CODEWITHSUNDEEP
eigeneigen3
T Devel
T Devel

Reputation: 23

Eigen Library: Compiler error when passing Block reference to templated function

I am getting a compilation error when trying to call templated functions with arguments that are Eigen references.

For example, consider the following simplified example:

 template <typename derived>
 inline void fcn(Eigen::MatrixBase<derived> &M){
     // ...
 
     M.template block<3,3>(0,0) = something here
     // ...
 
 }
  
// I get compiler errors when trying to call this function as follows:
 
Eigen::MatrixXd A(100,100);
 
// call the above function with a Block as an argument
 fcn(A.block<9,6>(10,10));

The compiler complains that I am trying to instantiate a non-const reference with an object passed by value (if my undersdanding of the following is correct):

error: invalid initialization of non-const reference of type ‘Eigen::MatrixBase<Eigen::Block<Eigen::Matrix<double, -1, -1>, 9, 6, false> >&’ from an rvalue of type ‘Eigen::MatrixBase<Eigen::Block<Eigen::Matrix<double, -1, -1>, 9, 6, false> >’

If I try to declare the argument of fcn as const then I am getting an error when I try to modify it inside the function.

The only solution I've found to work to fix this issue is to declare the arument of the function to be a const &, and then to use const_cast to "remove the const qualifier" when accessing M inside the function.

But this is hacky and I makes the code extremely messy. Am I missing some obvious solution to this problem? Any help would be appreciated. Thanks!

T

Upvotes: 2

Views: 1244

Answers (1)

ggael
ggael

Reputation: 29255

This is because in c++ you cannot bind a temporary object (here the proxy object returned by .block()) to a non-const reference. The solution is to name it:

auto A_block = A.block<9,6>(10,10);
fcn(A_block);

Upvotes: 4

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