No known expression from value to value&… Why?

Question

I have tried writing a function which takes a ColXpr value as input:

typedef Eigen::Array Signal2D;

vo         


        

Answer 1

In C++, a temporary object cannot bind to a non-const reference.

I'm assuming that arr.col(0) returns an object by value. The returned value is a temporary object. This means it cannot match a parameter of type T &.

One solution:

auto temp = arr.col(0);
Threshold(temp);

I wonder if you intended for arr.col() to return a reference?

Answer 2

To add to the answer above:

Eigen return expressions are temporaries. E.g. ColXpr is just a small copyable object that gives you access to the array data. It is returned by value, ColXpr col(...). You can capture the ColXpr temporary with const reference but not otherwise.

However you can write:

void Threshold(Matrix::ColXpr col);

Generally, it is frowned upon to modify Eigen expression in functions. The preferred way is to write unary/binary functors/lambdas eg:

array.col(i) = array.col(i).unaryExpr(
  [](const float &value) {
    return float(value >= 0); 
  }
);

Btw, if you intend to write generic Eigen functions, use Eigen::EigenBase<B> as arguments to capture any eigen expression, see http://eigen.tuxfamily.org/dox/TopicFunctionTakingEigenTypes.html

Boost serialization requires non-temporary reference as well, even the operand is being written to an archive.

Answer 3

To complete previous answers, you're probably looking for the Ref<> class which will allow you to write a function that accept both columns of a matrix and vectors without templates nor copies:

void threshold(Ref<ArrayXf> params) {
    params = (params >= 0 ).cast<float>();
}
ArrayXf  a;
ArrayXXf A;
/* ... */
threshold(a);
threshold(A.col(j));