dynamic memory allocation (arrays)

[newHansen]

Hi,

I have recently bought a copy of "Jumping into C++" and have come to chapter 14 ( dynamic memory location) and have a question.

On page 153-154 an example of dynamic allocation is given for array's of int. How would the code look like for strings or structs ?

The allocation was given by:

 
int *growArray (int* p_values, int *size)
{
*size *= 2;
int *p_new_values = new int[ *size ];
for ( int i = 0; i < *size; ++i )
{
p_new_values[ i ] = p_values[ i ];
}
delete [] p_values;
return p_new_values;
}
Sample Code

I tried to use this for an array of structs but failed completely... please help.

I used the following struct:

struct user{
    int days; 
    string name;
};

and the allocation function (which does not work):


	
	
struct user *growarray (struct user *p_values, int *size)
{
*size *= 2;
struct user *p_new_values = new struct user[ *size ];
for ( int i = 0; i < *size; ++i )
{
p_new_values[ i ].name = p_values[ i ].name;
p_new_values[i].days = p_values[ i ].days;


}


delete [] p_values;
return p_new_values;
}

[Mycroft Holmes]

Why don't you use vectors? Is there any specific reason for which you're sticking to arrays?

[newHansen]

I haven't made it to vectors yet, so don't know how they work... secondly I clearly didn't understand pointers properly.

[laserlight]

Well, your mistake in the original growArray is that you loop using *size, which has already doubled by that point. In your second version, growarray should just assign user objects rather than assign individual members (and the *size bug is still present).

[newHansen]

Hi laserligh,
thanks for your reply!

There is still something I don't get. You say that *size is used wrong, but why does it then work when using type "int" in the version from the book? Can you give an example what you mean by assigning user objects ?

[grumpy]

There is still something I don't get. You say that *size is used wrong, but why does it then work when using type "int" in the version from the book?

You got unlucky. The bug is there for the int version, but didn't cause a crash in your circumstance. The problem is that you are copying twice the number of elements from p_values than are there.

The C++ standard does not define what happens when you access more array elements than an array has. With undefined behaviour, anything can happen. You just got unlucky, with your compiler/library/host system, that your code seemed to work with the int version. It could equally have crashed, or reformatted your hard drive, or fried your mouse, .....

Can you give an example what you mean by assigning user objects ?

If you want to assign a whole struct, use

    //  assume a_struct and b_struct are both of type struct some_type, which has members member1, member2, ....  memberN.
   a_struct = b_struct;

rather than

    a_struct.member1 = b_struct.member1;
    a_struct.member2 = b_struct.member2;

      //   assign other members

    a_struct.memberN = b_struct.memberN;

The whole point of a struct is that it can be treated as a logical unit. That extends to assignment.

[grumpy]

There is still something I don't get. You say that *size is used wrong, but why does it then work when using type "int" in the version from the book?

You got unlucky. The bug is there for the int version, but didn't cause a crash in your circumstance. The problem is that you are copying twice the number of elements from p_values than are there.

The C++ standard does not define what happens when you access more array elements than an array has. With undefined behaviour, anything can happen. You just got unlucky, with your compiler/library/host system, that your code seemed to work with the int version. It could equally have crashed, or reformatted your hard drive, or fried your mouse, .....

And if the code you quoted is unmodified from the book, Alex deserves a gentle spanking (unless he has specified that the code is illustrating a problem).

Can you give an example what you mean by assigning user objects ?

If you want to assign a whole struct, use

    //  assume a_struct and b_struct are both of type struct some_type, which has members member1, member2, ....  memberN.
   a_struct = b_struct;

rather than

    a_struct.member1 = b_struct.member1;
    a_struct.member2 = b_struct.member2;

      //   assign other members

    a_struct.memberN = b_struct.memberN;

The whole point of a struct is that it can be treated as a logical unit. That extends to assignment.

[newHansen]

Hi grumpy,

Thanks for your answer, it worked perfectly correcting the array length... if you come to Copenhagen I will buy you a beer!

[newHansen]

And also to you laserlight ... Thanks!