Why can't I access dynamically allocated memory in my for loop?

Question

I create new memory for my child class, which inherits from the base class tool, when I try to access the second element of my array, it throws an error. Everything is fine when I have a new array size 1

#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; } 

Answer 1

Polymorphism and arithmetic of pointers do not mix, since the location of objects inside the array depends on the derived size itself, and polymorphism loses this information. Dynamic allocation is a red herring, you can see the same problem:

 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)); 

Note that the pointer values ​​are moved forward by sizeof (Derived) using array , but pointer arithmetic using p moves forward sizeof (Base) and cannot find real objects.

You usually fix this with a Base* array instead of a single Base* in combination with pointer arithmetic.

 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 

Another option is to use an object that remembers the original type:

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

and use poly(i) instead of p[i]

But a warning, you CANNOT do delete [] &poly(0); , because delete[] also not polymorphic.

Using std::unique_ptr<Derived[]> and std::bind , you can arrange for automatic release when the accessory object finally goes out of scope.

Answer 2

Although Mr.Ben's method is absolutely right, but I completely feel that its C ++ and my C ++ are not the same language, it mixes some strange things here, so according to his idea, I tried to change my code, like this.

 #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; } 

Answer 3

tl; dr answer: do not convert the array of the subclass ( Stock[N] ) to a pointer to the base class ( pBase ). Either use Stock* , or create an array of pointers instead:

 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); 

Detailed reasons:

1) The array bracket operator is syntactic sugar: a[b] -> *(a + b) ;

2) Pointer arithmetic is not polymorphic; it is always based on static types:

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

3) This, however, is what you want:

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

4) As long as sizeof(Instrument) != sizeof(Stock) , you have problems.