testobj.h:
#ifndef __TESTOBJ_H #define __TESTOBJ_H #include <iostream> class TestObj{ private: int *p; public: static int count; TestObj(){ p = new int(1); count++; std::cout << "init p" << std::endl; }; TestObj(const TestObj & obj){ p = new int(1); count++; std::cout << "copy p" << std::endl; }; ~TestObj(){ if (p != NULL){ delete p; p = NULL; std::cout << "delete p , p="<< p<< std::endl; } }; }; int TestObj::count = 0; #endif
test.cc
#include <iostream> #include <vector> #include <windows.h> #include "testobj.h" using namespace std; int main(){ vector<TestObj> objvector; //objvector.reserve(10); TestObj obj1; objvector.push_back(obj1); Sleep(1000); cout << "------------" << endl; TestObj obj2; objvector.push_back(obj2); Sleep(1000); cout << "------------" << endl; TestObj obj3; objvector.push_back(obj3); Sleep(1000); cout << "------------" << endl; cout << "Count:" << TestObj::count << endl; }
运行结果为:
init p
copy p
------------
init p
copy p
delete p , p=00000000
copy p
------------
init p
copy p
copy p
delete p , p=00000000
delete p , p=00000000
copy p
------------
Count:9
delete p , p=00000000
delete p , p=00000000
delete p , p=00000000
delete p , p=00000000
delete p , p=00000000
delete p , p=00000000
按道理来说应该是3次普通的对象创建,和三次对象的复制构造,但是结果却多出了3次对象的复制构造。下面来分析原因。
首先得了解std::vector的元素存放方式,std::vector是以连续的方式存放数据的,当添加一个元素时,如果容器的空间不足,std::vector就需要重新分配存储空间,用来存放原来的元素和新添加的元素,存放在旧存储空间中的元素被复制到新的存储空间,接着插入新的元素,最后撤销旧的存储空间。如果提前就预分配了足够的空间(objvector.reserve(10);),就不会有额外的复制操作。下图说明了以上运行结果的行为:
最后的6个析构是创建的3个Testobj对象和objvector中的3个Testobj对象的析构。