi ; printf
mov eax, DWORD PTR [esi+4] ; operator--: get ->prev pointer
push DWORD PTR [eax+12]
push DWORD PTR [eax+8]
push OFFSET $SG40866 ; '1st element: %d %d'
call edi ; printf
add esp, 64
push OFFSET $SG40867 ; 'removing last element...'
call edi ; printf
mov edx, DWORD PTR [ebx+4]
add esp, 4
; prev=next?
; it is the only element, "garbage one"?
; if yes, do not delete it!
cmp edx, ebx
je SHORT $LN349@main
mov ecx, DWORD PTR [edx+4]
mov eax, DWORD PTR [edx]
mov DWORD PTR [ecx], eax
mov ecx, DWORD PTR [edx]
mov eax, DWORD PTR [edx+4]
push edx
mov DWORD PTR [ecx+4], eax
call ??3@YAXPAX@Z ; operator delete
add esp, 4
mov DWORD PTR _l$[esp+32], 2
$LN349@main:
lea eax, DWORD PTR _l$[esp+28]
push eax
call ?dump_List_val@@YAXPAI@Z ; dump_List_val
mov eax, DWORD PTR [ebx]
add esp, 4
mov DWORD PTR [ebx], ebx
mov DWORD PTR [ebx+4], ebx
cmp eax, ebx
je SHORT $LN412@main
$LL414@main:
mov esi, DWORD PTR [eax]
push eax
call ??3@YAXPAX@Z ; operator delete
add esp, 4
mov eax, esi
cmp esi, ebx
jne SHORT $LL414@main
$LN412@main:
push ebx
call ??3@YAXPAX@Z ; operator delete
add esp, 4
xor eax, eax
pop edi
pop esi
pop ebx
add esp, 16
ret 0
_main ENDP
在构造数据链的时候,MSVC通过“Buynode”函数分配了虚节点的空间;此后,这个函数同样用于分配其他节点的存储空间。相比之下,GCC则会在局部栈里存储链表的第一个节点。
指令清单51.32 整个程序的输出
* empty list:
_Myhead=0x003CC258, _Mysize=0
ptr=0x003CC258 _Next=0x003CC258 _Prev=0x003CC258 x=6226002 y=4522072
* 3-elements list:
_Myhead=0x003CC258, _Mysize=3
ptr=0x003CC258 _Next=0x003CC288 _Prev=0x003CC2A0 x=6226002 y=4522072
ptr=0x003CC288 _Next=0x003CC270 _Prev=0x003CC258 x=3 y=4
ptr=0x003CC270 _Next=0x003CC2A0 _Prev=0x003CC288 x=1 y=2
ptr=0x003CC2A0 _Next=0x003CC258 _Prev=0x003CC270 x=5 y=6
node at .begin:
ptr=0x003CC288 _Next=0x003CC270 _Prev=0x003CC258 x=3 y=4
node at .end:
ptr=0x003CC258 _Next=0x003CC288 _Prev=0x003CC2A0 x=6226002 y=4522072
* let's count from the begin:
1st element: 3 4
2nd element: 1 2
3rd element: 5 6
element at .end(): 6226002 4522072
* let's count from the end:
element at .end(): 6226002 4522072
3rd element: 5 6
2nd element: 1 2
1st element: 3 4
removing last element...
_Myhead=0x003CC258, _Mysize=2
ptr=0x003CC258 _Next=0x003CC288 _Prev=0x003CC270 x=6226002 y=4522072
ptr=0x003CC288 _Next=0x003CC270 _Prev=0x003CC258 x=3 y=4
ptr=0x003CC270 _Next=0x003CC258 _Prev=0x003CC288 x=1 y=2
C++11 std::forward_list单向链表
std::forward_list和std::list的结构基本相同,只是它是单向链,它的迭代器(链域)只有”next”字段而没有”prev”字段。虽然这种链表的内存开销变小了,但是无法进行逆向遍历。
我们将std::vector标准向量称为PODT[4]C数组的安全封装容器。其内部结构和标准字符串std::string十分相似(参见51.4.1节)。它有一个数据缓冲区的专用指针,一个指向数组尾部的专用指针,以及一个指向分配缓冲区尾部的专用指针。
这种数组采取的各个元素以彼此相邻的方式存储于内存之中,这点和常规数组没什么区别(参见第18章)。新推出的C++11标准,为其定义了内置函数.data()。std::vector的.data()的作用就和std::string中的.c_str()一样,用于返回缓冲区的地址。
这种数据结构使用堆/heap来存储数据缓冲区,而堆的空间消耗可能会比数组本身还大。
MSVC和GCC的实现机理基本相同,只是结构体的变量名称稍微有些不同。因此,所以这里使用同一个例子进行说明。下述源程序用于遍历std::vector的数据结构。
#include <stdio.h>
#include <vector>
#include <algorithm>
#include <functional>
struct vector_of_ints
{
// MSVC names:
int *Myfirst;
int *Mylast;
int *Myend;
// GCC structure is the same, but names are: _M_start, _M_finish, _M_end_of_storage
};
void dump(struct vector_of_ints *in)
{
printf ("_Myfirst=%p, _Mylast=%p, _Myend=%p\n", in->Myfirst, in->Mylast, in->Myend);
size_t size=(in->Mylast-in->Myfirst);
size_t capacity=(in->Myend-in->Myfirst);
printf ("size=%d, capacity=%d\n", size, capacity);
for (size_t i=0; i<size; i++)
printf ("element %d: %d\n", i, in->Myfirst[i]);
};
int main()
{
std::vector<int> c;
dump ((struct vector_of_ints*)(void*)&c);
c.push_back(1);
dump ((struct vector_of_ints*)(void*)&c);
c.push_back(2);
dump ((struct vector_of_ints*)(void*)&c);
c.push_back(3);
dump ((struct vector_of_ints*)(void*)&c);
c.push_back(4);
dump ((struct vector_of_ints*)(void*)&c);
c.reserve (6);
dump ((struct vector_of_ints*)(void*)&c);
c.push_back(5);
dump ((struct vector_of_ints*)(void*)&c);
c.push_back(6);
dump ((struct vector_of_ints*)(void*)&c);
printf ("%d\n", c.at(5)); // with bounds checking
printf ("%d\n", c[8]); // operator[], without bounds checking
};
下面是采用MSVC编译后的程序输出。
_Myfirst=00000000, _Mylast=00000000, _Myend=00000000
size=0, capacity=0
_Myfirst=0051CF48, _Mylast=0051CF4C, _Myend=0051CF4C
size=1, capacity=1
element 0: 1
_Myfirst=0051CF58, _Mylast=0051CF60, _Myend=0051CF60
size=2, capacity=2
element 0: 1
element 1: 2
_Myfirst=0051C278, _Mylast=0051C284, _Myend=0051C284
size=3, capacity=3
element 0: 1
element 1: 2
element 2: 3
_Myfirst=0051C290, _Mylast=0051C2A0, _Myend=0051C2A0
size=4, capacity=4
element 0: 1
element 1: 2
element 2: 3
element 3: 4
_Myfirst=0051B180, _Mylast=0051B190, _Myend=0051B198
size=4, capacity=6
element 0: 1
element 1: 2
element 2: 3
element 3: 4
_Myfirst=0051B180, _Mylast=0051B194, _Myend=0051B198
size=5, capacity=6
element 0: 1
element 1: 2
element 2: 3
element 3: 4
element 4: 5
_Myfirst=0051B180, _Mylast=0051B198, _Myend=0051B198
size=6, capacity=6
element 0: 1
element 1: 2
element 2: 3
element 3: 4
element 4: 5
element 5: 6
6
6619158
从程序中我们可以看到,当主函数main()开始运行后并没有立即分配数据缓冲区。在第一次调用push_back()函数之后,它才分配了缓冲区。此后,每调用一次push_back()函数,数组的空间和size(capacity)都增大一次。不仅空间容量逐渐增长,而且缓冲区的地址也会同期改变。这是因为每次调用push_back()函数时,都会在堆里重新分配空间。所以这种操作的时间开销很大。要想提高效率,就必须预测数组的大小并使用.reserve()方式为其预留存储空间。
最后一个数值是随机的噪音。它不属于链表的某个节点,只是一个随机数。从这一点我们可以看出,std::vector(标准向量)的下标运算符“[]”不会检测索引值是否超越下标界限。要进行这种数组边界检查,可以使用内置函数.at()。虽然.at()方法的运行速度较慢,但是它能在下标越 std::out_of_range (越界)的错误提示。
我们来看它的汇编指令。使用MSVC 2012(启用 /GS- /Ob1选项)编译上述源程序,可得到:
指令清单51.33 MSVC 2012 /GS- /Ob1
$SG52650 DB '%d', 0aH, 00H
$SG52651 DB '%d', 0aH, 00H
_this$ = -4 ; size = 4
__Pos$ = 8 ; size = 4
?at@?$vector@HV?$allocator@H@std@@@std@@QAEAAHI@Z PROC ; std::vector<int,std::allocator<int> ↙
↘ >::at, COMDAT
; _this$ = ecx
push ebp
mov ebp, esp
push ecx
mov DWORD PTR _this$[ebp], ecx
mov eax, DWORD PTR _this$[ebp]
mov ecx, DWORD PTR _this$[ebp]
mov edx, DWORD PTR [eax+4]
sub edx, DWORD PTR [ecx]
sar edx, 2
cmp edx, DWORD PTR __Pos$[ebp]
ja SHORT $LN1@at
push OFFSET ??_C@_0BM@NMJKDPPO@invalid?5vector?$DMT?$DO?5subscript?$AA@
call DWORD PTR __imp_?_Xout_of_range@std@@YAXPBD@Z
$LN1@at:
mov eax, DWORD PTR _this$[ebp]
mov ecx, DWORD PTR [eax]
mov edx, DWORD PTR __Pos$[ebp]
lea eax, DWORD PTR [ecx+edx*4]
$LN3@at:
mov esp, ebp
pop ebp
ret 4
?at@?$vector@HV?$allocator@H@std@@@std@@QAEAAHI@Z ENDP ; std::vector<int,std::allocator<int> ↙
↘ >::at
_c$ = -36 ; size = 12
$T1 = -24 ; size = 4
$T2 = -20 ; size = 4
$T3 = -16 ; size = 4
$T4 = -12 ; size = 4
$T5 = -8 ; size = 4
$T6 = -4 ; size = 4
_main PROC
push ebp
mov ebp, esp
sub esp, 36
mov DWORD PTR _c$[ebp], 0 ; Myfirst
mov DWORD PTR _c$[ebp+4], 0 ; Mylast
mov DWORD PTR _c$[ebp+8], 0 ; Myend
lea eax, DWORD PTR _c$[ebp]
push eax
call ?dump@@YAXPAUvector_of_ints@@@Z ; dump
add esp, 4
mov DWORD PTR $T6[ebp], 1
lea ecx, DWORD PTR $T6[ebp]
push ecx
lea ecx, DWORD PTR _c$[ebp]
call ?push_back@?$vector@HV?$allocator@H@std@@@std@@QAEX$$QAH@Z ; std::vector<int,std:: ↙
↘ allocator<int> >::push_back
lea edx, DWORD PTR _c$[ebp]
push edx
call ?dump@@YAXPAUvector_of_ints@@@Z ; dump
add esp, 4
mov DWORD PTR $T5[ebp], 2
lea eax, DWORD PTR $T5[ebp]
push eax
lea ecx, DWORD PTR _c$[ebp]
call ?push_back@?$vector@HV?$allocator@H@std@@@std@@QAEX$$QAH@Z ; std::vector<int,std:: ↙
↘ allocator<int> >::push_back
lea ecx, DWORD PTR _c$[ebp]
push ecx
call ?dump@@YAXPAUvector_of_ints@@@Z ; dump
add esp, 4
mov DWORD PTR $T4[ebp], 3
lea edx, DWORD PTR $T4[ebp]
push edx
lea ecx, DWORD PTR _c$[ebp]
call ?push_back@?$vector@HV?$allocator@H@std@@@std@@QAEX$$QAH@Z ; std::vector<int,std:: ↙
↘ allocator<int> >::push_back
lea eax, DWORD PTR _c$[ebp]
push eax
call ?dump@@YAXPAUvector_of_ints@@@Z ; dump
add esp, 4
mov DWORD PTR $T3[ebp], 4
lea ecx, DWORD PTR $T3[ebp]
push ecx
lea ecx, DWORD PTR _c$[ebp]
call ?push_back@?$vector@HV?$allocator@H@std@@@std@@QAEX$$QAH@Z ; std::vector<int,std:: ↙
↘ allocator<int> >::push_back
lea edx, DWORD PTR _c$[ebp]
push edx
call ?dump@@YAXPAUvector_of_ints@@@Z ; dump
add esp, 4
push 6
lea ecx, DWORD PTR _c$[ebp]
call ?reserve@?$vector@HV?$allocator@H@std@@@std@@QAEXI@Z ; std::vector<int,std::allocator< ↙
↘ int> >::reserve
lea eax, DWORD PTR _c$[ebp]
push eax
call ?dump@@YAXPAUvector_of_ints@@@Z ; dump
add esp, 4
mov DWORD PTR $T2[ebp], 5
lea ecx, DWORD PTR $T2[ebp]
push ecx
lea ecx, DWORD PTR _c$[ebp]
call ?push_back@?$vector@HV?$allocator@H@std@@@std@@QAEX$$QAH@Z ; std::vector<int,std:: ↙
↘ allocator<int> >::push_back
lea edx, DWORD PTR _c$[ebp]
push edx
call ?dump@@YAXPAUvector_of_ints@@@Z ; dump
add esp, 4
mov DWORD PTR $T1[ebp], 6
lea eax, DWORD PTR $T1[ebp]
push eax
lea ecx, DWORD PTR _c$[ebp]
call ?push_back@?$vector@HV?$allocator@H@std@@@std@@QAEX$$QAH@Z ; std::vector<int,std:: ↙
↘ allocator<int> >::push_back
lea ecx, DWORD PTR _c$[ebp]
push ecx
call ?dump@@YAXPAUvector_of_ints@@@Z ; dump
add esp, 4
push 5
lea ecx, DWORD PTR _c$[ebp]
call ?at@?$vector@HV?$allocator@H@std@@@std@@QAEAAHI@Z ; std::vector<int,std::allocator<int ↙
↘ > >::at
mov edx, DWORD PTR [eax]
push edx
push OFFSET $SG52650 ; '%d'
call DWORD PTR __imp__printf
add esp, 8
mov eax, 8
shl eax, 2
mov ecx, DWORD PTR _c$[ebp]
mov edx, DWORD PTR [ecx+eax]
push edx
push OFFSET $SG52651 ; '%d'
call DWORD PTR __imp__printf
add esp, 8
lea ecx, DWORD PTR _c$[ebp]
call ?_Tidy@?$vector@HV?$allocator@H@std@@@std@@IAEXXZ ; std::vector<int,std::allocator<int ↙
↘ > >::_Tidy
xor eax, eax
mov esp, ebp
pop ebp
ret 0
_main ENDP
从中我们可以看到.at()方法检查下标边界以及异常处理的详细细节。最后一个输出值是printf()函数直接从内存中提取的数值。它在提取数值的时候没有做任何越界检查。
有读者可能会问了,为什么标准向量的数据结构为什么不像标准函数std::string中的那样、单独用一个字段记录size(大小)和capacity(体积)这类的信息呢?虽然笔者无法查证,但是笔者相信大概是.at()在进行边界检查时效率更好吧。
GCC生成的汇编指令也十分雷同,不过它以内联函数的形式实现.at()。
指令清单51.34 GCC 4.8.1 –fno –inline –small –functions –O1编译
main proc near
push ebp
mov ebp, esp
push edi
push esi
push ebx
and esp, 0FFFFFFF0h
sub esp, 20h
mov dword ptr [esp+14h], 0
mov dword ptr [esp+18h], 0
mov dword ptr [esp+1Ch], 0
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints *)
mov dword ptr [esp+10h], 1
lea eax, [esp+10h]
mov [esp+4], eax
lea eax, [esp+14h]
mov [esp], eax
call _ZNSt6vectorIiSaIiEE9push_backERKi ; std::vector<int,std::allocator<int>>::push_back( ↙
↘ int const&)
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints *)
mov dword ptr [esp+10h], 2
lea eax, [esp+10h]
mov [esp+4], eax
lea eax, [esp+14h]
mov [esp], eax
call _ZNSt6vectorIiSaIiEE9push_backERKi ; std::vector<int,std::allocator<int>>::push_back( ↙
↘ int const&)
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints *)
mov dword ptr [esp+10h], 3
lea eax, [esp+10h]
mov [esp+4], eax
lea eax, [esp+14h]
mov [esp], eax
call _ZNSt6vectorIiSaIiEE9push_backERKi ; std::vector<int,std::allocator<int>>::push_back( ↙
↘ int const&)
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints *)
mov dword ptr [esp+10h], 4
lea eax, [esp+10h]
mov [esp+4], eax
lea eax, [esp+14h]
mov [esp], eax
call _ZNSt6vectorIiSaIiEE9push_backERKi ; std::vector<int,std::allocator<int>>::push_back( ↙
↘ int const&)
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints *)
mov ebx, [esp+14h]
mov eax, [esp+1Ch]
sub eax, ebx
cmp eax, 17h
ja short loc_80001CF
mov edi, [esp+18h]
sub edi, ebx
sar edi, 2
mov dword ptr [esp], 18h
call _Znwj ; operator new(uint)
mov esi, eax
test edi, edi
jz short loc_80001AD
lea eax, ds:0[edi*4]
mov [esp+8], eax ; n
mov [esp+4], ebx ; src
mov [esp], esi ; dest
call memmove
loc_80001AD: ; CODE XREF: main+F8
mov eax, [esp+14h]
test eax, eax
jz short loc_80001BD
mov [esp], eax ; void *
call _ZdlPv ; operator delete(void *)
loc_80001BD: ; CODE XREF: main+117
mov [esp+14h], esi
lea eax, [esi+edi*4]
mov [esp+18h], eax
add esi, 18h
mov [esp+1Ch], esi
loc_80001CF: ; CODE XREF: main+DD
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints *)
mov dword ptr [esp+10h], 5
lea eax, [esp+10h]
mov [esp+4], eax
lea eax, [esp+14h]
mov [esp], eax
call _ZNSt6vectorIiSaIiEE9push_backERKi ; std::vector<int,std::allocator<int>>::push_back( ↙
↘ int const&)
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints *)
mov dword ptr [esp+10h], 6
lea eax, [esp+10h]
mov [esp+4], eax
lea eax, [esp+14h]
mov [esp], eax
call _ZNSt6vectorIiSaIiEE9push_backERKi ; std::vector<int,std::allocator<int>>::push_back( ↙
↘ int const&)
lea eax, [esp+14h]
mov [esp], eax
call _Z4dumpP14vector_of_ints ; dump(vector_of_ints *)
mov eax, [esp+14h]
mov edx, [esp+18h]
sub edx, eax
cmp edx, 17h
ja short loc_8000246
mov dword ptr [esp], offset aVector_m_range ; "vector::_M_range_check"
call _ZSt20__throw_out_of_rangePKc ; std::__throw_out_of_range(char const*)
loc_8000246: ; CODE XREF: main+19C
mov eax, [eax+14h]
mov [esp+8], eax
mov dword ptr [esp+4], offset aD ; "%d\n"
mov dword ptr [esp], 1
call __printf_chk
mov eax, [esp+14h]
mov eax, [eax+20h]
mov [esp+8], eax
mov dword ptr [esp+4], offset aD ; "%d\n"
mov dword ptr [esp], 1
call __printf_chk
mov eax, [esp+14h]
test eax, eax
jz short loc_80002AC
mov [esp], eax ; void *
call _ZdlPv ; operator delete(void *)
jmp short loc_80002AC
mov ebx, eax
mov edx, [esp+14h]
test edx, edx
jz short loc_80002A4
mov [esp], edx ; void *
call _ZdlPv ; operator delete(void *)
loc_80002A4: ; CODE XREF: main+1FE
mov [esp], ebx
call _Unwind_Resume
loc_80002AC: ; CODE XREF: main+1EA
; main+1F4
mov eax, 0
lea esp, [ebp-0Ch]
pop ebx
pop esi
pop edi
pop ebp
locret_80002B8: ; DATA XREF: .eh_frame:08000510
; .eh_frame:080005BC
retn
main endp
编译器也对.reserve()函数进行了内联处理。如果缓冲区不大,程序会调用new()方法分配缓冲区的存储空间;使用memmove()方法来复制缓冲区中的内容;最后通过delete()方法来释放缓冲区空间。
如果采用GCC编译的话,编译程序的输出为:
_Myfirst=0x(nil), _Mylast=0x(nil), _Myend=0x(nil)
size=0, capacity=0
_Myfirst=0x8257008, _Mylast=0x825700c, _Myend=0x825700c
size=1, capacity=1
element 0: 1
_Myfirst=0x8257018, _Mylast=0x8257020, _Myend=0x8257020
size=2, capacity=2
element 0: 1
element 1: 2
_Myfirst=0x8257028, _Mylast=0x8257034, _Myend=0x8257038
size=3, capacity=4
element 0: 1
element 1: 2
element 2: 3
_Myfirst=0x8257028, _Mylast=0x8257038, _Myend=0x8257038
size=4, capacity=4
element 0: 1
element 1: 2
element 2: 3
element 3: 4
_Myfirst=0x8257040, _Mylast=0x8257050, _Myend=0x8257058
size=4, capacity=6
element 0: 1
element 1: 2
element 2: 3
element 3: 4
_Myfirst=0x8257040, _Mylast=0x8257054, _Myend=0x8257058
size=5, capacity=6
element 0: 1
element 1: 2
element 2: 3
element 3: 4
element 4: 5
_Myfirst=0x8257040, _Mylast=0x8257058, _Myend=0x8257058
size=6, capacity=6
element 0: 1
element 1: 2
element 2: 3
element 3: 4
element 4: 5
element 5: 6
6
0
从程序我们可以看到,GCC的缓冲区buffer的增长方式与MSVC不同。具体来说,通过一个简单的例子实践可以看到:由MSVC编译的程序每次会把缓冲区增加50%;而由GCC编译的程序则会将缓冲区扩张100%——也就是翻倍。
“二叉树”是另外一种常见的基础数据结构。二叉树是每个节点最多有两个子树的有序树,每个节点都有自己的关键字(key)-值(value)。
二叉树通常会用于构造联合数组(associative arrays)。联合数组又称为词典(dictionary)或 hash 表,它由一系列的“关键字-值”构成。
二叉制树最少具备三个重要的特性:
所有的关键字都以有序方式存储。
任何类型的关键字都能构成二叉树。二叉树的算法与关键字的数据类型无关。只要能够定义一个比较二叉树关键字的排序函数,就可以构造一个二叉树。
与链表和数组相比,在二叉树中搜索特定关键字的速度比较快。
现在开始举例说明:我们将存储数列0, 1, 2, 3, 5, 6, 9, 10, 11, 12, 20, 99, 100, 101, 107, 1001, 1010,并形成下述二叉树结构:
从下面的图中,我们很容易发现这样一个规律:所有左边节点的值都比其右边节点的值小,而所有右边节点的值都比左边的节点的值大。
在遵守以上规则的前提下,我们做起查询算法来就相对简单了。我们只需要关注当前节点的数值以及我们要查询的值,将这两者进行比较,当前者比后者小时,那么我们只需要向右搜索对比查找;反之,当前者比后者大时,我们就会向左搜索对比查找。一直到找到节点的值与要搜索的值相等为止。这就是为什么刚才说只需要一个对比函数即可进行节点数或者字符串对比。
所有的键都有一个唯一的值,不能重复。
我们可以计算需要花费的步数,公式大约为:log2n(n是二进制树中键的个数),这个公式计算的是我们要找到一个平衡树的搜索步数。这就意味着在一个1000个键的二进制树中,通过大约10步我们就能找到需要的值;而10000个键的二进制树中则需要13步。看起来还是很不错的。树总是需要像这样平衡一下,也就是说,键必须在所有水平上平衡分布。插入和移除一些节点可能都需要保持树的平衡状态。
有几个可用的流行平衡算法,包括AVL树和红黑树。后面的算法采用的办法是将每个节点标注为red(红)或者black(黑),以简化结点的平衡过程。
不管是GCC还是MSVC的模板函数std::map()和std::set()都使用了红黑树(red-black)的算法。
在std::set()中只含有键,而std::map()中除了键(std::set)还有相应的每个节点的数值。
MSVC
#include <map>
#include <set>
#include <string>
#include <iostream>
// structure is not packed!
struct tree_node
{
struct tree_node *Left;
struct tree_node *Parent;
struct tree_node *Right;
char Color; // 0 - Red, 1 - Black
char Isnil;
//std::pair Myval;
unsigned int first; // called Myval in std::set
const char *second; // not present in std::set
};
struct tree_struct
{
struct tree_node *Myhead;
size_t Mysize;
};
void dump_tree_node (struct tree_node *n, bool is_set, bool traverse)
{
printf ("ptr=0x%p Left=0x%p Parent=0x%p Right=0x%p Color=%d Isnil=%d\n",
n, n->Left, n->Parent, n->Right, n->Color, n->Isnil);
if (n->Isnil==0)
{
if (is_set)
printf ("first=%d\n", n->first);
else
printf ("first=%d second=[%s]\n", n->first, n->second);
}
if (traverse)
{
if (n->Isnil==1)
dump_tree_node (n->Parent, is_set, true);
else
{
if (n->Left->Isnil==0)
dump_tree_node (n->Left, is_set, true);
if (n->Right->Isnil==0)
dump_tree_node (n->Right, is_set, true);
};
};
};
const char* ALOT_OF_TABS="\t\t\t\t\t\t\t\t\t\t\t";
void dump_as_tree (int tabs, struct tree_node *n, bool is_set)
{
if (is_set)
printf ("%d\n", n->first);
else
printf ("%d [%s]\n", n->first, n->second);
if (n->Left->Isnil==0)
{
printf ("%.*sL-------", tabs, ALOT_OF_TABS);
dump_as_tree (tabs+1, n->Left, is_set);
};
if (n->Right->Isnil==0)
{
printf ("%.*sR-------", tabs, ALOT_OF_TABS);
dump_as_tree (tabs+1, n->Right, is_set);
};
};
void dump_map_and_set(struct tree_struct *m, bool is_set)
{
printf ("ptr=0x%p, Myhead=0x%p, Mysize=%d\n", m, m->Myhead, m->Mysize);
dump_tree_node (m->Myhead, is_set, true);
printf ("As a tree:\n");
printf ("root----");
dump_as_tree (1, m->Myhead->Parent, is_set);
};
int main()
{
// map
std::map<int, const char*> m;
m[10]="ten";
m[20]="twenty";
m[3]="three";
m[101]="one hundred one";
m[100]="one hundred";
m[12]="twelve";
m[107]="one hundred seven";
m[0]="zero";
m[1]="one";
m[6]="six";
m[99]="ninety-nine";
m[5]="five";
m[11]="eleven";
m[1001]="one thousand one";
m[1010]="one thousand ten";
m[2]="two";
m[9]="nine";
printf ("dumping m as map:\n");
dump_map_and_set ((struct tree_struct *)(void*)&m, false);
std::map<int, const char*>::iterator it1=m.begin();
printf ("m.begin():\n");
dump_tree_node ((struct tree_node *)*(void**)&it1, false, false);
it1=m.end();
printf ("m.end():\n");
dump_tree_node ((struct tree_node *)*(void**)&it1, false, false);
// set
std::set<int> s;
s.insert(123);
s.insert(456);
s.insert(11);
s.insert(12);
s.insert(100);
s.insert(1001);
printf ("dumping s as set:\n");
dump_map_and_set ((struct tree_struct *)(void*)&s, true);
std::set<int>::iterator it2=s.begin();
printf ("s.begin():\n");
dump_tree_node ((struct tree_node *)*(void**)&it2, true, false);
it2=s.end();
printf ("s.end():\n");
dump_tree_node ((struct tree_node *)*(void**)&it2, true, false);
};
指令清单51.35 MSVC 2012
dumping m as map:
ptr=0x0020FE04, Myhead=0x005BB3A0, Mysize=17
ptr=0x005BB3A0 Left=0x005BB4A0 Parent=0x005BB3C0 Right=0x005BB580 Color=1 Isnil=1
ptr=0x005BB3C0 Left=0x005BB4C0 Parent=0x005BB3A0 Right=0x005BB440 Color=1 Isnil=0
first=10 second=[ten]
ptr=0x005BB4C0 Left=0x005BB4A0 Parent=0x005BB3C0 Right=0x005BB520 Color=1 Isnil=0
first=1 second=[one]
ptr=0x005BB4A0 Left=0x005BB3A0 Parent=0x005BB4C0 Right=0x005BB3A0 Color=1 Isnil=0
first=0 second=[zero]
ptr=0x005BB520 Left=0x005BB400 Parent=0x005BB4C0 Right=0x005BB4E0 Color=0 Isnil=0
first=5 second=[five]
ptr=0x005BB400 Left=0x005BB5A0 Parent=0x005BB520 Right=0x005BB3A0 Color=1 Isnil=0
first=3 second=[three]
ptr=0x005BB5A0 Left=0x005BB3A0 Parent=0x005BB400 Right=0x005BB3A0 Color=0 Isnil=0
first=2 second=[two]
ptr=0x005BB4E0 Left=0x005BB3A0 Parent=0x005BB520 Right=0x005BB5C0 Color=1 Isnil=0
first=6 second=[six]
ptr=0x005BB5C0 Left=0x005BB3A0 Parent=0x005BB4E0 Right=0x005BB3A0 Color=0 Isnil=0
first=9 second=[nine]
ptr=0x005BB440 Left=0x005BB3E0 Parent=0x005BB3C0 Right=0x005BB480 Color=1 Isnil=0
first=100 second=[one hundred]
ptr=0x005BB3E0 Left=0x005BB460 Parent=0x005BB440 Right=0x005BB500 Color=0 Isnil=0
first=20 second=[twenty]
ptr=0x005BB460 Left=0x005BB540 Parent=0x005BB3E0 Right=0x005BB3A0 Color=1 Isnil=0
first=12 second=[twelve]
ptr=0x005BB540 Left=0x005BB3A0 Parent=0x005BB460 Right=0x005BB3A0 Color=0 Isnil=0
first=11 second=[eleven]
ptr=0x005BB500 Left=0x005BB3A0 Parent=0x005BB3E0 Right=0x005BB3A0 Color=1 Isnil=0
first=99 second=[ninety-nine]
ptr=0x005BB480 Left=0x005BB420 Parent=0x005BB440 Right=0x005BB560 Color=0 Isnil=0
first=107 second=[one hundred seven]
ptr=0x005BB420 Left=0x005BB3A0 Parent=0x005BB480 Right=0x005BB3A0 Color=1 Isnil=0
first=101 second=[one hundred one]
ptr=0x005BB560 Left=0x005BB3A0 Parent=0x005BB480 Right=0x005BB580 Color=1 Isnil=0
first=1001 second=[one thousand one]
ptr=0x005BB580 Left=0x005BB3A0 Parent=0x005BB560 Right=0x005BB3A0 Color=0 Isnil=0
first=1010 second=[one thousand ten]
As a tree:
root----10 [ten]
L-------1 [one]
L-------0 [zero]
R-------5 [five]
L-------3 [three]
L-------2 [two]
R-------6 [six]
R-------9 [nine]
R-------100 [one hundred]
L-------20 [twenty]
L-------12 [twelve]
L-------11 [eleven]
R-------99 [ninety-nine]
R-------107 [one hundred seven]
L-------101 [one hundred one]
R-------1001 [one thousand one]
R-------1010 [one thousand ten]
m.begin():
ptr=0x005BB4A0 Left=0x005BB3A0 Parent=0x005BB4C0 Right=0x005BB3A0 Color=1 Isnil=0
first=0 second=[zero]
m.end():
ptr=0x005BB3A0 Left=0x005BB4A0 Parent=0x005BB3C0 Right=0x005BB580 Color=1 Isnil=1
dumping s as set:
ptr=0x0020FDFC, Myhead=0x005BB5E0, Mysize=6
ptr=0x005BB5E0 Left=0x005BB640 Parent=0x005BB600 Right=0x005BB6A0 Color=1 Isnil=1
ptr=0x005BB600 Left=0x005BB660 Parent=0x005BB5E0 Right=0x005BB620 Color=1 Isnil=0
first=123
ptr=0x005BB660 Left=0x005BB640 Parent=0x005BB600 Right=0x005BB680 Color=1 Isnil=0
first=12
ptr=0x005BB640 Left=0x005BB5E0 Parent=0x005BB660 Right=0x005BB5E0 Color=0 Isnil=0
first=11
ptr=0x005BB680 Left=0x005BB5E0 Parent=0x005BB660 Right=0x005BB5E0 Color=0 Isnil=0
first=100
ptr=0x005BB620 Left=0x005BB5E0 Parent=0x005BB600 Right=0x005BB6A0 Color=1 Isnil=0
first=456
ptr=0x005BB6A0 Left=0x005BB5E0 Parent=0x005BB620 Right=0x005BB5E0 Color=0 Isnil=0
first=1001
As a tree:
root----123
L-------12
L-------11
R-------100
R-------456
R-------1001
s.begin():
ptr=0x005BB640 Left=0x005BB5E0 Parent=0x005BB660 Right=0x005BB5E0 Color=0 Isnil=0
first=11
s.end():
ptr=0x005BB5E0 Left=0x005BB640 Parent=0x005BB600 Right=0x005BB6A0 Color=1 Isnil=1
因为结构设计得并不紧密,所以2个char型值各占用了4字节空间。
对于std::map结构来说,first和second的数据数据组合可被视为std::pair的一个数据元素。而std::set的每个节点则只有1个值。
51.4.2节介绍过,MSVC编译器在构造std::list的时候会保链表的长度信息。我们可以在程序看到它也存储了这种数据类型的具体尺寸。
这两种数据结构和std::list的相同之处是,迭代器都是指向节点的指针。.begin()迭代函数指向最小的键。实际上,整个数据结构里都没有最小键的指针(和链表的情况一样)每当程序调用这个迭代器的时候,它就遍历所有键、查找出最小值。单目递增运算符operator --和单目递减运算符operator++可将指针指向树里的前一个或后一个节点。MIT出版社出版的《Introduction to Algorithms, Third Edition》(Thomas H. Cormen等人著)介绍了这两个运算符的具体算法。
迭代器.end()指向了一个隐蔽的虚节点。其Isnil为1,即是说它没有对应的关键字(key)或值(value)、不是真正意义上的数据结点,仅是一个存储控制信息的容器。这个虚节点的父节点是真正的根节点的指针,也就是整个信息树的顶点。