[stmt.for]

The for statement

{o p t; expression {o p t) statement}_{o p t) statement}}_{o p t; expression {o p t) statement}_{o p t) statement}}

is equivalent to

{
init-statement
while ( condition ) {
statement
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).