C結構與結構指針

Question

我想定義一個數組使用指針而不是大小聲明，因爲我不知道有多少個地圖可能有的元素。嘗試鏈接列表，但沒有成功。如果這是一份報告，我很抱歉。如果看起來像一個愚蠢的問題，我是新手請原諒。

#include<stdio.h> 

typedef struct _keyValue 
{ 
    char *key; 
    char *value; 
} _keyValue; 

typedef struct _keyValues 
{ 
    /* _keyValue keyValue[5]; - This works*/ 
    _keyValue *keyValue; 
    int size; 
} _keyValues; 

_keyValues map; 

main() 
{ 
    map.keyValue[0].key  = "Key One"; 
    map.keyValue[0].value = "Value One"; 

    map.keyValue[1].key  = "Key Two"; 
    map.keyValue[1].value = "Value Two"; 

    map.size = 2; 

    printf("Key: %s Value: %s", map.keyValue[0].key, map.keyValue[0].value); 
} 

Answer 1

如果你不知道他們將會有多大，然後使用動態分配。這裏的主要功能是malloc和free。

下面是關於如何使用您的結構，如果你不知道他們有多大是一個建議：

首先有以下的代碼包括：

#include <stdio.h> /* for printf */ 
#include <string.h> /* for strcpy, as you cannot directly assign strings to a malloc'd pointer */ 
#include <stdlib.h> /* for malloc and free, for managing memory dynamically */ 

然後我們定義鍵和值大小：

const int key_size = 10; /* let's define how big our keys and values will be */ 
const int value_size = 25; 

這裏是你如何使用您的結構：

map.size = 30; /* decide how many keyValues we will have */ 

map.keyValue = malloc(sizeof(_keyValue) * map.size); /* create storage big enough for 30 _keyValue structs 
                 * malloc will allow you to assign memory to key and treat it as an array 
                 * malloc assigns memory from the heap 
                 * equal to the size specified (30), 
                 * this can be potentially as large as your computer's memory */ 

map.keyValue[0].key = malloc(sizeof(char) * key_size); /* let's create a key at position 0 */ 

strcpy(map.keyValue.key, "some key"); /* copying some values into key */ 

map.keyValue[0].value = malloc(sizeof(char) * value_size); /* let's create some space for a value for the 0th element */ 

strcpy(map.keyValue.value, "some value"); 


... /* you process and work with those values as you see fit */ 

free(map.keyValue[0]) /* malloc assigned memory needs to be returned to the OS as it's manually managed, 
         * here we free the element at position 0 we created earlier 
         * if you have more than element here use a loop e.g: 
         * for (int i = 0; i < map.size; i++) { free(map.KeyValue[i]) } 
         */ 


free(map.keyValue); /* free the keyValue itself that stored all the keyValue structs*/ 

一個技巧，以下劃線開頭的聲明因爲它們爲語言保留而不鼓勵。

Answer 2

map.keyValue在你的例子中是一個未初始化的指針。您需要爲數組分配的內存使用malloc

main() 
{ 
    map.keyValue = malloc(sizeof(*map.keyValue) * 2); 
    map.size = 2; 

    map.keyValue[0].key  = "Key One"; 
    map.keyValue[0].value = "Value One"; 

以後，您可以延長使用realloc

int newMapSize = ... 
_keyValue* temp = realloc(map.keyValue, sizeof(*map.keyValue) * newMapSize); 
if (temp == NULL) { 
    /* allocation failed. Handle out of memory error and exit */ 
} 
map.keyValue = temp; 
map.size = newMapSize; 
// map.keyValue[0..newMapSize-1] are now available 

Answer 3

#include <stdlib.h> 

int main(void){ 
    map.size = 2; 
    map.keyValue = malloc(sizeof(_keyValue)*map.size); 

    map.keyValue[0].key  = "Key One"; 
    map.keyValue[0].value = "Value One"; 

    map.keyValue[1].key  = "Key Two"; 
    map.keyValue[1].value = "Value Two"; 


    printf("Key: %s Value: %s", map.keyValue[0].key, map.keyValue[0].value); 

    return 0; 
} 

陣列提供存儲，如果我不知道大小

#include <stdlib.h> 
#include <string.h> 

int main(void){ 
    char buff[128] = ""; 

    map.size = 5;//decide the size temporarily 
    map.keyValue = malloc(sizeof(_keyValue)*map.size); 

    int count = 0, retv; 
    while(1){ 
     printf("input key : "); 
     retv=scanf(" %127s", buff); 
     if(retv != 1 || strcmp(buff, "end")==0) break; 
     map.keyValue[count].key = strdup(buff); 
     printf("input value : "); 
     scanf(" %127s", buff); 
     map.keyValue[count].value = strdup(buff); 
     ++count; 
     if(count == map.size)//full 
      map.keyValue = realloc(map.keyValue, sizeof(_keyValue)*(map.size+=5));//increase the size 
    } 

    int i; 
    for(i=0;i<count;++i) 
     printf("Key: %s Value: %s\n", map.keyValue[i].key, map.keyValue[i].value); 

    //dealloc 
    for(i=0;i<count;++i){ 
     free(map.keyValue[i].key); 
     free(map.keyValue[i].value); 
    } 
    free(map.keyValue); 

    return 0; 
}