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