Why can't you declare a variable inside the expression portion of a do while loop?

Question

The following syntax is valid:

while (int i = get_data()) { } 

But the following is not:

do { } while (int i = get_data()); 

We can see why via the draft standard N4140 section 6.4:

1 [...]

condition:      expression      attribute-specifier-seqoptdecl-specifier-seq declarator = initializer-clause      attribute-specifier-seqoptdecl-specifier-seq declarator braced-init-list

2 The rules for conditions apply both to selection-statements and to the for and while statements (6.5). [...]

and section 6.5

1 Iteration statements specify looping.

      iteration-statement:               while ( condition ) statement              do statement while ( expression ) ;

Instead, you're forced to do something ugly like:

int i = get_data(); do { } while ((i = get_data())); // double parentheses sic 

What is the rationale for this?

Answer 1

It seems like scoping would be the issue, what would be the scope of i declared in the while portion of a do while statement? It would seem rather unnatural to have a variable available within the loop when the declaration is actually below the loop itself. You don't have this issue with the other loops since the declarations comes before the body of the loop.

If we look at the draft C++ standard section [stmt.while]p2 we see that for the while statement that:

while (T t = x) statement 

is equivalent to:

label: { // start of condition scope     T t = x;     if (t) {         statement     goto label;     } } // end of condition scope 

and:

The variable created in a condition is destroyed and created with each iteration of the loop.

How would we formulate this for the do while case?

and as cdhowie points out if we look at section [stmt.do]p2 it says (emphasis mine):

In the do statement the substatement is executed repeatedly until the value of the expression becomes false. The test takes place after each execution of the statement.

which means the body of the loop is evaluated before we would even reach the declaration.

While we could create an exception for this case it would violate our intuitive sense that in general the point of declaration for a name is after we see the complete declaration(with some exceptions for example class member variables) with unclear benefits. Point of declaration is covered in section 3.3.2.