Does this code contain UB?

Operators such as the comma operator, the logical AND operator (&&) and the logical OR operator (||) before calculating the next operand perform all the side effects associated with the expression for the first operand.

For example, you can write

 std::cout << ( i++, i++, i++ ) << std::endl; 

or

 if ( i++ > 0 && i++ < 10 ) { /* ... */ } 

or

 if ( i++ == 0 || i++ % 3 == 0 ) { /* ... */ } 

The same applies to initialization lists in C ++ . For example,

 int a[] = { i++, ++i }; 

Or in the class constructor

 struct A { int x; int y; A( int i ) : x( i++ ), y( ++i ) { //,,, } }; 

If the behavior of the listed operators with respect to side effects is the same in both C ++ and C, then there is an important difference regarding the initialization lists between these two languages. In C, the order of calculating the side effects of initializers in the initialization list is not defined.

From standard C (6.7.9 Initialization)

This is an unspecified .152 23 evaluations of the initialization list.

Compare this quote with a quote from the C ++ standard (8.5.4 List-initialization)

4 Within the initializer-list-of-braced-init-list, the initializer-clauses are evaluated. This is where each of the initializer compiled with the initializer-clause is associated with the initializer-list. [Note: This evaluation evaluation of holdingsregardlessofthesemanticsoftheinitialization; forexample, itapplies when it comes to the constructor call, even though there are no sequencing constraints. —End note]