malloc/free和new/delete的區別
- malloc/free是C/C++標准庫的函數；new/delete是C++操作符。
- malloc/free只是動態分配內存空間/釋放空間；new/delete除了分配空間還會調用構造函數和析構函數進行初始化與清理資源。
- malloc/free需要手動計算類型大小且返回值類型為
void*；new/delete可自動計算類型的大小，返回對應類型的指針。 - malloc/free管理內存失敗會返回0；new/delete等的方式管理內存失敗會拋出異常。
在C++ Primer書中有提到說： new/delete的表達式與標准庫函數同名了，所以系統並沒有重載new或delete表達式。new/delete真正的實現其實是依賴下面這幾個內存管理接口的。
void *operator new(size_t); //allocate an object
void *operator delete(void *); //free an object
void *operator new[](size_t); //allocate an array
void *operator delete[](void *); //free an array
malloc/free和new/delete的底層實現
new的底層實現
// new.cpp
#include <cstdlib>
#include <new>
_C_LIB_DECL
int __cdecl _callnewh(size_t size) _THROW1(_STD bad_alloc);
_END_C_LIB_DECL
void *__CRTDECL operator new(size_t size) _THROW1(_STD bad_alloc)
{ // try to allocate size bytes
void *p;
while ((p = malloc(size)) == 0)
if (_callnewh(size) == 0)
{ // report no memory
_THROW_NCEE(_XSTD bad_alloc, );
}
return (p);
}
delete的底層實現
#include <cruntime.h>
#include <malloc.h>
#include <new.h>
#include <windows.h>
#include <rtcsup.h>
void operator delete( void * p )
{
RTCCALLBACK(_RTC_Free_hook, (p, 0));
free( p );
}
new[]的底層實現
#include <new>
void *__CRTDECL operator new[](size_t count) _THROW1(std::bad_alloc)
{ // try to allocate count bytes for an array
return (operator new(count));
}
delete[]的底層實現
#ifdef CRTDLL
#undef CRTDLL
#endif
#ifdef MRTDLL
#undef MRTDLL
#endif
#define _USE_ANSI_CPP // suppress defaultlib directive for Std C++ Lib
#include <new>
extern void __CRTDECL operator delete[](void *ptr) _THROW0();
void __CRTDECL operator delete[](void *ptr,
const std::nothrow_t&) _THROW0()
{ // free an allocated object
operator delete[](ptr);
}
malloc/free和new/delete的執行過程
new的執行過程
new(int size) --> operator new() --> malloc() --> constructor function --> return ptr
delete的執行過程
delete ptr --> destructor function --> operator delete() --> free
new[]的執行過程
new[count] --> operator new[]() --> operator new() --> malloc() --> constructor function --> return ptr
delete[]的執行過程
delete[] ptr --> destructor function --> operator delete[]() --> operator delete() --> free
- 依次調用指針指向對象數組中每個對象的析構函數
- 調用
operator delete[]()，operator delete[]()再調用operator delete - 底層用free執行
operator delete表達式，依次釋放內存 operator delete[]()數組的個數存放在指針的前4位