#include<iostream>
#include<string>
#include<type_traits>
using std::string;
using std::cerr;
using std::endl;
// base trait class stores data we want to access & modify in a uniform manner at runtime
struct TraitBase{
string _name, _label;
};
// template derived class stores data that are accessed at compile-time
template<int _flags>
struct Trait: public TraitBase{
// saves compile-time flags for easy access
enum{flags=_flags};
// named parameter idiom; must be defined here (not in TraitBase) so that ref to ourselves is returned
// in that way, chained expression retains template parameters
Trait& name(const string& s){ _name=s; return *this; }
Trait& label(const string& s){ _label=s; return *this; }
};
struct Foo{
#define aTrait Trait<23>().name("a").label("[original label]")
int a;
static decltype(aTrait)& makeTrait_a(){
// remove reference in case named parameters are used in aTrait
static std::remove_reference<decltype(aTrait)>::type _a=aTrait;
return _a;
}
// we need std::remove_reference for decltype (otherwise error with gcc 4.5)
//
// this typedef also works around limited support for decltype in gcc-4.5;
// e.g. std::remove_reference<decltype(Foo::makeTrait_a())>::type::flags would fail
typedef std::remove_reference<decltype(Foo::makeTrait_a())>::type TraitType_a;
};
template<int val> void printFlags(){ cerr<<"[compile-time flags "<<val<<"]\n"; }
int main(void){
// show compile-time resolution
printFlags<Foo::TraitType_a::flags>();
// show that static initialization works
cerr<<Foo::makeTrait_a()._name<<": "<<Foo::makeTrait_a()._label<<endl;
// show that reference to the single object is passed around properly
Foo::makeTrait_a().label("[changed label]");
cerr<<Foo::makeTrait_a()._name<<": "<<Foo::makeTrait_a()._label<<endl;
}