[討論] Array size 的問題

[ric2k1 (Ric)]

有同學寫了這樣的程式:

   int x;
   cin >> x;
   int a[x];
   // ... 使用 a[x]...

結果沒有問題且執行正確.


理論上 int a[x] 的 size 要是 constant 然後 compiler 才能在 compile time 決定

sizeof(a). (還記得 sizeof 是 compile time operator).

那這裡是怎麼回事呢?


我做了一個實驗:

==============================================
int main()
{
   int x;
   cin >> x;

   int a[x];

   cout << "sizeof(a) = " << sizeof(a) << endl;
   cout << "&a    = " << &a << endl;
   cout << " a    = " <<  a << endl;
   cout << "&a[0] = " << &a[0] << endl;
   cout << "&a[1] = " << &a[1] << endl;
   cout << "&a[9] = " << &a[9] << endl;

   int b[10];

   cout << "sizeof(b) = " << sizeof(b) << endl;
   cout << "&b    = " << &b << endl;
   cout << " b    = " <<  b << endl;
   cout << "&b[0] = " << &b[0] << endl;
   cout << "&b[1] = " << &b[1] << endl;
   cout << "&b[9] = " << &b[9] << endl;

   int c[10];

   cout << "&c    = " << &c << endl;
   cout << " c    = " <<  c << endl;
   cout << "&c[0] = " << &c[0] << endl;
   cout << "&c[1] = " << &c[1] << endl;
   cout << "&c[9] = " << &c[9] << endl;
}

=============================================

結果輸出是 (on Linux):

10
sizeof(a) = 40
&a    = 0xfef7e770
 a    = 0xfef7e770
&a[0] = 0xfef7e770
&a[1] = 0xfef7e774
&a[9] = 0xfef7e794
sizeof(b) = 40
&b    = 0xfef7e7f0
 b    = 0xfef7e7f0
&b[0] = 0xfef7e7f0
&b[1] = 0xfef7e7f4
&b[9] = 0xfef7e814
&c    = 0xfef7e7c0
 c    = 0xfef7e7c0
&c[0] = 0xfef7e7c0
&c[1] = 0xfef7e7c4
&c[9] = 0xfef7e7e4


We can see:

1. sizeof(a) = 40!! 所以顯然 sizeof(a) 並不是 compile time 就決定的, 因為

   x 是 cin 才指定為 10 的.

   查了許多網頁都是一面倒的說 sizeof is a compile time operator...

   只有一個 IBM Mac OS X/XL C++ compiler 說:

   "At compile time, the compiler analyzes the expression to determine its

   type, but does not evaluate it...."

   Anyway, it's a mystery...


2. a 的 memory address 比 b and c 小!!

   Since stack memory gives away its memory in the reverse addressing order,

   that means 程式先給了 b[10] and c[10] 的 memory space 才給 a[10].

   也就是說, 程式進入 main() 之後就先將 local varible b[10], c[10] 的 memory

   準備好, 然後執行到 a[x] 才去決定要用多少 memory 來存 a.


===================================

喔, 對了, 對 int a[x] 如果 x is not a constant 是不能用 initializer list 來

initialize a[x] 的值 (e.g. int a[x] = { 0 } ).

因為 sizeof({0}) 還是有可能比 sizeof(a[x]) 還要大...

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◆ From: 59.121.132.121
※ 編輯: ric2k1          來自: 59.121.132.121       (12/05 22:28)
推 ilway25:選課程式有用到這樣，G++過，VC不給過..
推 ric2k1:不過忘了說, 這樣寫是不好的...
推 ilway25:所以還是用pointer new比較好? 話說strtok後要不要delete?
推 timrau:This is a GCC extension.
推 timrau:http://tinyurl.com/yt9gr3
推 ric2k1:Cool, so it is safe if the compiler supports it.
推 ric2k1:strtok 不用 "額外" 去 delete.
推 lionel20002:原來如此 話說我常這樣寫...