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Diffstat (limited to 'eigen/doc/TopicAliasing.dox')
| -rw-r--r-- | eigen/doc/TopicAliasing.dox | 30 |
1 files changed, 26 insertions, 4 deletions
diff --git a/eigen/doc/TopicAliasing.dox b/eigen/doc/TopicAliasing.dox index c2654ae..a8f1644 100644 --- a/eigen/doc/TopicAliasing.dox +++ b/eigen/doc/TopicAliasing.dox @@ -153,10 +153,11 @@ not necessary to evaluate the right-hand side explicitly. \section TopicAliasingMatrixMult Aliasing and matrix multiplication -Matrix multiplication is the only operation in %Eigen that assumes aliasing by default. Thus, if \c matA is a -matrix, then the statement <tt>matA = matA * matA;</tt> is safe. All other operations in %Eigen assume that -there are no aliasing problems, either because the result is assigned to a different matrix or because it is a -component-wise operation. +Matrix multiplication is the only operation in %Eigen that assumes aliasing by default, <strong>under the +condition that the destination matrix is not resized</strong>. +Thus, if \c matA is a \b squared matrix, then the statement <tt>matA = matA * matA;</tt> is safe. +All other operations in %Eigen assume that there are no aliasing problems, +either because the result is assigned to a different matrix or because it is a component-wise operation. <table class="example"> <tr><th>Example</th><th>Output</th></tr> @@ -198,6 +199,27 @@ may get wrong results: \verbinclude TopicAliasing_mult3.out </td></tr></table> +Moreover, starting in Eigen 3.3, aliasing is \b not assumed if the destination matrix is resized and the product is not directly assigned to the destination. +Therefore, the following example is also wrong: + +<table class="example"> +<tr><th>Example</th><th>Output</th></tr> +<tr><td> +\include TopicAliasing_mult4.cpp +</td> +<td> +\verbinclude TopicAliasing_mult4.out +</td></tr></table> + +As for any aliasing issue, you can resolve it by explicitly evaluating the expression prior to assignment: +<table class="example"> +<tr><th>Example</th><th>Output</th></tr> +<tr><td> +\include TopicAliasing_mult5.cpp +</td> +<td> +\verbinclude TopicAliasing_mult5.out +</td></tr></table> \section TopicAliasingSummary Summary |