胡乱写的一些代码，感觉应该有一定的用处，可是还没有想好用在什么情况下，赠与有缘人。
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#include "new" using namespace std; class COneInstance { typedef COneInstance thisClass; int* _piCount; public: COneInstance() { static bool bConstructing = false; if(bConstructing) return; bConstructing = true; thisClass* p = new thisClass; if(p != NULL) delete p; bConstructing = false; if(this != p) // you [may] allocate this object on stack throw bad_alloc(); if(*_piCount == 1) // the very first instance { /// TODO: do some thing printf("Aha, the first constructing!\n"); } } void* operator new(size_t size) { static int iCount = 0; static void* pOnlyInstance = NULL; void* p = NULL; if(iCount == 0) { p = ::operator new(size); pOnlyInstance = p; } else { p = pOnlyInstance; } if(p != NULL) { iCount++; ((thisClass*)p)->_piCount = &iCount; } return p; } void operator delete(void* p) { if(p == NULL) return; int& riCount = *((thisClass*)p)->_piCount; riCount--; if(riCount == 0) ::operator delete(p); } private: void* operator new[](size_t size); void operator delete[](void* p); }; typedef COneInstance COneInstanceOnHeap; class COneInstanceOnStack { public: COneInstanceOnStack() { static int iCount = 0; if(iCount != 0) throw bad_alloc(); iCount++; } private: void* operator new(size_t size); void operator delete(void* p); void* operator new[](size_t size); void operator delete[](void* p); }; void TestOneInstance() { COneInstanceOnHeap* pooh1 = new COneInstanceOnHeap; COneInstanceOnHeap* pooh2 = new COneInstanceOnHeap; printf("COneInstanceOnHeap instance 1: 0x%p\n", pooh1); printf("COneInstanceOnHeap instance 2: 0x%p\n", pooh2); delete pooh1; delete pooh2; try { COneInstanceOnHeap ooh; } catch(bad_alloc& /*rba*/) { printf("we wanna place a COneInstanceOnHeap instance on stack, so exception fired!\n"); } COneInstanceOnStack oos1; try { COneInstanceOnStack oos2; } catch(bad_alloc& /*rba*/) { printf("we wanna place a COneInstanceOnStack instance on stack again, so exception fired!\n"); } // You cannot do this, the compiler will stop you... // COneInstanceOnStack* poos = new COneInstanceOnStack; } |