Modeling with Data

Tip 17: Define a string type

04 November 11. [link]

Part of a series of tips on POSIX and C. Start from the tip intro page, or get 21st Century C, the book based on this series.

level: still basic
purpose: slightly less confusing declarations

A tip every other day on POSIX and C. Start from the tip intro page.

Here's a line to paste into the unified header you wrote to include at the head of all your programs back in Tip 6:

typedef char* string;

Pretty simple, but it will make your code more readable.

The sample code from last time is pretty short, so I'll reprint the whole program. You may find it to be hard to read (I was using it as an example of a point I was calling obscure), but we'll try to fix that:

#include <stdio.h>

int main(){
    char *list[] = {"first", "second", "third", NULL};
    for (char **p=list; *p != NULL; p++){
        printf("%s\n", *p); 
    }
}

Do the declarations communicate to you that char *list[] is a list of strings, and that *p is a string?

Now use typedef to replace char * with string. There are fewer stars floating around; p is more clearly a pointer and *p the data at the pointer:

#include <stdio.h>
typedef char* string;

int main(){
    string list[] = {"first", "second", "third", NULL};
    for (string *p=list; *p != NULL; p++){
        printf("%s\n", *p); 
    }
}

The declaration line for list is now as easy as C gets, and clearly indicates that it is a list of strings, and the snippet string *p should indicate to you that p is a pointer-to-string, so *p is a string.

In the end, you'll still have to remember that a string is a pointer-to-char; for example, NULL is a valid value.

I find that most folks (myself included) have no serious problem with pointers until they get to pointers-to-pointers and further depth beyond that. With strings as char*s, you hit that multiple-star wall much earlier than there's any reason to. And since Tips 10 through 12 already went over how to deal with strings without ever calling malloc, we might as well start using a non-pointer type name for them.

To do:
Try declaring a 2-D array of strings, using the typedef above plus

typedef stringlist string*

Is it easier to work out how to allocate its parts?

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Modeling With Data

Tip 17: Define a string type