CODEWITHSUNDEEP
c++pointersstruct
Yoda
Yoda

Reputation: 18068

How to create a pointer and an instance of a structure just after it's definition

Is it possible to initialize in dynamic way using pointers but just after the struct definition? Please at the example. I tried that but I get an exception when I try cout << a->val << endl;

#include "stdafx.h"
#include <iostream>

using namespace std;


struct A{
public:
    int val = 0;
    A(){}

}*a,b,c;

int _tmain(int argc, _TCHAR* argv[]){

    cout << a->val << endl;//EXCEPTION
    cout << b.val << endl;
    int x;
    cin >> x;
    return 0;
}

Upvotes: 0

Views: 486

Answers (2)

kfsone
kfsone

Reputation: 24269

Your code

struct A{
public:
    int val = 0;
    A(){}
}*a,b,c;

defines the structure A and declares three global variables, a, b, c.

a is of type 'A*' - i.e. pointer to type A, b and c are instances of type A.

Global variables of simple, base types (e.g. ints, pointers, etc), default to 0.

The result is equivalent to this:

struct A{
public:
    int val = 0;
    A(){}
};
A* a = NULL;
A b;
A c;

When your code then tries to use a without assigning a value to it, it is dereferencing a NULL pointer and crashes.

Upvotes: 1

masoud
masoud

Reputation: 56549

Yes

struct A
{
  // ...

} *a { new A };

and somewhere suitable:

delete a;

Generally bare pointers are not recommended, you should avoid them and if you really need pointers try to use smart pointers such as std::unique_ptr and std::shared_ptr

Upvotes: 2

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