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;
}
運行結果爲:
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
按道理來說應該是3次普通的對象創建,和三次對象的複製構造,但是結果卻多出了3次對象的複製構造。下面來分析原因。
首先得了解std::vector的元素存放方式,std::vector是以連續的方式存放數據的,當添加一個元素時,如果容器的空間不足,std::vector就需要重新分配存儲空間,用來存放原來的元素和新添加的元素,存放在舊存儲空間中的元素被複制到新的存儲空間,接着插入新的元素,最後撤銷舊的存儲空間。如果提前就預分配了足夠的空間(objvector.reserve(10);),就不會有額外的複製操作。下圖說明了以上運行結果的行爲:
最後的6個析構是創建的3個Testobj對象和objvector中的3個Testobj對象的析構。