how to avoid many similar overloads for C strings - c++

Question

Here is the code:
template <typename L, typename R> bool eq (const L& lhs, const R& rhs) { return lhs == rhs; }
template<int N> bool eq(char* lhs, const char(&rhs)[N]) { return String(lhs).compare(rhs) == 0; }
template<int N> bool eq(const char(&lhs)[N], char* rhs) { return String(lhs).compare(rhs) == 0; }
inline bool eq(char* lhs, char* rhs) { return String(lhs).compare(rhs) == 0; }
inline bool eq(const char* lhs, const char* rhs) { return String(lhs).compare(rhs) == 0; }
inline bool eq(char* lhs, const char* rhs) { return String(lhs).compare(rhs) == 0; }
inline bool eq(const char* lhs, char* rhs) { return String(lhs).compare(rhs) == 0; }
I have to do this for neq/lt/gt/lte/gte and not just for equality. Maybe I've already missed something.
Is there a way to not list all the possible combinations of C string types?
Also C++98.
EDIT: >> here << is an online demo with the problem

Answer 1

Decay an array type to pointer:
template<class T>
struct decay_array { typedef T type; };
template<class T, size_t N>
struct decay_array<T[N]> { typedef T* type; };
template<class T>
struct decay_array<T[]> { typedef T* type; };
Check that a type is not a pointer to (possibly const) char:
template<class T>
struct not_char_pointer { enum { value = true }; };
template<>
struct not_char_pointer<char*> { enum { value = false }; };
template<>
struct not_char_pointer<const char*> { enum { value = false }; };
Now check that a type is not a pointer to or array of (possibly const) char:
template<class T>
struct can_use_op : not_char_pointer<typename decay_array<T>::type> {};
Reimplement std::enable_if:
template<bool, class = void>
struct enable_if {};
template<class T>
struct enable_if<true, T> { typedef T type; };
and use it to constrain your template:
template <typename L, typename R>
typename enable_if<can_use_op<L>::value || can_use_op<R>::value, bool>::type
eq (const L& lhs, const R& rhs) { return lhs == rhs; }
Then just one overload is enough:
inline bool eq(const char* lhs, const char* rhs) { return String(lhs).compare(rhs) == 0; }

Answer 2

namespace details{
template<template<class...>class Z,class,class...Ts>
struct can_apply:std::false_type{};
template<template<class...>class Z,class...Ts>
struct can_apply<Z,std::void_t<Z<Ts...>>,Ts...>:std::true_type{};
}
template<template<class...>class Z,class...Ts>
using can_apply=details::can_apply<Z,void,Ts...>;
This tests if a template can be applied some types.
namespace strcmp{
bool eq(const char*lhs, const char*rhs){/* body */}
}
template<class L, class R>
using str_eq_r=decltype(strcmp::eq(std::declval<L>(),std::declval<R>()));
template<class L, class R>
using can_str_eq=can_apply<str_eq_r,L,R>;
can_str_eq is truthy iff we can call stdcmp::eq on it.
namespace details {
bool eq(const char* lhs, const char* rhs, std::true_type){
return strcmp::eq(lhs,rhs);
}
template<class L,class R>
bool eq(L const& l, R const&r,std::false_type){
return l==r;
}
}
template<class L,class R>
bool eq(L const& l, R const&r){
return details::eq(l,r,can_str_eq<L const&,R const&>{});;
}
We could also use a static_if trick to do it inline, if you like:
template<class L,class R>
bool eq(L const& l, R const&r){
return static_if<can_str_eq>( l, r )(
strcmp::eq,
[](auto&& l, auto&& r){return l==r;}
);
}
After writing a static_if:
template<class...Ts>
auto functor(Ts...ts){
return [=](auto&& f){
return f(ts...);
};
}
namespace details{
template<class Functor>
auto switcher(std::true_type, Functor functor){
return [=](auto&& t, auto&&){
return functor(t);
};
}
template<class Functor>
auto switcher(std::false_type, Functor functor){
return [=](auto&&, auto&& f){
return functor(f);
};
}
}
template<template<class...>class test, class...Ts>
auto static_if(Ts...ts){
return details::switcher(
test<Ts...>{},
functor(ts...)
);
}
now, what are the odds that works? (Written on phone, not compiled yet) Also not optimal: lots of perfect forwarding, some of which requires de-lamdaing, required.