---
tags: C
---

C語言的變數的宣告、定義
(含指標與自訂型別)
===
+ [執行程式](https://repl.it/join/jvusmyza-aidanlu)
+ Forward Declaration[~[1]~](https://en.wikipedia.org/wiki/Forward_declaration),[~[2]~](https://zh.wikipedia.org/wiki/%E5%89%8D%E5%90%91%E5%A3%B0%E6%98%8E)
+ 延伸學習：[C語言的指標範例](https://hackmd.io/@6djlj9vRR1GZP5OhFxxMWA/H1ZY_i35w)
___

### Case#1

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int a;

    printf("a = %d\n", a);
}

:::spoiler 結果
:::success
> a = 0[color=green]
:::

### Case#2

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int* a;

    printf("*a = %d\n", *a);
}

:::spoiler 結果
:::danger
可通過編譯，但是<font color="#00f">**執行時會錯誤</font>**
:::

### Case#3

#include <stdio.h>
#include <stdlib.h>

int main()
{
    int* a = malloc(sizeof(int));

    *a = 10;
    printf("*a = %d\n", *a);
}

:::spoiler 結果
:::success
> *a = 10[color=green]
:::

### Case#4

#include <stdio.h>
#include <stdlib.h>

struct Point; //Forward Declaration

int main()
{
    struct Point p;
}

:::spoiler 結果

:::danger
無法通過編譯
:::

### Case#5

#include <stdio.h>
#include <stdlib.h>

struct Point; //Forward Declaration

struct Point
{
    int x, y;
};
int main()
{
    struct Point p;

    p.x = 10;
    printf("p.x = %d\n", p.x);
}

:::spoiler 結果
:::success
> p.x = 10[color=green]
:::

### Case#6

#include <stdio.h>
#include <stdlib.h>

struct Point; //Forward Declaration

struct Point
{
    int x, y;
};
int main()
{
    struct Point* p;

    p->x = 10;
    printf("p->x = %d\n", p->x);
}

:::spoiler 結果
:::danger
可通過編譯，但是<font color="#00f">**執行時會錯誤</font>**
:::

### Case#7

#include <stdio.h>
#include <stdlib.h>

struct Point;

struct Point
{
    int x, y;
};
int main()
{
    struct Point* p = malloc(sizeof(struct Point));

    p->x = 10;
    printf("p->x = %d\n", p->x);
}

:::spoiler 結果
:::success
> p->x = 10[color=green]
:::

## 延伸學習：[C語言的指標範例](https://hackmd.io/@6djlj9vRR1GZP5OhFxxMWA/H1ZY_i35w)