Hi,

I have a function that gets a VectorXf as an argument:

Code: Select all

void foo(const VectorXf& v)
{
...
}

Now, when I pass a row of a matrix to this function by:

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Matrix<float, 3, Dynamic> m(10, 100);
...
foo(m.row(1)); // Doesn't work


It doesn't work.
But when I use:

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Matrix<float, 3, Dynamic> m(10, 100);
VectorXf m_row_1 = m.row(1);
foo(m_row_1); // Works

My question is why doesn't the first option work, and what's the best way to pass a row of a matrix to a function.

Thanks in advance,
Yoav

because .row() or .col() does not return a VectorXf but a lightweight Block<> object. See this page to write generic functions:

http://eigen.tuxfamily.org/dox/TopicFun ... Types.html

Thanks a lot! This was very helpful.

Now, assuming that I want to write a template function that is limited to get only vectors, or BlockVectors.
1. Is there a common abstract class for all vectors (/segments)?
2. How can I write a function that deduces the size of the vector as a constant in compile time?

Thanks in advance,
Yoav

There is no common base class for that, but you can use a static assert or boost::enable_if. e.g.:

foo(const MatrixBase<Derived>& vec) {
EIGEN_STATIC_ASSERT_VECTOR_ONLY(Derived);
}

The underlying compile time test is:

Derived::IsVectorAtCompileTime

You can use this with enable_if.


Useful compile-time information:

Derived::SizeAtCompileTime
Derived::RowsAtCompileTime
Derived::ColsAtCompileTime

Note that they might be equal to Dynamic if the size is not known at compile time!