陣列？指標？

聽說c++打算廢棄指標了，誰讓指標這麼難呢！

我的環境：

>uname -a
CYGWIN_NT-10.0-WOW DESKTOP-499IG24 2.10.0(0.325/5/3) 2018-02-02 15:21 i686 Cygwin

可見為32bit核心，也就是指標所佔空間的sizeof為4.

那麼這段程式：

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

/*main函式*/
int main(int argc, char**argv)
{
    char s1[] = "abcdefg";
    char s2[] = {'a','b','c','d','e','f','g'};
    char s3[] = {"abcdefg"};
    char *s4 = "abcdefg";
    //char *s5 = {'a','b','c','d','e','f','g'};//error
    char *s6 = {"abcdefg"};
    
    printf("s:%s, length:%2d, sizeof:%2d\n",s1,strlen(s1),sizeof(s1));
    printf("s:%s, length:%2d, sizeof:%2d\n",s2,strlen(s2),sizeof(s2));
    printf("s:%s, length:%2d, sizeof:%2d\n",s3,strlen(s3),sizeof(s3));
    printf("s:%s, length:%2d, sizeof:%2d\n",s4,strlen(s4),sizeof(s4));
    //printf("s:%s, length:%2d, sizeof:%2d\n",s5,strlen(s5),sizeof(s5));//error
    printf("s:%s, length:%2d, sizeof:%2d\n",s6,strlen(s6),sizeof(s6));
    
    s1[0] = 'z';
    s2[0] = 'z';
    s3[0] = 'z';
    //s4[0] = 'z';//error
    //s5[0] = 'z';//error
    //s6[0] = 'z';//error
    
    return 1;
}
 

的輸出為：

>gcc string.c
>a.exe
s:abcdefg, length: 7, sizeof: 8
s:abcdefgabcdefg, length:14, sizeof: 7
s:abcdefg, length: 7, sizeof: 8
s:abcdefg, length: 7, sizeof: 4
s:abcdefg, length: 7, sizeof: 4

那麼這幾個的sizeof為什麼不一樣呢？

32bit系統核心的指標大小為4，後面的兩個4不難解釋，上面的兩個8可以解釋為自動補齊（類似於struct，struct是按照最大位元組數補齊），而大小為7的則是對應的陣列。

sizeof(結構體)

一下均採用如下程式列印：

printf("%d+%d+%d+%d = %d\n",
        sizeof(int),sizeof(char),sizeof(float),sizeof(double),sizeof(S));

需要搞明白的是struct的自動對齊機制：

例1：

struct{
    int a;
    char ch;
    float f;
    double d;
}S;

輸出結果為：

>gcc string.c
>a.exe
4+1+4+8 = 24

例2：

struct{
    int a;
    char ch;
    float f;
}S;
 

結果：

>gcc string.c
>a.exe
4+1+4+8 = 12

例3：

struct{
    float f;
    double d;
}S;

結果：

>gcc string.c
>a.exe
4+1+4+8 = 16

例4：

struct{
    int a;
    double d;
}S;

結果：

>gcc string.c
>a.exe
4+1+4+8 = 16