
November 21st, 2012, 04:25 AM
#1
get return type of function
Hi,
Say I have overloaded functions with different arguments AND return types. Each set of argument has only one corresponding return type.
Code:
Vector grad(Scalar)
Tensor grad(Vector)
Later I have:
Code:
template <class T>
void test(T x) {
... Y = grad(x)
}
Then how do I automatically declare the type of Y. Do I need to make the grad function a template and specialize each of them ?
Thanks for any help

November 21st, 2012, 04:51 AM
#2
Re: get return type of function
in c++11, you can simply write
"auto Y = grad(x);"
"auto const& Y = grad(x);"
...
or
"typedef decltype(grad(x)) GradType;
GradType Y = grad(x);"
etc... depending on what's the expected type of Y.
with c++2003 compilers, things get more complex ...

November 21st, 2012, 05:05 AM
#3
Re: get return type of function
Thanks a lot.
Unfortunately I am not using c++11 so I can not use those solutions.
I get 'error C3861: 'decltype': identifier not found' for the second solution.
Do you have the solution that works for in Visual studio 2008?
I appreciate your help.
Last edited by dshawul; November 21st, 2012 at 05:13 AM.

November 21st, 2012, 05:50 AM
#4
Re: get return type of function
the easiest way is to write a trait class
Code:
template <class T>
struct grad_type {};
template <>
struct grad_type<Scalar> { typedef Vector type; };
template <>
struct grad_type<Vector> { typedef Tensor type; };
template <class T>
void test(T x) {
typename grad_type<Y>::type Y = grad(x)
}
of course, you need to specialize the trait for each possible parameter type. It would be easier if Scalar,Vector,Tensor had some highlevel info in them ( like a rank grading ), in this way the trait could compute the grad_type automatically at compile time.
Anyway, I think emulating "auto"/"decltype" is possible in this case via some template machinery like SFINAE, but I wouldn't advise it if you don't know what you're doing ...
moreover, another simple solution could be to refactor the code, if possible; something like
Code:
template <class T,class G>
void test_impl(T& x,G y) {
// ...
}
template <class T>
void test(T x) {
test_impl( x, grad(x) );
}

November 21st, 2012, 06:46 AM
#5
Re: get return type of function
You are a life saver. I will use the traits method for now.
Scalar,vector and tensor are 1st,2nd and 3rd rank tensors but they are of completely different types represented by
double, double[], and double[][]. So I don't think I can automatically compute them. But anyway it is important to know
the higher or lower ranks in many places of the code so I will use that.
Thanks again.
Last edited by dshawul; November 21st, 2012 at 06:48 AM.
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