何时使用指向C中嵌套结构中另一个结构的指针

Question

I'm confused as when should I use a pointer to another struct or contain a copy. For instance, should I use Products *prods; or Products prods; within Inventory? and how do I malloc ?

typedef struct Products Products;
struct Products
{
    int  id;
    char *cat;
    char *name
};

typedef struct Inventory Inventory;
struct Inventory
{
    char* currency;
    int size;
    Products prods; // or Products *prods;
};

Answer 1

You should use a pointer when the size of the array is unknown at compile time. If you know each Inventory struct will contain exactly one Products struct or 10 or 100, then just declare Products prods[100] . But if you're reading arbitrary records at runtime and can't know at compile time how many Products records an Inventory struct will contain, then use Products *prods . You'll also need size and count struct elements to keep track of how much you've malloc'd or realloc'd and how much of that memory you've filled with Products structs.

Answer 2

Complementing Kyle's answer, the decision about whether or not using a pointer to Products , you must think of the following:

If you don't know how many elements you'll have, your Inventory struct should have at least:

typedef struct Inventory Inventory;
struct Inventory
{
    char *currency;
    int size, count;
    Products* prods;
    ... // other elements you should need
};

and the pointer should be defined as (when instantiating an Inventory element):

...
Inventory inv;
inv.size = _total_elems_you_will_need_
inv.prods = (Products *)malloc(inv.size * sizeof(Products));
...

On the other hand, if that amount is always fixed, then you can define your struct Inventory with something like this (instead of the pointer defined above):

Products prods;      // if you'll need only one element.
Products prods[10];  // if you'll need only ten.