return an array of structs or an array of struct p

问题:

If you need to return a struct from a function, you would normally return a pointer to the struct instead.

If you then want to return an array of structs, is it recommended to:

1. return an array of structures (pointer to the first element)
2. or return an array of struct pointers?

I have drawn a diagram for the two options below:

1:

2:

Given the following struct definition

struct values {
    int a;
    int b;
};

here is some sample code for accessing the fields of the structs from the two options:

Option #1:

 struct values *vals = get_values1();
 printf("%d, %d\n", values[0].a, values[1].b);

Option #2:

 struct values **vals = get_values2();
 printf("%d, %d\n", values[0]->a, values[1]->b);

回答1:

The only issue I see to not use version 1, is that it might be easier to identify the number of structures (returned) in the second version, as a NULL pointer can be used as a stopper element, whereas in the first version it might not be possible to define a stopper element.

回答2:

Unless you have a good reason to return a pointer to pointers (for example, if you want to modify the pointers themselves later on), I think a pointer to the struct is good enough. No need to trick around with double pointers.

回答3:

As an example for a beginners book on C, the question would clearly be satisfied better by option 1 (no indirection) as was pointed out by the answers so far. If there is more to the question however (e.g. which is the best way to pass around structs in a large framework) then I would certainly go for option 2. Why? structs are complex units of data and tend to become standalone entities which are allocated and passed around and disposed in differing ways - all code that handles them by direct access will need a rewrite if data handling becomes more complex.

回答4:

For me, as the number of indirections grow, the code complexity grows even faster. Thus I prefer your Option #1: struct values *foo().

Should data requirements push toward #2: struct values **foo(), suggest creating a new type typedef struct values *ValuesSet and returning a pointer to that: ValuesSet *foo().