Static Reflection

Reflection lets one examine objects and their properties in a program at runtime.

Typically, this is provided by a runtime. Using TemplateMetaprogrammingTechniques, compile-time reflection emerges - the ability to examine, for example, whether a member function (with a specific name and signature) is present for a given object.

This can be used to create object hierarchies in CeePlusPlus that do not rely on inheritance - rather, objects are organized by their function names, signatures and the convention that a name and signature in an object describes loosely its semantic meaning.

A CeePlusPlus example follows:

struct food;
struct animal
{
void eat(food*);
void sleep(double);
};
struct path;
struct biped
{
void walk(path*);
};
struct statement;
struct human
{
// animal
void eat(food*);
void sleep(double d);
// biped
void walk(path*);
// human
void talk(statement*);
};
FW_HAS_FUNCTION(eat);
FW_HAS_FUNCTION(talk);
FW_HAS_FUNCTION(walk);
FW_HAS_FUNCTION(sleep);
template bool looks_like_an_animal()
{
return
has_function_eat::value &&
has_function_sleep::value;
}
template bool looks_like_a_biped()
{
return has_function_walk::value;
}
template bool looks_like_a_human()
{
return
looks_like_an_animal() &&
looks_like_a_biped () &&
has_function_talk ::value;
}
template void act_like_an_animal(T& t)
{
food* nothing = 0;
t.eat(nothing);
t.sleep(34.45);
}
template void act_like_a_biped(T& t)
{
path* nowhere = 0;
t.walk(nowhere);
}
template void act_like_a_human(T& t)
{
statement* ergocognisum = 0;
act_like_an_animal(t);
act_like_a_biped(t);
t.talk(ergocognisum);
}
void test_has()
{
_ASSERT(true == looks_like_an_animal());
_ASSERT(false == looks_like_an_animal());
_ASSERT(true == looks_like_an_animal());
_ASSERT(false == looks_like_a_biped());
_ASSERT(true == looks_like_a_biped());
_ASSERT(true == looks_like_a_biped());
_ASSERT(false == looks_like_a_human());
_ASSERT(false == looks_like_a_human());
_ASSERT(true == looks_like_a_human());
// let's be ourselves
animal aardvark; biped chimp; human harry;
act_like_an_animal(aardvark);
act_like_an_animal(harry);
act_like_a_biped(chimp);
act_like_a_biped(harry);
act_like_a_human(harry);
}


See also TemplateMetaprogrammingTechniques, TemplateMetafunctionHasFunction, ReadMacro

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CategoryCppTemplates CategoryMetaprogramming