Ref< PlainObjectType, Options, StrideType > Class Template Reference
[Core module]

A matrix or vector expression mapping an existing expressions. More...

#include <Ref.h>

Inherits Eigen::RefBase< Ref< PlainObjectType, Options, StrideType > >.

Detailed Description

template<typename PlainObjectType, int Options, typename StrideType>
class Eigen::Ref< PlainObjectType, Options, StrideType >

A matrix or vector expression mapping an existing expressions.

Template Parameters:

PlainObjectType	the equivalent matrix type of the mapped data
Options	specifies whether the pointer is Aligned, or Unaligned. The default is Unaligned.
StrideType	optionally specifies strides. By default, Ref implies a contiguous storage along the inner dimension (inner stride==1), but accept a variable outer stride (leading dimension). This can be overridden by specifying strides. The type passed here must be a specialization of the Stride template, see examples below.

This class permits to write non template functions taking Eigen's object as parameters while limiting the number of copies. A Ref<> object can represent either a const expression or a l-value:

 // in-out argument:
 void foo1(Ref<VectorXf> x);

 // read-only const argument:
 void foo2(const Ref<const VectorXf>& x);

In the in-out case, the input argument must satisfies the constraints of the actual Ref<> type, otherwise a compilation issue will be triggered. By default, a Ref<VectorXf> can reference any dense vector expression of float having a contiguous memory layout. Likewise, a Ref<MatrixXf> can reference any column major dense matrix expression of float whose column's elements are contiguously stored with the possibility to have a constant space inbetween each column, i.e.: the inner stride mmust be equal to 1, but the outer-stride (or leading dimension), can be greater than the number of rows.

In the const case, if the input expression does not match the above requirement, then it is evaluated into a temporary before being passed to the function. Here are some examples:

 MatrixXf A;
 VectorXf a;
 foo1(a.head());             // OK
 foo1(A.col());              // OK
 foo1(A.row());              // compilation error because here innerstride!=1
 foo2(A.row());              // The row is copied into a contiguous temporary
 foo2(2*a);                  // The expression is evaluated into a temporary
 foo2(A.col().segment(2,4)); // No temporary

The range of inputs that can be referenced without temporary can be enlarged using the last two template parameter. Here is an example accepting an innerstride!=1:

 // in-out argument:
 void foo3(Ref<VectorXf,0,InnerStride<> > x);
 foo3(A.row());              // OK

The downside here is that the function foo3 might be significantly slower than foo1 because it won't be able to exploit vectorization, and will involved more expensive address computations even if the input is contiguously stored in memory. To overcome this issue, one might propose to overloads internally calling a template function, e.g.:

 // in the .h:
 void foo(const Ref<MatrixXf>& A);
 void foo(const Ref<MatrixXf,0,Stride<> >& A);

 // in the .cpp:
 template<typename TypeOfA> void foo_impl(const TypeOfA& A) {
     ... // crazy code goes here
 }
 void foo(const Ref<MatrixXf>& A) { foo_impl(A); }
 void foo(const Ref<MatrixXf,0,Stride<> >& A) { foo_impl(A); }

See also:

PlainObjectBase::Map(), TopicStorageOrders

Definition at line 188 of file Ref.h.