Re: [問題] 新手關於字元的問題

[loveme00835 (朴髮箍)]

※ 引述《Dirichlet (微風輕吹)》之銘言：
: 請問為啥
: #include<iostream>
: using namespace std;
: int main()
: {
: cout<<'10'<<endl;
: }
: 執行後會出現 12592 呢？
: 而 '10' 中的 10 如果改成 0~9 就是 0~9，@@?

(只是)編譯會過, 這時要注意的是: 「'10' 的型態為何?」, 可以
使用下面程式碼來確定:

    assert( typeid('10') == typeid(int) );
    // [Warning] multi-character character constant 

它是被當成整數來印...而且其實它也不是一個合法的寬/窄字元內
容:

    wchar_t wc = L'10';
    // [Warning] character constant too long for its type 

    char c = L'10';
    // [Warning] character constant too long for its type 


另外 C99 6.4.4.4/10 中有提到:

An integer character constant has type int. [...] The value
of an integer character constant containing more than one 
character (e.g. 'ab'), or containing a character or escape
sequence that does not map to a single-byte execution
character, is implementation-defined. [...]

(110423 補充)
C++  2.13.2/1
[...] An ordinary character literal that contains more
than one c-char is a multicharacter literal. A multichara- 
cter literal has type int and implementation-defined value.

所以標準並沒有保證你「包好幾個字元」這件事在不同情境下都有
絕對相同的行為!唯一例外是這些字元s是用來表達字元變數的內存
值, 而非顯示的內容:

    cout << '\x41' << endl; // 0x41 == 65 == 'A'


順帶一提, 要記住下面程式碼:

    assert( (warning == bug) &&
            (undefined behavior == bug) );


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◆ From: 140.121.197.115
→ james732:其實我想問板主，為什麼會想去閱讀規格書呢？
→ james732:那樣的東西我實在沒有耐心看...XD
推 purpose:板主是有開過真理之門嗎
之前看到 loveflames大PO一些規格書內文, 覺得很有趣我就 K 下
去了, 就無聊當小說來讀囉~ 之前一陣子被 n3291 的 lambda
expression 其中一段搞死:

   int  a  =  1,  b  =  1,  c  =  1;
   auto  m1  =  [a,  &b,  &c]()  mutable  {
      auto  m2  =  [a,  b,  &c]()  mutable  {
         std::cout  <<  a  <<  b  <<  c;
         a  =  4;  b  =  4;  c  =  4;
      };
      a  =  3;  b  =  3;  c  =  3;
      m2();
   };
   a  =  2;  b  =  2;  c  =  2;
   m1();
   std::cout  <<  a  <<  b  <<  c;

看了會比較知道為什麼某些地方要設計成那樣, 不需去死背, 看懂
也會有成就感!
推 tomap41017:那個 lambda...也太XD
※ 編輯: loveme00835     來自: 140.121.197.115      (04/23 00:18)