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为什么我在for循环中无法访问动态分配的内存？

c++pointerspolymorphism
5

我为我的子类类型股票（stock）新建了一个继承自基类工具（instrument）的内存，但当我尝试访问数组的第二个元素时，它会抛出错误。当我的新数组大小为1时，情况是好的。

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

class Instrument{

public:
    virtual void display(){}
    virtual void output(){}
    virtual void readFile(){}
    virtual ~Instrument(){}
};
class Stock : 
    public Instrument{
public:
    Stock(){

    }
    virtual void input(){
        cout << "This is stock, please input its information: ";
        cin >> name >> bidPrice >> askPrice >> lastPrice >> issueExchange;
    }
    virtual void display(){
        cout <<"This is to display stock: "<< name << " "
            << bidPrice << " "
            << askPrice << " "
            << lastPrice << " "
            << issueExchange << " "
            << endl;
    }
    virtual void output(){
        ofstream myfile;
        myfile.open("Stock.txt", ios::out | ios::app);
        if (myfile.is_open()){
            myfile << "This is a stock: "
                << name << " "
                << bidPrice << " "
                << askPrice << " "
                << lastPrice << " "
                << issueExchange << " "
                << endl;
        }
        else cout << "Unable to open file";
    }
    virtual void readFile(){
        string line;
        ifstream myfile("Stock.txt");
        cout << "\nThis is file stored\n";
        if (myfile.is_open())
        {
            while (getline(myfile, line))
            {
                cout << line << '\n';
            }
            myfile.close();
        }
    }
    virtual ~Stock(){}
private:
    char name[13];
    double bidPrice;
    double askPrice;
    double lastPrice;
    int issueExchange;

};


int main(){

    const int N = 5;//it works fine if I use N=1;
    Instrument *pBase = NULL;
    pBase = new Stock[N];

    for (int i = 0; i < N; i++){
        pBase[i].input();// here throws an exception and ends the program
        pBase[i].display();
        pBase[i].output();
    }
    pBase[N - 1].readFile();
    delete[] pBase;

    system("pause");
    return 0;

}
@GermaineJason 这不是Java或C#。有对象，而空指针异常不存在。 - deviantfan
@MooingDuck，它已经从字符串更改为char [13]，你看到我的#include<string>了吗？ - user4592991
1
问题在于您正在将 Stock 数组访问，就好像它是一个 Instrument 数组。多态不能在整个数组之间起作用，您需要指向每个单独元素的指针。 - Galik
1
@Galik：请不要在评论区回答问题。那是用来回答的_answer_部分。谢谢。 - Lightness Races in Orbit
@deviantfan：是的，他错了。 - Lightness Races in Orbit
显示剩余2条评论
3个回答
7

多态和指针算术不搭配，因为对象在数组中的排列取决于最终派生类的大小，而多态会丢失这些信息。动态分配只是一个误导，你可以在以下情况下看到相同的问题：

Derived array[2];
Base* p = array;

printf("%p\n", &array[0]);
printf("%p\n", p);
printf("%p\n", &array[1]);
printf("%p\n", p + 1);

printf("%z\n", sizeof (array[0]));
printf("%z\n", sizeof (*p));

注意，使用 array 的指针值向前移动 sizeof (Derived)，但是使用 p 的指针算术运算向前移动 sizeof (Base)，并且无法找到真正的对象。
通常您会使用 Base* 数组来修复这个问题，而不是单个的 Base* 与指针算术相结合。
Base* pp[2];
for( auto& elem : array ) pp[&elem - array] = &elem;

printf("%p\n", &array[1]);
printf("%p\n", pp[1]);

// use (*pp[1]) or pp[1]->whatever

另一种选项是使用记住原始类型的对象：

Derived* allocated = new Derived[N];
std::function<Base& (int)> poly = [allocated](int i){ return allocated[i]; };

使用poly(i)代替p[i]。

但是要注意，不能执行delete [] &poly(0);，因为delete[]也不具有多态性。

使用std::unique_ptr<Derived[]>和std::bind，可以在访问器对象最终超出范围时自动释放内存。

3
虽然准确，但这个答案在解释方面有些不足，不是吗？ - Mooing Duck
是的，这是关键，但我的任务要求我同时使用多态和数组，我尝试使用instrument *p[N]，并访问p[i]，但在这里它无法工作。这是我尝试的新方法。 - user4592991
1
@Leewis 你可以使用指向单个对象的指针数组（任何类型）来代替对象本身的数组。 - deviantfan
是的，我在 Stack Overflow 上看到初学者代码中经常出现这种模式。但我不喜欢它。n 次 new，而且没有内存局部性 :( - Lightness Races in Orbit
2
@LightnessRacesinOrbit：你还可以使用指针数组，每个指针对应数组中的相应元素。 - Ben Voigt
显示剩余2条评论
0

虽然Ben先生的方法绝对是正确的，但我完全感觉他的C++和我的C++不是同一种语言，他在这里混合了一些奇怪的东西，因此根据他的想法，我尝试修改我的代码如下。

#include <iostream>
#include <fstream>
#include <string>
#include <vector>
#include <memory>
using namespace std;

class Instrument{

public:
    virtual void display() = 0;
    virtual void output() = 0;
    virtual void readFile() = 0;
    virtual ~Instrument(){};
};
class Stock :
    public Instrument{
public:
    Stock(){
        cout << "This is stock, please input its information: ";
        cin >> name >> bidPrice >> askPrice >> lastPrice >> issueExchange;
    }
    virtual void display(){
        cout << "This is to display stock: " << name << " "
            << bidPrice << " "
            << askPrice << " "
            << lastPrice << " "
            << issueExchange << " "
            << endl;
    }
    virtual void output(){
        ofstream myfile;
        myfile.open("Stock.txt", ios::out | ios::app);
        if (myfile.is_open()){
            myfile << "This is a stock: "
                << name << " "
                << bidPrice << " "
                << askPrice << " "
                << lastPrice << " "
                << issueExchange << " "
                << endl;
        }
        else cout << "Unable to open file";
    }
    virtual void readFile(){
        string line;
        ifstream myfile("Stock.txt");
        cout << "\nThis is file stored\n";
        if (myfile.is_open())
        {
            while (getline(myfile, line))
            {
                cout << line << '\n';
            }
            myfile.close();
        }
    }
    virtual ~Stock(){}
private:
    string name;
    double bidPrice;
    double askPrice;
    double lastPrice;
    int issueExchange;

};
class Option :
    public Instrument{
public:
    Option(){
        cout << "This is option, please input its information: ";
        cin >> name >> uname >> bidPrice >> askPrice >> lastPrice >> contractSize >> exp;
    }
    virtual void display(){
        cout << "This is to display option: "
            << name << " "
            << uname << " "
            << bidPrice << " "
            << askPrice << " "
            << lastPrice << " "
            << contractSize << " "
            << exp << " "
            << endl;
    }
    virtual void output(){
        ofstream myfile;
        myfile.open("Option.txt", ios::out | ios::app);
        if (myfile.is_open()){
            myfile << "This is an option: "
                << name << " "
                << uname << " "
                << bidPrice << " "
                << askPrice << " "
                << lastPrice << " "
                << contractSize << " "
                << exp << " "
                << endl;
        }
        else cout << "Unable to open file";
    }
    virtual void readFile(){
        string line;
        ifstream myfile("Option.txt");
        cout << "\nThis is file stored\n";
        if (myfile.is_open())
        {
            while (getline(myfile, line))
            {
                cout << line << '\n';
            }
            myfile.close();
        }
    }
    virtual ~Option(){}
private:
    string name;
    string uname;
    double bidPrice;
    double askPrice;
    double lastPrice;
    int contractSize;
    double exp;
};
class Future :
    public Instrument{
public:
    Future(){
        cout << "This is option, please input its information: ";
        cin >> name >> uname >> bidPrice >> askPrice >> lastPrice >> contractSize >> tickSize >> contractMonth;
    }
    virtual void display(){
        cout << "This is to display option: "
            << name << " "
            << uname << " "
            << bidPrice << " "
            << askPrice << " "
            << lastPrice << " "
            << contractSize << " "
            << tickSize << " "
            << contractMonth << " "
            << endl;
    }
    virtual void output(){
        ofstream myfile;
        myfile.open("Future.txt", ios::out | ios::app);
        if (myfile.is_open()){
            myfile << "This is a future: "
                << name << " "
                << uname << " "
                << bidPrice << " "
                << askPrice << " "
                << lastPrice << " "
                << contractSize << " "
                << tickSize << " "
                << contractMonth << " "
                << endl;
        }
        else cout << "Unable to open file";
    }
    virtual void readFile(){
        string line;
        ifstream myfile("Future.txt");
        cout << "\nThis is file stored\n";
        if (myfile.is_open())
        {
            while (getline(myfile, line))
            {
                cout << line << '\n';
            }
            myfile.close();
        }
    }
    virtual ~Future(){}
private:
    string name;
    string uname;
    double bidPrice;
    double askPrice;
    double lastPrice;
    int contractSize;
    int tickSize;
    int contractMonth;
};

int main(){

    int N = 20;
    //shared_ptr<Instrument> pBase[N];
    vector<shared_ptr<Instrument>> pBase(N);
    int i = 5;
    for (i = 0; i < N; i++) pBase[i] = make_shared<Stock>();
    for (i = 0; i < N; i++){
        pBase[i]->display();
        pBase[i]->output();
    }
    pBase[N - 1]->readFile();

    for (i = 0; i < N; i++) pBase[i] = make_shared<Option>();
    for (i = 0; i < N; i++){
        pBase[i]->display();
        pBase[i]->output();
    }
    pBase[N - 1]->readFile();

    for (i = 0; i < N; i++) pBase[i] = make_shared<Future>();
    for (i = 0; i < N; i++){
        pBase[i]->display();
        pBase[i]->output();
    }
    pBase[N - 1]->readFile();

    system("pause");
    return 0;

}
0

简短回答：不要将子类数组（Stock[N]）转换为基类指针（pBase）。直接使用Stock*，或者创建一个指针数组。

auto arr = new Stock*[N];
for (int i = 0; i < N; i++) {
  arr[i] = new Stock();
}

// unrelated but suggested: C++11 unique_ptr is recommended:
vector<unique_ptr<Stock>> v(N);

详细原因：
1）数组方括号运算符是语法糖：a[b] -> *(a + b)；
2）指针算术不具有多态性，它始终基于静态类型：
pBase[i] -> *(pBase+i) -> *(pBase*)((char*)pBase + sizeof(Instrument) * i);

3) 然而，这正是你想要的：

pBase[i] -> *(pBase+i) -> *(pBase*)((char*)pBase + sizeof(Stock) * i);

4) 只要 sizeof(Instrument) != sizeof(Stock)，你就会遇到麻烦。

我做了，看我的答案，vector<shared_ptr<Instrument>> pBase(N); - user4592991
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