C++中new的原始碼與行為分析
阿新 • • 發佈:2019-01-28
這是我在處女面中遇到的一個問題,當時回答的不太好,因此下來研究、學習了一下。C++中operator new的行為及其與malloc的區別。當時我回答二者的區別是new在實現中會呼叫malloc並且由編譯器安插呼叫建構函式的程式碼,並在面試官前輩的提醒下又答出malloc失敗返回0,而new直接拋異常。但不清楚有沒有不拋異常的new和不呼叫建構函式的new這一點。下面結合程式碼分析一下。
19: CTest *pTest = new CTest(3, 2);
0027652D 6A 08 push 8
0027652F E8 8D AD FF FF call operator new (02712C1h) ; 預設的operator new
00276534 83 C4 04 add esp,4
00276537 89 85 08 FF FF FF mov dword ptr [ebp-0F8h],eax
0027653D C7 45 FC 00 00 00 00 mov dword ptr [ebp-4],0
00276544 83 BD 08 FF FF FF 00 cmp dword ptr [ebp-0F8h],0
0027654B 74 17 je main+74 h (0276564h)
0027654D 6A 02 push 2
0027654F 6A 03 push 3
00276551 8B 8D 08 FF FF FF mov ecx,dword ptr [ebp-0F8h]
00276557 E8 B4 AE FF FF call CTest::CTest (0271410h) ; 呼叫建構函式
0027655C 89 85 E8 FE FF FF mov dword ptr [ebp-118h],eax
00276562 EB 0A jmp main+7Eh (027656Eh)
00276564 C7 85 E8 FE FF FF 00 00 00 00 mov dword ptr [ebp-118h],0
0027656E 8B 85 E8 FE FF FF mov eax,dword ptr [ebp-118h]
00276574 89 85 14 FF FF FF mov dword ptr [ebp-0ECh],eax
0027657A C7 45 FC FF FF FF FF mov dword ptr [ebp-4],0FFFFFFFFh
00276581 8B 8D 14 FF FF FF mov ecx,dword ptr [ebp-0ECh]
00276587 89 4D EC mov dword ptr [pTest],ecx
20:
21: CTest *pTest2 = new (std::nothrow) CTest(4, 6);
0027658A 68 D0 C2 27 00 push offset std::nothrow (027C2D0h)
0027658F 6A 08 push 8
00276591 E8 63 AC FF FF call operator new (02711F9h) ;多個引數,不拋異常的operator new
00276596 83 C4 08 add esp,8
00276599 89 85 F0 FE FF FF mov dword ptr [ebp-110h],eax
0027659F C7 45 FC 01 00 00 00 mov dword ptr [ebp-4],1
002765A6 83 BD F0 FE FF FF 00 cmp dword ptr [ebp-110h],0
002765AD 74 17 je main+0D6h (02765C6h)
002765AF 6A 06 push 6
002765B1 6A 04 push 4
002765B3 8B 8D F0 FE FF FF mov ecx,dword ptr [ebp-110h]
002765B9 E8 52 AE FF FF call CTest::CTest (0271410h) ; 呼叫建構函式
002765BE 89 85 E8 FE FF FF mov dword ptr [ebp-118h],eax
20:
21: CTest *pTest2 = new (std::nothrow) CTest(4, 6);
002765C4 EB 0A jmp main+0E0h (02765D0h)
002765C6 C7 85 E8 FE FF FF 00 00 00 00 mov dword ptr [ebp-118h],0
002765D0 8B 85 E8 FE FF FF mov eax,dword ptr [ebp-118h]
002765D6 89 85 FC FE FF FF mov dword ptr [ebp-104h],eax
002765DC C7 45 FC FF FF FF FF mov dword ptr [ebp-4],0FFFFFFFFh
002765E3 8B 8D FC FE FF FF mov ecx,dword ptr [ebp-104h]
002765E9 89 4D E0 mov dword ptr [pTest2],ecx
22:
23: pTest->m_dw1 = 3;
002765EC 8B 45 EC mov eax,dword ptr [pTest]
002765EF C7 00 03 00 00 00 mov dword ptr [eax],3
24: pTest2->m_dw2 = 5;
002765F5 8B 45 E0 mov eax,dword ptr [pTest2]
002765F8 C7 40 04 05 00 00 00 mov dword ptr [eax+4],5
25:
26:
27: return 0;
002765FF 33 C0 xor eax,eax
28: }
operator new的原始碼:
void* __CRTDECL operator new(size_t const size)
{
for (;;)
{
if (void* const block = malloc(size))
{
return block;
}
if (_callnewh(size) == 0)
{
if (size == SIZE_MAX)
{
__scrt_throw_std_bad_array_new_length();
}
else
{
__scrt_throw_std_bad_alloc();
}
}
// The new handler was successful; try to allocate again...
}
}
operator new的原始碼,嗯嗯,果然不可能拋異常,因為異常都被接住了:
void* __CRTDECL operator new(size_t const size, std::nothrow_t const&) noexcept
{
try
{
return operator new(size);
}
catch (...)
{
return nullptr;
}
}
delete操作都是一樣的,呼叫了帶兩個引數的operator delete,這個應該也是後來搞出來的一個函式,我印象還比較深。之前在嘗試將VS2017編譯的obj與WDK中lib版CRT庫匹配時曾遇到過這個帶了兩個引數的delete找不到匹配的符號,結果用了/Zc:sizedDealloc-開關才避免其生成。詳細過程請見這篇文章
26: delete(pTest);
013865FF 8B 45 EC mov eax,dword ptr [pTest]
01386602 89 85 E4 FE FF FF mov dword ptr [ebp-11Ch],eax
01386608 6A 08 push 8
0138660A 8B 8D E4 FE FF FF mov ecx,dword ptr [ebp-11Ch]
01386610 51 push ecx
01386611 E8 3F AA FF FF call operator delete (01381055h)
01386616 83 C4 08 add esp,8
27: delete(pTest2);
01386619 8B 45 E0 mov eax,dword ptr [pTest2]
0138661C 89 85 D8 FE FF FF mov dword ptr [ebp-128h],eax
01386622 6A 08 push 8
01386624 8B 8D D8 FE FF FF mov ecx,dword ptr [ebp-128h]
0138662A 51 push ecx
0138662B E8 25 AA FF FF call operator delete (01381055h)
01386630 83 C4 08 add esp,8
在其中呼叫了帶一個引數的operator delete:
28: void __CRTDECL operator delete(void* const block, size_t const) noexcept
29: {
01384FF0 55 push ebp
01384FF1 8B EC mov ebp,esp
30: operator delete(block);
01384FF3 8B 45 08 mov eax,dword ptr [block]
30: operator delete(block);
01384FF6 50 push eax
01384FF7 E8 36 C0 FF FF call operator delete (01381032h)
01384FFC 83 C4 04 add esp,4
31: }
01384FFF 5D pop ebp
01385000 C3 ret
而這個operator delete則呼叫了free:
void __CRTDECL operator delete(void* const block) noexcept
{
#ifdef _DEBUG
_free_dbg(block, _UNKNOWN_BLOCK);
#else
free(block);
#endif
}
結論:可以通過std::nothrow讓new不拋異常,VS17下的做法是在該new函式中用try捕獲了異常,並不再丟擲,同時讓其返回0;對於有建構函式的類物件,new時都會呼叫其建構函式,是由編譯器自動插入程式碼的。