• If either operand is of type long double, the other operand is converted to type long double.

• If the above condition is not met and either operand is of type double, the other operand is converted to type double. 

• If the above two conditions are not met and either operand is of type float, the other operand is converted to type float.

//如果不是浮点数精度

1. If the above three conditions are not met (none of the operands are of floating types), then integral conversions are performed on the operands as follows: 

   • If either operand is of type unsigned long, the other operand is converted to type unsigned long.

   • If the above condition is not met and either operand is of type long and the other of type unsigned int, both operands are converted to type unsigned long.

   • If the above two conditions are not met, and either operand is of type long, the other operand is converted to type long.

   • If the above three conditions are not met, and either operand is of type unsigned int, the other operand is converted to type unsigned int.

   • If none of the above conditions are met, both operands are converted to type int.    

   •  二元运算不考虑枚举的话，如果两边类型不一样，且有一边是浮点类型，则两边都转换成等级更高的浮点类型。否则两边进行integral promotion，然后如果两边类型不一样且同是signed或者同是unsigned，等级低的类型转换成等级高的类型。否则如果unsigned一边等级高于或等于signed一边，signed那边转换成unsigned那边的类型。否则如果signed那边的类型能容纳unsigned那边的范围，unsigned那边转换成signed那边的类型。否则两边都转换成signed那边对应的unsigned类型。

c/c++基本数据类型转换