iterate over tuple

[nielsp]

Hello!

How can I iterate over a tuple (using C++0x)? I tried the following, but that doesn't work:

for(int i=0; i<std::tuple_size<T...>::value; ++i) 
    std::get<i>(my_tuple).do_sth();
 

Error 1: sorry, unimplemented: cannot expand ‘Listener ...’ into a fixed-length argument list.
Error 2: i cannot appear in a constant expression.

So, how do I correctly iterate over the elements of a tuple?

Thanks!

Niels.

[weaknessforcats]

http://www.boost.org/doc/libs/1_39_0...ers_guide.html

[nielsp]

The document does not say anything about iterating over tuples. By the way, I am using tuples of C++0x. Although those tuples are probably derived from the boost library, it might be not absolutly the same.

Niels.

[nielsp]

Is it impossible or what?

[Banfa]

@nielsp
Assuming error 1 is associated with your first line of code and error 2 is associated with the second line of code (on a side note it would be helpful if you clearly identified what line of code each error is associated with when posting code and errors in the future) then

Error 2: The problem is with this std::get<i> you are trying to pass i as a parameter to a template but templates are resolved at compile time. You can only pass constants (or types were appropriate) as template parameters.

As to error 1 and code line 1 the <T ...> construct is not something I am familiar with and there appears to be no type or variable or symbol with the name Listener in your code.

[nielsp]

Hi!

I understood the mistakes I made. The Listener-related error has nothing to do with the problem, that the symbol does'nt exists, I just wanted to simplify the code in order to make it easer to read, so I renamed Listener to T, but the error exists although (cannot expand 'T' ... instead of cannot expand 'Listener') .
I got some helpful answers at stackoverflow.com, so here is the version that works:

template<class TupleType, size_t N>
struct do_iterate 
{
    static void call(TupleType& t) 
    {
        std::get<N>(t).do_sth(); 
        do_iterate<TupleType, N-1>::call(t); 
    }
}; 
 
template<class TupleType>
struct do_iterate<TupleType, 0> 
{
    static void call(TupleType& t) 
    {
        std::get<0>(t).do_sth(); 
    }
}; 
 
template<class TupleType>
void iterate_tuple(TupleType& t)
{
    do_iterate<TupleType, std::tuple_size<TupleType>::value-1>::call(t);
}
 

By the way: This will execute the do_sth()-operations of the tuple elements in reverse order.

Niels

[Banfa]

I kind of thought there would be some clever template way of achieving this, glad you found it.