Order of evaluation of elements in initializer-list

[Vlad from Moscow]

I tried the following test program with GCC 4.8.1 at www.ideone.com

 

[code]

#include <utility>
#include <iostream>

int main()
{
 int x = 0;
 
 std::pair<int, int> p { ( x = 10 ), x + 10 };
 std::cout << p.first << '\t' << p.second << std::endl;
 
 return 0;
}

[/code]

 

and I got the following result

 

10   10

 

Am I right that it is a bug of the compiler and the correct result should be 10 20 or I missed something?

 

 

[Richard Smith]

Yes, that's a compiler bug, and still seems to be present in a recent g++ trunk checkout. Clang produces '10      20' as expected (in C++11 mode).

[Vlad from Moscow]

It seems that the problem of the compiler is that it evaluates initializer-clauses from right to left applying side effects. For example the output of the following test program

 

[code]

#include <utility>
#include <iostream>

int main()
{
 int x = 0;
 

 std::pair<int, int> p { x + 10, x++ };

 std::cout << p.first << '\t' << p.second << std::endl;
 
 return 0;
}

[/code]

 

is

 

11 0

 

though the correct result should be

 

10 10

среда, 11 сентября 2013 г., 2:28:45 UTC+4 пользователь Richard Smith написал:

[Vlad from Moscow]

I am sorry I have made a typo. :)

 

The correct resuslt will be

 

10 0

среда, 11 сентября 2013 г., 13:31:28 UTC+4 пользователь Vlad from Moscow написал:

[Xeo]

Yes, GCC doesn't yet evaluate braced-init-lists left-to-right as demanded by the standard. Surprisingly, it *does* evaluate aggregate-init left-to-right.

[David Krauss]

On Wednesday, September 11, 2013 5:58:40 PM UTC+8, Xeo wrote:

Yes, GCC doesn't yet evaluate braced-init-lists left-to-right as demanded by the standard. Surprisingly, it *does* evaluate aggregate-init left-to-right.

To be sure, aggregate initialization is just one semantic that may be selected to interpret the braced-init-list syntax.

It probably maps the braced-init-list directly to a normal function argument list and forgets the special semantics. IIRC aggregate initializers are sequenced since C++98.