我有以下结构：
typedef struct _chess {
int **array;
int size;
struct _chess *parent;
} chess;
我有：
typedef struct _chess *Chess;
现在，我想创建一个动态长度数组来存储指向国际象棋结构的指针，所以我执行以下操作：
Chess array [] = malloc(size * sizeof(Chess));
这给了我一个错误：无效的初始化程序。
如果我放弃 [] 并这样做：
Chess array = malloc(size * sizeof(Chess));
它编译没有错误，但是当我尝试通过执行以下操作将此数组的元素设置为 NULL 时：
array[i]=NULL;
我收到一个错误：从类型“void *”分配给类型“struct _chess”时类型不兼容</p>
知道我在做什么错吗？谢谢。
array是一个有点误导的名字。对于一个动态分配的指针数组，malloc将返回一个指向内存块的指针。您需要使用Chess*而不是Chess[]保存指向数组的指针。
Chess *array = malloc(size * sizeof(Chess));
array[i] = NULL;
也许以后：
/* create new struct chess */
array[i] = malloc(sizeof(struct chess));
/* set up its members */
array[i]->size = 0;
/* etc. */
There's a lot of typedef going on here. Personally I'm against "hiding the asterisk", i.e. typedef:ing pointer types into something that doesn't look like a pointer. In C, pointers are quite important and really affect the code, there's a lot of difference between foo and foo *.
Many of the answers are also confused about this, I think.
Your allocation of an array of Chess values, which are pointers to values of type chess (again, a very confusing nomenclature that I really can't recommend) should be like this:
Chess *array = malloc(n * sizeof *array);
Then, you need to initialize the actual instances, by looping:
for(i = 0; i < n; ++i)
array[i] = NULL;
This assumes you don't want to allocate any memory for the instances, you just want an array of pointers with all pointers initially pointing at nothing.
If you wanted to allocate space, the simplest form would be:
for(i = 0; i < n; ++i)
array[i] = malloc(sizeof *array[i]);
See how the sizeof usage is 100% consistent, and never starts to mention explicit types. Use the type information inherent in your variables, and let the compiler worry about which type is which. Don't repeat yourself.
Of course, the above does a needlessly large amount of calls to malloc(); depending on usage patterns it might be possible to do all of the above with just one call to malloc(), after computing the total size needed. Then you'd still need to go through and initialize the array[i] pointers to point into the large block, of course.
恕我直言，这看起来更好：
Chess *array = malloc(size * sizeof(Chess)); // array of pointers of size `size`
for ( int i =0; i < SOME_VALUE; ++i )
{
array[i] = (Chess) malloc(sizeof(Chess));
}
I agree with @maverik above, I prefer not to hide the details with a typedef. Especially when you are trying to understand what is going on. I also prefer to see everything instead of a partial code snippet. With that said, here is a malloc and free of a complex structure.
The code uses the ms visual studio leak detector so you can experiment with the potential leaks.
#include "stdafx.h"
#include <string.h>
#include "msc-lzw.h"
#define _CRTDBG_MAP_ALLOC
#include <stdlib.h>
#include <crtdbg.h>
// 32-bit version
int hash_fun(unsigned int key, int try_num, int max) {
return (key + try_num) % max; // the hash fun returns a number bounded by the number of slots.
}
// this hash table has
// key is int
// value is char buffer
struct key_value_pair {
int key; // use this field as the key
char *pValue; // use this field to store a variable length string
};
struct hash_table {
int max;
int number_of_elements;
struct key_value_pair **elements; // This is an array of pointers to mystruct objects
};
int hash_insert(struct key_value_pair *data, struct hash_table *hash_table) {
int try_num, hash;
int max_number_of_retries = hash_table->max;
if (hash_table->number_of_elements >= hash_table->max) {
return 0; // FULL
}
for (try_num = 0; try_num < max_number_of_retries; try_num++) {
hash = hash_fun(data->key, try_num, hash_table->max);
if (NULL == hash_table->elements[hash]) { // an unallocated slot
hash_table->elements[hash] = data;
hash_table->number_of_elements++;
return RC_OK;
}
}
return RC_ERROR;
}
// returns the corresponding key value pair struct
// If a value is not found, it returns null
//
// 32-bit version
struct key_value_pair *hash_retrieve(unsigned int key, struct hash_table *hash_table) {
unsigned int try_num, hash;
unsigned int max_number_of_retries = hash_table->max;
for (try_num = 0; try_num < max_number_of_retries; try_num++) {
hash = hash_fun(key, try_num, hash_table->max);
if (hash_table->elements[hash] == 0) {
return NULL; // Nothing found
}
if (hash_table->elements[hash]->key == key) {
return hash_table->elements[hash];
}
}
return NULL;
}
// Returns the number of keys in the dictionary
// The list of keys in the dictionary is returned as a parameter. It will need to be freed afterwards
int keys(struct hash_table *pHashTable, int **ppKeys) {
int num_keys = 0;
*ppKeys = (int *) malloc( pHashTable->number_of_elements * sizeof(int) );
for (int i = 0; i < pHashTable->max; i++) {
if (NULL != pHashTable->elements[i]) {
(*ppKeys)[num_keys] = pHashTable->elements[i]->key;
num_keys++;
}
}
return num_keys;
}
// The dictionary will need to be freed afterwards
int allocate_the_dictionary(struct hash_table *pHashTable) {
// Allocate the hash table slots
pHashTable->elements = (struct key_value_pair **) malloc(pHashTable->max * sizeof(struct key_value_pair)); // allocate max number of key_value_pair entries
for (int i = 0; i < pHashTable->max; i++) {
pHashTable->elements[i] = NULL;
}
// alloc all the slots
//struct key_value_pair *pa_slot;
//for (int i = 0; i < pHashTable->max; i++) {
// // all that he could see was babylon
// pa_slot = (struct key_value_pair *) malloc(sizeof(struct key_value_pair));
// if (NULL == pa_slot) {
// printf("alloc of slot failed\n");
// while (1);
// }
// pHashTable->elements[i] = pa_slot;
// pHashTable->elements[i]->key = 0;
//}
return RC_OK;
}
// This will make a dictionary entry where
// o key is an int
// o value is a character buffer
//
// The buffer in the key_value_pair will need to be freed afterwards
int make_dict_entry(int a_key, char * buffer, struct key_value_pair *pMyStruct) {
// determine the len of the buffer assuming it is a string
int len = strlen(buffer);
// alloc the buffer to hold the string
pMyStruct->pValue = (char *) malloc(len + 1); // add one for the null terminator byte
if (NULL == pMyStruct->pValue) {
printf("Failed to allocate the buffer for the dictionary string value.");
return RC_ERROR;
}
strcpy(pMyStruct->pValue, buffer);
pMyStruct->key = a_key;
return RC_OK;
}
// Assumes the hash table has already been allocated.
int add_key_val_pair_to_dict(struct hash_table *pHashTable, int key, char *pBuff) {
int rc;
struct key_value_pair *pKeyValuePair;
if (NULL == pHashTable) {
printf("Hash table is null.\n");
return RC_ERROR;
}
// Allocate the dictionary key value pair struct
pKeyValuePair = (struct key_value_pair *) malloc(sizeof(struct key_value_pair));
if (NULL == pKeyValuePair) {
printf("Failed to allocate key value pair struct.\n");
return RC_ERROR;
}
rc = make_dict_entry(key, pBuff, pKeyValuePair); // a_hash_table[1221] = "abba"
if (RC_ERROR == rc) {
printf("Failed to add buff to key value pair struct.\n");
return RC_ERROR;
}
rc = hash_insert(pKeyValuePair, pHashTable);
if (RC_ERROR == rc) {
printf("insert has failed!\n");
return RC_ERROR;
}
return RC_OK;
}
void dump_hash_table(struct hash_table *pHashTable) {
// Iterate the dictionary by keys
char * pValue;
struct key_value_pair *pMyStruct;
int *pKeyList;
int num_keys;
printf("i\tKey\tValue\n");
printf("-----------------------------\n");
num_keys = keys(pHashTable, &pKeyList);
for (int i = 0; i < num_keys; i++) {
pMyStruct = hash_retrieve(pKeyList[i], pHashTable);
pValue = pMyStruct->pValue;
printf("%d\t%d\t%s\n", i, pKeyList[i], pValue);
}
// Free the key list
free(pKeyList);
}
int main(int argc, char *argv[]) {
int rc;
int i;
struct hash_table a_hash_table;
a_hash_table.max = 20; // The dictionary can hold at most 20 entries.
a_hash_table.number_of_elements = 0; // The intial dictionary has 0 entries.
allocate_the_dictionary(&a_hash_table);
rc = add_key_val_pair_to_dict(&a_hash_table, 1221, "abba");
if (RC_ERROR == rc) {
printf("insert has failed!\n");
return RC_ERROR;
}
rc = add_key_val_pair_to_dict(&a_hash_table, 2211, "bbaa");
if (RC_ERROR == rc) {
printf("insert has failed!\n");
return RC_ERROR;
}
rc = add_key_val_pair_to_dict(&a_hash_table, 1122, "aabb");
if (RC_ERROR == rc) {
printf("insert has failed!\n");
return RC_ERROR;
}
rc = add_key_val_pair_to_dict(&a_hash_table, 2112, "baab");
if (RC_ERROR == rc) {
printf("insert has failed!\n");
return RC_ERROR;
}
rc = add_key_val_pair_to_dict(&a_hash_table, 1212, "abab");
if (RC_ERROR == rc) {
printf("insert has failed!\n");
return RC_ERROR;
}
rc = add_key_val_pair_to_dict(&a_hash_table, 2121, "baba");
if (RC_ERROR == rc) {
printf("insert has failed!\n");
return RC_ERROR;
}
// Iterate the dictionary by keys
dump_hash_table(&a_hash_table);
// Free the individual slots
for (i = 0; i < a_hash_table.max; i++) {
// all that he could see was babylon
if (NULL != a_hash_table.elements[i]) {
free(a_hash_table.elements[i]->pValue); // free the buffer in the struct
free(a_hash_table.elements[i]); // free the key_value_pair entry
a_hash_table.elements[i] = NULL;
}
}
// Free the overall dictionary
free(a_hash_table.elements);
_CrtDumpMemoryLeaks();
return 0;
}