how to use const std::function< T(const T &, const T &)> & combiner

[ztdep...@gmail.com]

I want to use the mpi::all_reduce to add some values from every processes. 

but i don't know how to set the "const std::function< T(const T &, const T &)> &   combiner". 

Could you please give me some help about it?

 Utilities::MPI::all_reduce ( const T &   local_value,
const MPI_Comm &   comm,
const std::function< T(const T &, const T &)> &   combiner
)

const std::function< T(const T &, const T &)> &   combiner 

[Daniel Arndt]

It's simply a function that takes two arguments of the same type by reference and returns a new value of that type.

For a sum reduction using doubles, you could pass in in a lambda, for example:

[](const double& a, const double& b) {return a+b;}

Best,

Daniel

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