I'm trying to write a wrapper function that returns an expression operating on a mapped array, but it looks like I'm doing it wrong. Here's a sample program:

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#include <iostream>
#include <Eigen/Core>

typedef Eigen::ArrayXd ColArray;
typedef ColArray::ConstAlignedMapType ColArrayMap;
typedef Eigen::CwiseUnaryOp< Eigen::internal::scalar_add_op<double>, const ColArray > PlusExpr;
typedef Eigen::CwiseUnaryOp< Eigen::internal::scalar_add_op<double>, ColArrayMap > PlusMapExpr;

ColArray v(3);

ColArrayMap mymap()
{
        return Eigen::ArrayXd::MapAligned(const_cast<const double*>(v.data()), v.size());
}

PlusExpr plusone()
{
        return v+1.0;
}

PlusMapExpr mapplusone()
{
        return mymap()+1.0;
}

int main()
{
        v << 1, 2, 3;

        std::cout << "v,        rows = " << v.rows() << "\n";
        std::cout << "map(v),   rows = " << mymap().rows() << "\n";
        std::cout << "v+1,      rows = " << plusone().rows() << "\n";
        std::cout << "map(v)+1, rows = " << mapplusone().rows() << "\n";

        return 0;
}


When I compile and run this, using the current development branch of Eigen, I get:

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$ g++ -Wall -W -I eigen testmap.cc -o testmap && ./testmap
v,        rows = 3
map(v),   rows = 3
v+1,      rows = 3
map(v)+1, rows = 4199135


So it looks like something is overwritten where it shouldn't. Doing anything more involved with the resulting expression than simply printing the number of rows will usually result in a segfault or a failed assertion.

So, is it possible to use maps in this way, and if so, what is the correct incantation? Any tips on how to fix this will be greatly appreciated.

the problem is that mymap returns a temporary while Map objects are currenty nested by reference... Honestly I don't see why Map objects are not nested by value, I'll investigate.

ggael wrote:the problem is that mymap returns a temporary while Map objects are currenty nested by reference...

Right. That would explain the symptoms.

ggael wrote:Honestly I don't see why Map objects are not nested by value, I'll investigate.

Thanks for looking into this!