C++ Primer, Fifth Edition

Remember That Operands Can Be Evaluated in Any Order

Most operators give no guarantee as to the order in which operands will be evaluated (§ 4.1.3, p. 137). This lack of guaranteed order often doesn’t matter. The cases where it does matter are when one subexpression changes the value of an operand that is used in another subexpression. Because the increment and decrement operators change their operands, it is easy to misuse these operators in compound expressions.

To illustrate the problem, we’ll rewrite the loop from § 3.4.1 (p. 108) that capitalizes the first word in the input. That example used a for loop:

for (auto it = s.begin(); it != s.end() && !isspace(*it); ++it)    *it = toupper(*it); // capitalize the current character

which allowed ...