vector预分配空间溢出

vector

当一个vector预分配的存储空间用完之后，为维护其连续的对象数组，它必须在另外一个地方重新分配大块新的（更大的）存储空间，并把以前已有的对象拷贝到新的存储空间中去。

// A class to track various object activities
#ifndef NOISY_H
#define NOISY_H
#include<iostream>
using std::endl;
using std::cout;
using std::ostream;
class Noisy{
    //设置一些static变量用来跟踪所有的创建，赋值（使用运算符operator=), 拷贝构造和析构操作
    static long create, assign, copycons, destroy;
    long id;
public:
    Noisy() :id(create++)//
    {
        cout << "d[" << id << "]" << endl;
    }
    Noisy(const Noisy& rv) :id(rv.id)
    {
        cout << "c[" << id << "]" << endl;
        ++copycons;
    }
    Noisy& operator=(const Noisy& rv){//赋值运算符operator=
        cout << "(" << id << ")=[" << rv.id << "]" << endl;
        id = rv.id;
        ++assign;
        return *this;
    }
    //为了支持排序和查找，Noisy 必须有运算符operator< 和 operator=
    //这仅仅是比较id值
    friend bool operator==(const Noisy& lv, const Noisy& rv)
    {
        return lv.id == rv.id;
    }
    friend bool operator<(const Noisy& lv, const Noisy& rv)
    {
        return lv.id < rv.id;
    }
    ~Noisy(){
        cout << "~[" << id << "]" << endl;
        ++destroy;
    }
    friend ostream& operator<<(ostream& os, const Noisy& n)
    {
        return os << n.id;
    }
    friend class NoisyReport;
};
//Ojects  of type NoisyGen are funciton objects(since there is an operator()) 
//that produce Noisy objects during testing
struct NoisyGen{
    Noisy operator()(){
        return Noisy();
    }
};
//a singleton will automatically report the statistics as the program terminates
class NoisyReport{
    static NoisyReport nr;
    NoisyReport(){}//private constructor
    NoisyReport& operator=(NoisyReport&);//disallowed
    NoisyReport(const NoisyReport&);//disallowed
public:
    //beause we only want one report printed at program termination. 
    //It has a private constructor  so no additional NoisyReport objects can be created
    //it disallows assignment and copy-construction, 
    //and it has a single static instance of NoisyReport called nr. 
    //the only executable statements are in the destructor, which is called as the program
    //exits 
    //and static destructors are called. 
    //the distructor printss the statistics captured by the static variables in Noisy
    ~NoisyReport()
    {
        cout << "
.................
"
            << "Noisy creations: " << Noisy::create
            << "
Copy-Constructions: " << Noisy::copycons
            << "
Assignment: " << Noisy::assign
            << "
Destructions: " << Noisy::destroy << endl;
    }
};
#endif

#include"Noisy.h"
long Noisy::create = 0, Noisy::assign = 0, Noisy::copycons = 0, Noisy::destroy = 0;
NoisyReport NoisyReport::nr;

//using Noisy.h, the following program shows a vector overflowing
#include<iostream>
#include<string>
#include<vector>
#include<cstdlib>
#include"Noisy.h"
using namespace  std;
int main(int argc, char* argv[])
{
    int size = 1000;
    if (argc >= 2) size = atoi(argv[1]);
    vector<Noisy> vn;
    Noisy n;
    for (int i = 0; i < size; i++)
        vn.push_back(n);
    cout << "
 clearning up" << endl;

}

测试结果

Note that the use of reserve( ) is different from using the vector constructor with an integral first argument; the latter initializes a prescribed number of elements using the element type’s default constructor.

是不是说

vector<Noisy> v（100）这用形式调用100次构造函数

而reverse其实是不调用的，只预留空间

The deque also allows random access with operator[ ], but it’s not quite as fast as vector’s operator[ ].