在前面看过了一个vectorcoredump的样例,接触了vector的iterator,能够知道vector的iterator仅仅有一个成员_M_current指向vector某一个元素.
先看一个样例:
1 #include <vector>
2
3 void init( std::vector<int>& vec )
4 {
5 for ( int i = 0; i < 0x10; i++ )
6 {
7 vec.push_back( i );
8 }
9 }
10
11 int getSum( std::vector<int>& vec )
12 {
13 std::vector<int>::iterator iter;
14 int result = 0;
15
16 for ( iter = vec.begin(); iter != vec.end(); iter++ )
17 {
18 result += *iter;
19 }
20
21 return result;
22 }
23
24 int main()
25 {
26 std::vector<int> vec;
27 init( vec );
28
29 return getSum( vec );
30 }
因为仅仅是考察iterator,仅仅看getSum的汇编:
(gdb) disassemble getSum Dump of assembler code for function _Z6getSumRSt6vectorIiSaIiEE: 0x080486cd <+0>: push %ebp 0x080486ce <+1>: mov %esp,%ebp 0x080486d0 <+3>: sub $0x38,%esp 0x080486d3 <+6>: lea -0x18(%ebp),%eax 0x080486d6 <+9>: mov %eax,(%esp) 0x080486d9 <+12>: call 0x8048840 <_ZN9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEC2Ev> 0x080486de <+17>: movl $0x0,-0xc(%ebp) 0x080486e5 <+24>: lea -0x1c(%ebp),%eax 0x080486e8 <+27>: mov 0x8(%ebp),%edx 0x080486eb <+30>: mov %edx,0x4(%esp) 0x080486ef <+34>: mov %eax,(%esp) 0x080486f2 <+37>: call 0x804884e <_ZNSt6vectorIiSaIiEE5beginEv> 0x080486f7 <+42>: sub $0x4,%esp 0x080486fa <+45>: mov -0x1c(%ebp),%eax 0x080486fd <+48>: mov %eax,-0x18(%ebp) 0x08048700 <+51>: jmp 0x804872f <_Z6getSumRSt6vectorIiSaIiEE+98> 0x08048702 <+53>: lea -0x18(%ebp),%eax 0x08048705 <+56>: mov %eax,(%esp) 0x08048708 <+59>: call 0x80488f8 <_ZNK9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEdeEv> 0x0804870d <+64>: mov (%eax),%eax 0x0804870f <+66>: add %eax,-0xc(%ebp) 0x08048712 <+69>: lea -0x10(%ebp),%eax 0x08048715 <+72>: movl $0x0,0x8(%esp) 0x0804871d <+80>: lea -0x18(%ebp),%edx 0x08048720 <+83>: mov %edx,0x4(%esp) 0x08048724 <+87>: mov %eax,(%esp) 0x08048727 <+90>: call 0x80488c4 <_ZN9__gnu_cxx17__normal_iteratorIPiSt6vectorIiSaIiEEEppEi> 0x0804872c <+95>: sub $0x4,%esp 0x0804872f <+98>: lea -0x14(%ebp),%eax 0x08048732 <+101>: mov 0x8(%ebp),%edx 0x08048735 <+104>: mov %edx,0x4(%esp) 0x08048739 <+108>: mov %eax,(%esp) 0x0804873c <+111>: call 0x8048872 <_ZNSt6vectorIiSaIiEE3endEv> 0x08048741 <+116>: sub $0x4,%esp 0x08048744 <+119>: lea -0x14(%ebp),%eax 0x08048747 <+122>: mov %eax,0x4(%esp) 0x0804874b <+126>: lea -0x18(%ebp),%eax 0x0804874e <+129>: mov %eax,(%esp) 0x08048751 <+132>: call 0x8048898 <_ZN9__gnu_cxxneIPiSt6vectorIiSaIiEEEEbRKNS_17__normal_iteratorIT_T0_EESA_> 0x08048756 <+137>: test %al,%al 0x08048758 <+139>: jne 0x8048702 <_Z6getSumRSt6vectorIiSaIiEE+53> 0x0804875a <+141>: mov -0xc(%ebp),%eax 0x0804875d <+144>: leave 0x0804875e <+145>: ret End of assembler dump.
在0x0804874b打断点.由上面汇编可知,iter的this指针是ebp-0x18,而vec的this指针放在ebp+0x8.
运行到断点处,看一下vec的内容:
(gdb) x $ebp+8 0xbffff5a0: 0xbffff5b4 (gdb) x /4x 0xbffff5b4 0xbffff5b4: 0x0804b068 0x0804b0a8 0x0804b0a8 0x080491f0 (gdb) x /16x 0x0804b068 0x804b068: 0x00000000 0x00000001 0x00000002 0x00000003 0x804b078: 0x00000004 0x00000005 0x00000006 0x00000007 0x804b088: 0x00000008 0x00000009 0x0000000a 0x0000000b 0x804b098: 0x0000000c 0x0000000d 0x0000000e 0x0000000f
看一下iter的内容怎样变化,运行完几次c命令之后(注意,断点地址在运行iter++之后):
Breakpoint 1, 0x0804874b in getSum(std::vector<int, std::allocator<int> >&) () (gdb) x /4x $ebp-0x18 0xbffff580: 0x0804b068 0x0804b0a8 0x00000001 0x00000000 (gdb) c Continuing. Breakpoint 1, 0x0804874b in getSum(std::vector<int, std::allocator<int> >&) () (gdb) x /4x $ebp-0x18 0xbffff580: 0x0804b06c 0x0804b0a8 0x0804b068 0x00000000 (gdb) c Continuing. Breakpoint 1, 0x0804874b in getSum(std::vector<int, std::allocator<int> >&) () (gdb) x /4x $ebp-0x18 0xbffff580: 0x0804b070 0x0804b0a8 0x0804b06c 0x00000001 (gdb) c Continuing. Breakpoint 1, 0x0804874b in getSum(std::vector<int, std::allocator<int> >&) () (gdb) x /4x $ebp-0x18 0xbffff580: 0x0804b074 0x0804b0a8 0x0804b070 0x00000003 (gdb) c Continuing. Breakpoint 1, 0x0804874b in getSum(std::vector<int, std::allocator<int> >&) () (gdb) x /4x $ebp-0x18 0xbffff580: 0x0804b078 0x0804b0a8 0x0804b074 0x00000006
可见vector的iterator确实仅仅有一个成员_Ptr,它的取值范围是
vec. _M_start? <= _M_current < vec. _M_finish
