C++ Primer, Fifth Edition

Beware of Missing or Extraneous Semicolons

Because a null statement is a statement, it is legal anywhere a statement is expected. For this reason, semicolons that might appear illegal are often nothing more than null statements. The following fragment contains two statements—the expression statement and the null statement:

ival = v1 + v2;; // ok: second semicolon is a superfluous null statement

Although an unnecessary null statement is often harmless, an extra semicolon following the condition in a while or if can drastically alter the programmer’s intent. For example, the following code will loop indefinitely:

// disaster: extra semicolon: loop body is this null statementwhile (iter != svec.end()) ; // the while body is the empty statement