Certain expressions are ambiguous in their meaning. These expressions occur most frequently when an object’s value is modified more than once in the same expression. These expressions rely on a particular order of evaluation where the language does not define one. Consider the following example:
int i = 7;
func( i, ++i );
The C++ language does not guarantee the order in which arguments to a function call are evaluated. Therefore, in the preceding example, func could receive the values 7 and 8, or 8 and 8 for its parameters, depending on whether the parameters are evaluated from left to right or from right to left.