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Doxygen comments for AttachedVirtualFunction

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Paul Licameli 2020-09-08 22:58:29 -04:00
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src/AttachedVirtualFunction.h
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@ -1,8 +1,9 @@
/**********************************************************************
/*!********************************************************************
Audacity: A Digital Audio Editor
AttachedVirtualFunction.h
@file AttachedVirtualFunction.h
@brief Utility for non-intrusive definition of a new method on a base class
Paul Licameli
@ -11,14 +12,29 @@ Paul Licameli
#ifndef __AUDACITY_ATTACHED_VIRTUAL_FUNCTION__
#define __AUDACITY_ATTACHED_VIRTUAL_FUNCTION__
/* \brief Define a "virtual" function with multiple bodies chosen by type-switch
on the runtime class of the first argument, leaving the set of functions
#include <functional>
#include <mutex>
#include <type_traits>
#include <utility>
#include "InconsistencyException.h"
//! Class template generates single-dispatch, open method registry tables
/*!
Defines a "virtual" function with multiple bodies chosen by type-switch
on the runtime class of the first argument, leaving the set of overrides
open-ended for extension, but also requiring no modification of the definition
of the base class of the type hierarchy that is switched on.
The invocation of the function is not as efficient as for true virtual functions
but the advantage of this utility is greater compilation decoupling. A client
can attach its own virtual functions to a class hierarchy in the core.
but the advantage of this utility is greater compilation decoupling. Client code
can attach its own virtual functions, non-intrusively, to the root of a class hierarchy
defined in the core.
There is no collection of overriding function pointers into virtual function tables by the
linker, but a registration of overrides into a table at static initialization time. This allows
those implementations to be defined wherever is convenient, even in dynamically loaded
libraries.
Beware that invocation of the function should not be done during initialization
of file scope static objects. Dispatch might not go to the correct subclass
@ -26,32 +42,25 @@ case if initializations are not yet complete.
Example usage:
////////
// Core classes in Host.h:
A core class:
```
// file Host.h
class AbstractHost
{
// ...
};
```
class SpecialHost : public Abstract Host
{
// ...
};
class ExtraSpecialHost : public SpecialHost
{
// ...
};
////////
// Declare the root of the attached function hierarchy in Client.h,
// which Host.cpp need not include at all:
Declare the attached function
(in a header that Host.cpp need not include at all)
as a specialization of the template:
```
// file Client.h
enum class ErrorCode { Ok, Bad, // ...
}; // a return type for our function
// First this empty structure serving just to distinguish among instantiations
// of AttachedVirtualFunction with otherwise identical arguments
// of AttachedVirtualFunction with otherwise identical parameters
// An incomplete type is enough
struct DoSomethingTag;
@ -63,9 +72,27 @@ AttachedVirtualFunction<
AbstractHost, // class to be switched on by runtime type
int, double // other arguments
>;
```
////////
// Usage of the "virtual function"
Definitions needed:
```
//file Client.cpp
// Define the default function body here (as a function returning a function!)
template<> auto DoSomething::Implementation() -> Function {
return [](AbstractHost &host, int arg1, double arg2) {
return ErrorCode::Ok;
};
// or you could return nullptr instead of a lambda to force InconsistencyException
// at runtime if the virtual function is invoked for a host subclass for which no override
// was defined.
}
// Must also guarantee construction of an instance of class DoSomething at least
// once before any use of DoSomething::Call()
static DoSomething registerMe;
```
Usage of the method somewhere else:
```
#include "Client.h"
void UseDoSomething( AbstractHost &host )
{
@ -73,39 +100,39 @@ void UseDoSomething( AbstractHost &host )
auto error = DoSomething::Call( host, 0, 1.0 );
// ...
}
```
////////
// Out-of-line registrations needed in Client.cpp:
Derived classes from AbstractHost, not needing Client.h:
```
// file SpecialHost.h
#include "Host.h"
class SpecialHost : public AbstractHost
{
// ...
};
// Define the default function body here (as a function returning a function!)
template<> auto DoSomething::Implementation() -> Function {
return [](AbstractHost &host, int arg1, double arg2) {
return ErrorCode::Ok;
};
// or you could return nullptr to force InconsistencyException at runtime if
// the virtual function is invoked for a host subclass for which no override
// was defined
}
// Must also guarantee construction of an instance of class DoSomething at least
// once before any use of DoSomething::Call()
static DoSomething registerMe;
class ExtraSpecialHost : public SpecialHost
{
// ...
};
```
////////
// An override of the function is defined in SpecialClient.cpp,
// building up a hierarchy of function bodies parallel to the host class
// hierarchy
Overrides of the method, defined in any other .cpp file:
```
#include "SpecialHost.h"
#include "Client.h"
// An override of the function, building up a hierarchy of function bodies parallel
// to the host class hierarchy
using DoSomethingSpecial = DoSomething::Override< SpecialHost >;
template<> template<> auto DoSomethingSpecial::Implementation() -> Function {
// The function can be defined assuming downcast of the first argument
// has been done
// The function can be defined without casting the first argument
return [](SpecialHost &host, int arg1, double arg2) {
return arg1 == 0 ? ErrorCode::Ok : ErrorCode::Bad;
};
}
static DoSomethingSpecial registerMe;
////////
// A further override in ExtraSpecialClient.cpp, demonstrating call-through too
// A further override, demonstrating call-through too
using DoSomethingExtraSpecial =
DoSomething::Override< ExtraSpecialHost, DoSomethingSpecial >;
template<> template<>
@ -120,48 +147,50 @@ auto DoSomethingExtraSpecial::Implementation() -> Function {
return result;
};
}
static DoSomethinExtraSpecial registerMe;
static DoSomethingExtraSpecial registerMe;
```
@tparam Tag an incomplete type, to distinguish methods with otherwise identical parameters
@tparam Return the value returned by each override
@tparam This type of the first argument, a class with at least one true virtual function, the root of the hierarchy for the run-time type-switch
@tparam Arguments any number of types for the second and later arguments
*/
#include <functional>
#include <mutex>
#include <type_traits>
#include <utility>
#include "InconsistencyException.h"
template< typename Tag, typename Return, typename This, typename... Arguments >
class AttachedVirtualFunction
{
public:
// These member names are declared in this class template and redeclared
// in each override
//! This member name is declared in this class template and redeclared in each override
using Object = This;
//! This member name is declared in this class template and redeclared in each override
using Function = std::function< Return( Object&, Arguments... ) >;
// A function returning a std::function, which you define elsewhere;
// it may return nullptr in case this must act somewhat as a "pure virtual",
// throwing InconsistencyException if the function is invoked on a subclass
// for which no override was defined
//! A function returning a std::function, which you must define so that the program links
/*! It may return nullptr in case this must act somewhat as a "pure virtual",
throwing InconsistencyException if the function is invoked on a subclass
for which no override was defined */
static Function Implementation();
// This class must be instantiated once at least to register the function in
// a table, but may be instantiated multiply (and will be if there are any
// overrides)
//! At least one static instance must be created; more instances are harmless
/*! (There will be others if there are any overrides.) */
AttachedVirtualFunction()
{
static std::once_flag flag;
std::call_once( flag, []{ Register<This>( Implementation() ); } );
}
// For defining overrides of the virtual function; template arguments
// are the more specific subclass and the immediately overridden version
// of the function, defaulting to the base version
//! For defining overrides of the method
/*!
@tparam Subclass the more specific subclass of @b This
@tparam Overridden The immediately overridden version, defaulting to the base version
*/
template<
typename Subclass, typename Overridden = AttachedVirtualFunction >
struct Override : Overridden
{
//! Shadowing Overridden::Object
using Object = Subclass;
//! Shadowing Overridden::Function, giving the first argument a more specific type
using Function = std::function< Return( Object&, Arguments... ) >;
// Check that inheritance is correct
@ -170,16 +199,17 @@ public:
"overridden class must be a base of the overriding class"
);
// A function returning a std::function that must be defined out-of-line
//! A function returning a std::function that must be defined so that the program links
static Function Implementation();
// May be used in the body of the overriding function, defining it in
// terms of the overridden one:
//! May be used in the body of the overriding function, defining it in terms of the overridden one
static Return Callthrough(
typename Overridden::Object &object, Arguments &&...arguments )
{
return Overridden::Implementation()(
object, std::forward< Arguments >( arguments )... );
}
//! At least one static instance must be created; more instances are harmless
/*! (There will be others if there are any further overrides.) */
Override()
{
static std::once_flag flag;
@ -195,7 +225,11 @@ public:
}
};
static Return Call( This &obj, Arguments &&...arguments )
//! Invoke the method -- but only after static initialization time
static Return Call(
This &obj, //!< Object on which to type-switch at run-time
Arguments &&...arguments //!< other arguments
)
{
try {
// Note that the constructors of this class and overrides cause
@ -211,9 +245,11 @@ public:
for ( ; iter != end; ++iter ) {
auto &entry = *iter;
if ( entry.predicate( &obj ) )
// This might throw std::bad_function_call on a null function
return entry.function(
obj, std::forward< Arguments >( arguments )... );
}
// If not found, also throw
throw std::bad_function_call{};
}
catch ( const std::bad_function_call& ) {
@ -236,6 +272,7 @@ private:
using Predicate = std::function< bool( This* ) >;
//! Member of registry of implementations of the method
struct Entry
{
Predicate predicate;