8 Statements [stmt.stmt]

8.6 Iteration statements [stmt.iter]

8.6.4 The for statement [stmt.for]

The for statement is equivalent to
   while ( condition ) {
      expression ;
except that the init-statement is in the same scope as the condition, and except that a continue in statement (not enclosed in another iteration statement) will execute expression before re-evaluating condition.
[Note 1: 
Thus the first statement specifies initialization for the loop; the condition ([stmt.pre]) specifies a test, sequenced before each iteration, such that the loop is exited when the condition becomes false; the expression often specifies incrementing that is sequenced after each iteration.
— end note]
Either or both of the condition and the expression can be omitted.
A missing condition makes the implied while clause equivalent to while(true).