482 lines
9.8 KiB
Plaintext
482 lines
9.8 KiB
Plaintext
#LyX 2.0 created this file. For more info see http://www.lyx.org/
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\lyxformat 413
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\textclass article
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\language english
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\index Index
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\shortcut idx
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\end_index
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\end_header
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\begin_body
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\begin_layout Section
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Template Classes
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\end_layout
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\begin_layout Standard
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Templated classes are great for writing generic code for multiple types
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(e.g.
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the same elimination algorithm code for symbolic, discrete, and Gaussian
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elimination) without the drawbacks of virtual inheritance (which include
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rigid class interfaces, downcasting from returned base class pointers,
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and additional runtime overhead).
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Depending on how they're used, though, templates can result in very slow
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compile times, large binary files, and hard-to-use code.
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This section describes the
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\begin_inset Quotes eld
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\end_inset
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best practices
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\begin_inset Quotes erd
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\end_inset
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we have developed for gaining the benefits of templates without the drawbacks.
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\end_layout
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\begin_layout Standard
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If you need to write generic code or classes, here are several programming
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patterns we have found to work very well:
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\end_layout
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\begin_layout Subsection
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The
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\begin_inset Quotes eld
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\end_inset
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Templated Base, Specialized Derived
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\begin_inset Quotes erd
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\end_inset
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Pattern
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\end_layout
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\begin_layout Standard
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This pattern is for when you have a generic class containing algorithm or
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data structure code that will be specialized to several types.
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The templated base class should never be used directly, instead only the
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specializations should be used.
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Some specialized types can be pre-compiled into the library, but the option
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remains to specialize new types in external libraries or projects.
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\end_layout
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\begin_layout Subsubsection
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Basic Class Structure
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\end_layout
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\begin_layout Standard
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We'll use
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\family typewriter
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FactorGraph
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\family default
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as an example.
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It is templated on the factor type stored in it and has several specializations.
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The templated base class
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\family typewriter
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FactorGraph<class FACTOR>
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\family default
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is divided into a header file (
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\family typewriter
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.h
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\family default
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) and an
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\begin_inset Quotes eld
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\end_inset
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instantiation
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\begin_inset Quotes erd
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\end_inset
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file (
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\family typewriter
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-inst.h
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\family default
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).
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The basic class structure is as follows.
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\begin_inset listings
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lstparams "basicstyle={\scriptsize\ttfamily},language={C++}"
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inline false
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status open
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\begin_layout Plain Layout
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// File FactorGraph.h
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\begin_layout Plain Layout
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%*
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\backslash
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bfseries{
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\backslash
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emph{
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\backslash
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color{red}{// Include a minimal set of headers.
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Do not include any '-inst.h' files (this is the key to fast compiles).}}}*)
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\end_layout
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\begin_layout Plain Layout
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#include <boost/serialization/nvp.hpp>
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\end_layout
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\begin_layout Plain Layout
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...
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\begin_layout Plain Layout
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namespace gtsam {
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\end_layout
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\begin_layout Plain Layout
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/** Class description */
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\end_layout
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\begin_layout Plain Layout
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template<class FACTOR>
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\end_layout
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\begin_layout Plain Layout
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class FactorGraph
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\end_layout
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\begin_layout Plain Layout
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{
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\end_layout
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\begin_layout Plain Layout
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%*
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\backslash
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bfseries{
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\backslash
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emph{
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\backslash
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color{red}{// Make 'private' any typedefs that must be redefined in derived
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classes.
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E.g.
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'This' in the context of the derived class should refer to the derived
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class.
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These typedefs will be used only by the generic code in this base class.}}}*)
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\end_layout
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\begin_layout Plain Layout
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private:
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\end_layout
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\begin_layout Plain Layout
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typedef FactorGraph<FACTOR> This; ///< Typedef for this class
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\end_layout
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\begin_layout Plain Layout
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typedef std::shared_ptr<This> shared_ptr; ///< Shared pointer to
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this
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\begin_layout Plain Layout
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%*
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\backslash
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bfseries{
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\backslash
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emph{
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\backslash
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color{red}{// Make 'public' the typedefs that will be valid in the derived
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class.}}}*)
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\end_layout
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\begin_layout Plain Layout
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public:
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\end_layout
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\begin_layout Plain Layout
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typedef FACTOR FactorType; ///< Factor type stored in this graph
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\end_layout
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\begin_layout Plain Layout
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typedef std::shared_ptr<FACTOR> sharedFactor; ///< Shared pointer
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to a factor
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\end_layout
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\begin_layout Plain Layout
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...
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\begin_layout Plain Layout
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%*
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\backslash
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bfseries{
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\backslash
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emph{
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\backslash
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color{red}{// Normally, data is 'protected' so the derived class can access
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it.}}}*)
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\end_layout
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\begin_layout Plain Layout
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protected:
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\end_layout
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\begin_layout Plain Layout
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/** Collection of factors */
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\end_layout
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\begin_layout Plain Layout
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std::vector<sharedFactor> factors_;
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\begin_layout Plain Layout
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%*
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\backslash
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bfseries{
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\backslash
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emph{
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\backslash
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color{red}{// Make 'protected' all constructors, named constructors, or
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methods returning the base class type.
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These are not public - the derived class will call them and properly convert
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returned base classes to the derived class.}}}*)
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\end_layout
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\begin_layout Plain Layout
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/// @name Standard Constructors
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\end_layout
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\begin_layout Plain Layout
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/// @{
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\begin_layout Plain Layout
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/** Default constructor */
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\end_layout
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\begin_layout Plain Layout
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FactorGraphUnordered() {}
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\begin_layout Plain Layout
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/** Named constructor from iterator over factors */
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\end_layout
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\begin_layout Plain Layout
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template<typename ITERATOR>
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\end_layout
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\begin_layout Plain Layout
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static This FromIterator(ITERATOR firstFactor, ITERATOR lastFactor);
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\end_layout
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\begin_layout Plain Layout
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/// @}
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\end_layout
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\begin_layout Plain Layout
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\end_layout
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\begin_layout Plain Layout
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%*
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\backslash
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bfseries{
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\backslash
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emph{
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\backslash
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color{red}{// Make 'public' standard methods that will be available in the
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derived class's API.}}}*)
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\end_layout
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\begin_layout Plain Layout
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public:
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\end_layout
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\begin_layout Plain Layout
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/// @name Adding Factors
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\end_layout
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\begin_layout Plain Layout
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/// @{
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\end_layout
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\begin_layout Plain Layout
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/** ...
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*/
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\end_layout
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\begin_layout Plain Layout
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void reserve(size_t size);
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\end_layout
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\begin_layout Plain Layout
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...
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\end_layout
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\begin_layout Plain Layout
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/// @}
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\end_layout
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\begin_layout Plain Layout
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};
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\end_layout
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\begin_layout Plain Layout
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}
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\end_layout
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\end_inset
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\end_layout
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\end_body
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\end_document
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