105 lines
3.6 KiB
Plaintext
105 lines
3.6 KiB
Plaintext
[/
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(C) Copyright Edward Diener 2012
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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http://www.boost.org/LICENSE_1_0.txt).
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]
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[section:tti_detail_has_function Introspecting an inner function]
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The TTI macro [macroref BOOST_TTI_HAS_FUNCTION] introspects
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an inner function of a class. The function can be either a member
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function or a static member function.
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BOOST_TTI_HAS_FUNCTION takes a single
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parameter which is the name of an inner function whose existence
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the programmer wants to check. The macro generates a metafunction
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called "has_function_'name_of_inner_function'".
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The metafunction can be invoked by passing it the enclosing type to introspect and a
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signature for the function as separate template arguments. The signature for the
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function consists of a return type, optional parameter types in the form of a boost::mpl
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forward sequence of types, and an optional Boost FunctionTypes tag type. A
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typical boost::mpl forward sequence of types is a boost::mpl::vector<>.
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The metafunction returns a single type called 'type', which is a
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boost::mpl::bool_. As a convenience the metafunction
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returns the value of this type directly as a compile time bool constant
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called 'value'. This is true or false depending on whether the inner
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function, of the specified signature, exists or not.
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[heading Generating the metafunction]
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You generate the metafunction by invoking the macro with the name
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of an inner function:
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BOOST_TTI_HAS_FUNCTION(AnInnerFunction)
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generates a metafunction called 'has_function_AnInnerFunction' in the current scope.
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[heading Invoking the metafunction]
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You invoke the metafunction by instantiating the template with an enclosing
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type to introspect and the signature of the function as a series of template
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parameters.
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A return value called 'value' is a compile time bool constant.
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has_function_AnInnerFunction
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<
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Enclosing_Type,
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Function_ReturnType,
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boost::mpl::vector<Function_ParameterTypes>, // optional, can be any mpl forward sequence
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boost::function_types::SomeTagType // optional, can be any FunctionTypes tag type
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>::value
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[heading Examples]
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First we generate metafunctions for various inner function names:
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#include <boost/tti/has_function.hpp>
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BOOST_TTI_HAS_FUNCTION(function1)
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BOOST_TTI_HAS_FUNCTION(function2)
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BOOST_TTI_HAS_FUNCTION(function3)
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Next let us create some user-defined types we want to introspect.
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struct AClass { };
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struct Top
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{
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static int function1();
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AClass function2(double,short *);
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};
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struct Top2
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{
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long function2(Top &,int,bool,short,float);
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static Top * function3(long,int,AClass &);
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};
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Finally we invoke our metafunction and return our value.
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This all happens at compile time, and can be used by
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programmers doing compile time template metaprogramming.
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has_function_function1<Top,int>::value; // true
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has_function_function1<Top2,int>::value; // false
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has_function_function2<Top,AClass,boost::mpl::vector<double,short *> >::value; // true
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has_function_function2<Top2,AClass,boost::mpl::vector<double,short *> >::value; // false
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has_function_function3<Top2,int>::value; // false
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has_function_function3<Top2,Top *,boost::mpl::vector<long,int,AClass &> >::value; // true;
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[heading Metafunction re-use]
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The macro encodes only the name of the function
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for which we are searching and the fact that we are
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introspecting for a function within an enclosing type.
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Because of this, once we create our metafunction for
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introspecting a function by name, we can reuse the
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metafunction for introspecting any enclosing type, having any
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function, for that name.
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[endsect]
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