Expressions in C C++

Operators, constants, and variables are the constituents of expressions. An expression in
C/C++ is any valid combination of these elements. Because most expressions tend to
follow the general rules of algebra, they are often taken for granted. However, a few
aspects of expressions relate specifically to C and C++.

Order of Evaluation
Neither C nor C++ specifies the order in which the subexpressions of an expression are
evaluated. This leaves the compiler free to rearrange an expression to produce more
optimal code. However, it also means that your code should never rely upon the order
in which subexpressions are evaluated. For example, the expression
x = f1() + f2();
does not ensure that f1() will be called before f2() .

Type Conversion in Expressions
When constants and variables of different types are mixed in an expression, they are
all converted to the same type. The compiler converts all operands up to the type of
the largest operand, which is called type promotion. First, all char and short int values
are automatically elevated to int. (This process is called integral promotion.) Once this
step has been completed, all other conversions are done operation by operation, as
described in the following type conversion algorithm:
IF an operand is a long double
THEN the second is converted to long double
ELSE IF an operand is a double
THEN the second is converted to double
ELSE IF an operand is a float
THEN the second is converted to float
ELSE IF an operand is an unsigned long
THEN the second is converted to unsigned long
ELSE IF an operand is long
THEN the second is converted to long
ELSE IF an operand is unsigned int
THEN the second is converted to unsigned int