In C++ why can't I write a for() loop like this: for( int i = 1, double i2 = 0;

Question

or, \"Declaring multiple variables in a for loop ist verboten\" ?!

My original code was

 for( int i = 1, int i2 = 1; 
      i2 < mid;
      i++, i         


        

Answer 1

int i = 1, double i2 = 0; is not a valid declaration statement, so it cannot be used inside the for statement. If the statement can't stand alone outside the for, then it can't be used inside the for statement.

Edit: Regarding your questions about comma operators, options 'A' and 'B' are identical and are both valid. Option 'C' is also valid, but will probably not do what you would expect. z will be assigned 1, and the statements 3 and 4 don't actually do anything (your compiler will probably warn you about "statements with no effect" and optimize them away).

Update: To address the questions in your edit, here is how the C++ spec (Sec 6.5) defines for:

for ( for-init-statement condition(opt) ; expression(opt) ) statement

It further defines for-init-statement as either expression-statement or simple-declaration. Both condition and expression are optional.

The for-init-statement can be anything that is a valid expression-statement (such as i = 0;) or simple-declaration (such as int i = 0;). The statement int i = 1, double i2 = 0; is not a valid simple-declaration according to the spec, so it is not valid to use with for. For reference, a simple-declaration is defined (in Section 7) as:

attribute-specifier(opt) decl-specifier-seq(opt) init-declarator-list(opt) ;

where decl-specifier-seq would be the data type plus keywords like static or extern and init-declarator-list would be a comma-separated list of declarators and their optional initializers. Attempting to put more than one data type in the same simple-declaration essentially places a decl-specifier-seq where the compiler expects a init-declarator-list. Seeing this element out of place causes the compiler to treat the line as ill-formed.

The spec also notes that the for loop is equivalent to:

{
    for-init-statement
    while ( condition ) {
        statement
        expression ;
    }
}

where condition defaults to "true" if it is omitted. Thinking about this "expanded" form may be helpful in determining whether a given syntax may be used with a for loop.