指向对象和初始化成员值的指针数组

Question

I'm attempting to allocate memory for each entry of the array and initialize the members part1 and part2 to 0.

#include <iostream>
using namespace std;

class Two {
public:
    int part1;
    int part2;
};


int main() {

    Two * dp[10]; //array of 10 pointers to objects of class Two

    part1 = 0;
    part2 = 0;

    for (int i = 0; i < 10; i++) { 
        dp[i] = 0;                        

    }
    return 0; 
}

Any help is appreciated. I'm new to c++ and I'm trying to understand the basic concepts. Thank you in advance!

Answer 1

Here's a really basic version of that code that uses new to allocate memory:

#include <iostream>

class Two {
public:
  Two() : part1(0), part2(0) { };
  int part1;
  int part2;
};


int main() {
  const size_t count = 10;
  Two *dp = new Two[count];

  // Do stuff?

  for (size_t i = 0; i < count; ++i) {
    std::cout << dp[i].part1 << "/" << dp[i].part2 << std::endl;
  }

  delete[] dp;

  return 0; 
}

Note that for a multitude of reasons this is a bad idea, but if you need to side-step the Standard Library because teacher then this is where you go.

Answer 2

In c++ for your example is preferable to have a default constructor that initialize your data and a std::vector to hold objects of your custom class. This will save you a lot of memory handling problems.

class Two {
public:
  Two() : part1(0), part2(0) { };
  int part1;
  int part2;
};


int main() {
    std::vector<Two> twoVec(10);
    return 0; 
}

Answer 3

You created 10 pointers to instances of Two , but those pointers are not initalized and doesn't point to any instances.

Here is one way to create those instances, store pointers to them in your array, and then set their part1 member to 1 and the part2 member to 2;

for (int i = 0; i < 10; i++) { 
    dp[i] = new Two();
    dp[i]->part1= 1;              
    dp[i]->part2= 2;              

}

Answer 4

You need to allocate objects of the type class Two pointed to by the elements of the array. Otherwise the program will have undefined behavior.

You can do it either using standard algorithm std::generate like this

#include <iterator>
#include <algorithm>

//...

Two * dp[10]; //array of 10 pointers to objects of class Two

std::generate( std::begin( dp ), std::end( dp ), 
               [] { return new Two { 0, 0 };} );    

Or you can use the range based for loop. For example

Two * dp[10]; //array of 10 pointers to objects of class Two

for ( Two * &p : dp ) p = new Two { 0, 0 }; 

Here is a demonstrative program

#include <iostream>

using namespace std;

class Two {
public:
    int part1;
    int part2;
};


int main() 
{

    Two * dp[10]; //array of 10 pointers to objects of class Two

    for ( Two * &p : dp ) p = new Two { 0, 0 }; 

    // processing of the array

    for ( Two * &p : dp ) 
    {
        delete p;
        p = nullptr;
    }       

    return 0; 
}