Thread: Dynamic array allocation

Here is the exercise I am trying to complete:

Write a function that takes a char* argument. Using new,
dynamically allocate an array of char that is the size of
the char array that’s passed to the function. Using array
indexing, copy the characters from the argument to the
dynamically allocated array (don’t forget the null
terminator) and return the pointer to the copy. In your
main( ), test the function by passing a static quoted
character array, then take the result of that and pass it
back into the function. Print both strings and both
pointers so you can see they are different storage. Using
delete, clean up all the dynamic storage.

I have a basic understanding of pointers, but I'm totally new to dynamic memory, so I really had no idea what to do with this one. Here's my failing code:

Code:

#include <iostream>
using namespace std;

char dynChar(char* a, int size){
    char* b = new char[size];
    for (int i=0; i<size; i++)
        b[i] = a[i];
    return *b;
    delete[] b;
}

int main(){
    char c[] = "Hello world!";
    char d[] = dynChar(c,13); //line 14, with the error
    cout << d;
    return 0;
}

There are two issues with this code. 1) I don't know how to find the size of the array, so the function has to be given the length of the array. How can I find an array's size?

2) It won't compile. It gives an error at line 14, saying the initializer can't determine the size of d. I don't really know what that means. Could someone help me out with this?

Making dynChar return a pointer and changing d to a pointer worked, though I still can't figure out how to get the array size from the inputted array.

I put the delete[] after return because the exercise requires the allocated space to be deallocated, and I assumed because it was allocated in the function that I would need to deallocate it in the function. As I said before, I'm very new to dynamic memory.

Okay, thanks. Let me see if I understand: the pointer inside the function IS the object in main(), so if you allocate the space for it in a function, it can be deallocated outside of the function?

Also, I got it working without strlen (I don't want to use any functions the book hasn't introduced yet):

Code:

char* dynChar(char* a){
    int size = 0;
    char c = '0';
    while(c != '\0'){
        c = a[size];
        size++;
    }
    cout << size << endl;
    char* b = new char[size];
    for (int i=0; i<size; i++)
        b[i] = a[i];
    return b;
}

Oh right, I forget to add an extra size++ for the \0 character. Thanks for all the help with this! I spent a good amount of time in class today sitting and trying to get this to work, and I have a much better understanding of arrays and dynamic memory now.

Code:

char* dynChar(char* a){
    int size = 0;
    char c;
    while((c = a[size])) size++;
    size++;

O_o

Why not simplify it further?

The parameter needs to be `const char *' if you don't intend to modify the data.

Soma

Code:

int size(0);
while(a[size++]);