Re: tuples in C++11

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=?UTF-8?B?TWFyY2VsIE3DvGxsZXI=?= <news.5.maazl@spamgourmet.com>

Newsgroups:

comp.lang.c++

Date:

Tue, 14 Aug 2012 20:43:07 +0200

Message-ID:

<502a9c3d$0$9520$9b4e6d93@newsspool1.arcor-online.net>

On 14.08.2012 20:17, Single Stage to Orbit wrote:

Is it even possible to have something like this:

for (auto& i : tuples)
{
         for (unsigned j = 0; j< 3; ++j)
         {
                 std::cout<< i.get<j>();
                 if (j< 3)
                         std::cout<< " ";
         }

         std::cout<< '\n';
}

When I try it, GCC 4.6.3 says it's illegal to have an non constant
expression as in 'i.get<j>'. Are there any workarounds for this one?

No, this is not possible. The compiler cannot know, which function to
call, because at the compile time j does not have a value. In fact you
want to call /another/ function at each loop iteration.
Furthermore i.get<0> and i.get<1> do not have the same type in general.
So the compiler cannot know the type of the expression get<j>.

Your code requires run time polymorphism rather than compile time
polymorphism.

If you want to iterate over the components of your tuple and if all
components have the same type then tuple<> is not the solution. This is
a vector (in the mathematical sense). The C++ equivalent of a vector
with a constant length is simply int[3]. I.e.:

typedef int tuple3[3];
....
tuples.push_back(tuple3({1, 2, 3}));
....
std::cout<< i[j];

(untested)

tuple<> is more something like an anonymous structure.

Marcel