Dear all,

I have been reading the documentation at http://eigen.tuxfamily.org/dox-devel/To ... Types.html but there is a topic that is missing. I would like to create a temporay matrix inside a templated function of a size different to any of which that are passed in. For example:

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template <typename Derived 1, typename Derived2>
void Foo(MatrixBase<Derived1>& y, const MatrixBase<Derived1>& x, const MatrixBase<Derived2>& A)
{
    // How do I declare B in terms of x.rows() and A.cols()?
    Matrix<rows of x, cols of A> B = SomeVeryExpensiveFunctionCall();
    y = x + B * A * B.transpose();
}


Previously, I have been passing in matricies of specific sizes, but I would like to use the generic functionality.

Thanks in advance,
Damien.

Every Eigen type has constants RowsAtCompileTime and ColsAtCompileTime, which are set to the number of rows and columns (or to Dynamic for types with a dynamic size). Similarly, there is a typedef Scalar which is the type of the entries that make up the matrix. Thus, if B should have the same entries as A, you can use you can use

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 Matrix<Derived1::RowsAtCompileTime, Derived2::ColsAtCompileTime, typename Derived2::Scalar> B = ...


Alternatively, you can just use a dynamic size matrix for B. If your expensive function call is really expensive, the extra overhead of dynamic size is negligible, and getting the template parameters right can sometimes be tricky.

This works, very nicely, but one comment: change

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 Matrix<Derived1::RowsAtCompileTime, Derived2::ColsAtCompileTime, typename Derived2::Scalar>


To:

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 Matrix<typename Derived2::Scalar, Derived1::RowsAtCompileTime, Derived2::ColsAtCompileTime>


Thanks!