Thanks Joaquin I will try boost::array. Lars ----- Original Message ---- From: "joaquin@tid.es" <joaquin@tid.es> To: "boost@lists.boost.org" <boost@lists.boost.org> Sent: Wednesday, May 21, 2008 4:05:50 PM Subject: Re: [boost] boost::get<i> Lars Schouw escribió:
I tried to use boost::tuple for the first time, and soon ran into a problem.
I can't get values using boost::get<i> where i is an int.
samplec code
typedef boost::tuple<double,double,double> tuple;
void foo() { std::vector<tuple> corr = boost::assign::tuple_list_of ( 1.0, 0.1, -0.1 ) ( 0.1, 1.0, 0.4 ) ( -0.1, 0.4, 1.0 );
for (int i = 0; i < 3; i++) for (int j = 0; j < 3; j++) { double val = boost::get<i>( corr[j] ); } }
Using Visual Studio 2005 this comes give me a compiler error: error C2971: 'boost::tuples::get' : template parameter 'N' : 'i' : a local variable cannot be used as a non-type argument
then when I move the int i declaration outside of the function foo. I get a new error: error C2975: 'N' : invalid template argument for 'boost::tuples::get', expected compile-time constant expression
What do I need to do to access values in a tuple with an index i?
Hello Lars, The problem is that you can't do that for very fundamental reasons. As GCC correctly complains about, i in get<i> must be a compile-time constant, not some value that can change during the execution of the program. To see why, consider the case where your tuple type had been defined as: // component 1 is not a double bout a char* typedef boost::tuple<double,char*,double> tuple; Now, take a look again at the expression: double val = boost::get<i>(corr[j])); What happens when i==1? The compiler cannot know in advance what values i will take, so it demands that i be a compile-time constant so as to be able to do its type checking. In your particular case where all the component types are the same, you can consider replacing the tuple with boost::array. HTH, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost