定位放置new操作的语法形式不同于普通的new操作。例如,一般都用如下语句A* p=new A;申请空间,而定位放置new操作则使用如下语句A* p=new (ptr) A;申请空间,其中ptr就是程序员指定的内存首地址。
通常,定位new负责在堆中找到一个足以能满足要求的内存块。new运算符还有另外一种变体,被称为定位(placement)new运算符,他让您能够制定要使用的位置。
例子如下:
#include<iostream> #include<string> #include<new> using namespace std; const int BUF = 512; class JustTesting { private: string words; int number; public: JustTesting(const string &s = "Just Testing", int n = 0) { words = s; number = n; cout << words << " constructed\n"; } ~JustTesting() { cout << words << " destroyed\n"; } void Show()const { cout << words << ", " << number << endl; } }; int main() { char * buffer = new char[BUF]; JustTesting *pc1, *pc2; pc1 = new (buffer) JustTesting; pc2 = new JustTesting("heap1", 20); cout << "Memory block addresses:\n" << "buffer: " << (void *)buffer << " heap: " << pc2 << endl; cout << "Memory contents:\n"; cout << pc1 << ": "; pc1->Show(); cout << pc2 << ": "; pc2->Show(); JustTesting *pc3, *pc4; pc3 = new(buffer) JustTesting("Bad Idea", 6); pc4 = new JustTesting("Heap2", 10); cout << "Memory contents:\n"; cout << pc3 << ": "; pc3->Show(); cout << pc4 << ": "; pc4->Show(); delete pc2; delete pc4; delete []buffer; cout << "Done\n"; return 0; }这个程序出现两个问题。第一:在创建第二个对象时,定位new运算符使用第一个新对象来覆盖用于第一个对象的内存单元。显然,如果类动态地为其成员分配内存,将引发问题。
第二:将delete用于pc2和pc4时,将自动调用为pc2和pc4指向的对象调用析构函数;然而,将delete[]用于buffer时,不会使用定位new运算符创建的对象调用析构函数。
不能使用delete pc1;delete pc3。原因在于delete可以和常规new运算符配合使用,但是不能和定位new运算符配合使用。
代码二:
#include<iostream> #include<string> #include<new> using namespace std; const int BUF = 512; class JustTesting { private: string words; int number; public: JustTesting(const string &s = "Just Testing", int n = 0) { words = s; number = n; cout << words << " constructed\n"; } ~JustTesting() { cout << words << " destroyed\n"; } void Show()const { cout << words << ", " << number << endl; } }; int main() { char * buffer = new char[BUF]; JustTesting *pc1, *pc2; pc1 = new (buffer)JustTesting; pc2 = new JustTesting("Heap1", 20); cout << "Memory block addresses:\n" << " buffer: " << (void*)buffer << " heap: " << pc2 << endl; cout << "Memory contents:\n"; cout << pc1 << ": "; pc1->Show(); cout << pc2 << ": "; pc2->Show(); JustTesting *pc3, *pc4; pc3 = new(buffer + sizeof(JustTesting)) JustTesting("Better Idea", 6); pc4 = new JustTesting("Heap2", 10); cout << "Memory contents:\n"; cout << pc3 << ":"; pc3->Show(); cout << pc4 << ": "; pc4->Show(); delete pc2; delete pc4; pc3->~JustTesting(); pc1->~JustTesting(); delete[]buffer; cout << "Done\n"; while (1); return 0; }解决这种问题方案是,显式地调用析构函数,必须指定要销毁的对象。需要注意一点是,正确的删除顺序。应以与创建顺序相反的顺序进行删除。原因在于,晚创建的对象可以依赖于早创建的对象。对象创建完成过后,才能释放用于存储这些对象的缓冲区。