What is the correct way to receive a pointer to a C-style string as an argument and be able to allocate memory or modify it?

Question

I have the following method signature:

int get_name(char* &name_out);

What I'd like to be able to do is allocate memory that is the size of name for name_out and then copy the contents of name into name_out.

Or would it be better to change it to:

int get_name(char &name_out[], int size);

and allow the user of the function to first allocate and track the memory, and return an error if the size of the given array isn't large enough to contain the string?

The only thing that I don't like about that is that it would require the user of the get_name() function to have knowledge about the length of the name string.

I feel it would be redundant to have two functions int get_name_length(); and get_name(char* name_out);

Since this is a piece of a programming assignment, there are stipulations:

1. The string class is not allowed.
2. C-style strings must be used.
3. Vectors cannot be used.
4. The function must return an int to indicate an error.
5. Exception handling is not allowed.

Thank you.

Answer 1

The standard C way to do this, is to pass two pointers to the function:

int getName(char** name_out, size_t* size_out);

This has several advantages:

• The caller is free to preallocate memory/reuse an allocation.

• The callee is free to adjust the allocation via realloc() (assuming that the C-style string is allocated with malloc()), or via a pair of delete[]/new[].

• The address taking is explicit. I.e. at the calling site you would write:

  char* name = null_ptr;
  size_t size = 0;
  if(getName(&name, &size)) handleError();
  

  The explicit & operator makes it very clear that the function getName() can change both variables. If you go with references, you cannot distinguish between call by reference and call by value without looking at the function declaration.

Also note, the type to use for allocation sizes is size_t: it is the type that is guaranteed to be able to hold the size of the entire usable address space.

Answer 2

If I understand correctly what you are trying to do is to implement a variant for 'strcpy'.

The major difference is that you pass the allocation responsibility to your copy function, while 'strcpy' leaves that to the user. If this is a production code then I recommend following the 'strcpy' approach which is what the industry is accustomed to.

If it's just for play then wrap strcpy with a function that does the allocation and stick to strcpy's interface.