什麼是integer promotion:

Integer Promotions

Integer types smaller than int are promoted when an operation is performed on them. If all values of the original type can be represented as an int, the value of the smaller type is converted to an int; otherwise, it is converted to an unsigned int.

Integer promotions are applied as part of the usual arithmetic conversions

char c1, c2;
c1 = c1 + c2;

Integer promotions require the promotion value of each variable (c1

Integer promotions are performed to avoid arithmetic errors resulting from the overflow of intermediate values. On line 5 of Figure 5-7, the value of c1 is added to the value of c2. The sum of these values is then added to the value of c3

Figure 5-7. Preventing arithmetic errors with implicit conversions

1. char cresult, c1, c2, c3;
2. c1 = 100;
3. c2 = 90;
4. c3 = -120;
5. cresult = c1 + c2 + c3;

更多例子：

1. 這個例子來自： http://blog.csdn.net/dyllove98/article/details/9004872

開門見山，先來看一段簡單的程式碼：

  1 #include <stdio.h>
  2 
  3 int array[] = {1, 2, 3, 4, 5, 6, 7};
  4 #define TOTAL_ELEMENTS (sizeof(array) / sizeof(array[0]))
  5 
  6 int main()
  7 {
  8     int i = -1;
  9     int x;
 10 
 11     if(i <= TOTAL_ELEMENTS - 2) {               //整型提升
 12         x = array[i + 1];
 13         printf("x = %d.\n", x);
 14     }
 15 
 16     printf("now i = %d.\n", TOTAL_ELEMENTS);
 17 
 18     return 0;
 19 }

執行結果：

[email protected]:~/C_Language$ ./a.out 

now i = 7.

是不是很奇怪？為什麼沒有打出line13的x = ？。

是這樣的。這個小例子有三點值得注意：

1.sizeof()是運算子，返回型別是無符號的，即非負數。

2.if語句在singned int和unsigned int之間進行判斷語句，根據C語言的整型提升規則，如果原始型別的所有值都可用int型別表示，則其值將被轉換為int型別；否則將被轉換為unsigned int型別。因此此時的i將會被升級成無符號型別。

3.i = -1被升級為無符號型，值究竟是多少？這要用到整型轉換規則：K＆R上這樣解釋，將任何整數轉換為某種指定的無符號數型別數的方法是：以該無符號數型別能夠表示的最大值加1為摸，找出與此整數同餘的最小的非負值。聽著很拗口，其實說白了，只要知道原整數的二進位制表達方法，再用要即將轉換的型別去解析，就得到升級後的值了。 比如-1，負數在計算機裡用補碼錶示，0xffffffff，那升級成無符號型之後，值就是0xffffffff，顯然比TOTAL_ELEMENTS(7)大。