How to define a contructor for <...auto...> - c++

Question

With the following program, which is an extract from something larger that I'm experimenting with, I get an error message that seems to be related to the constructor for fixed_string
#include <string>
#include <cstring>
template<std::size_t N>
struct fixed_string {
static const constexpr std::size_t size__ = N;
constexpr fixed_string(char const* s) :
buf("") {
for (std::size_t i = 0; i <= N; ++i)
buf[i] = s[i];
}
constexpr operator char const*() const {
return buf;
}
constexpr bool operator==(const char* other) const {
return ::strncmp(buf, other, N) == 0;
}
template<std::size_t M>
constexpr bool compare(const fixed_string<M>& other) const {
return (N == M && ::strncmp(buf, other.buf, N) == 0) ? std::true_type(): std::false_type();
}
char buf[N + 1];
};
template<std::size_t N>
fixed_string(char const (&)[N]) -> fixed_string<N - 1>;
////////////////////////////////////////////
template<fixed_string TARGET_NAME, fixed_string THIS_NAME>
concept NameMatches = (TARGET_NAME.compare(THIS_NAME));
template<fixed_string NAME, typename TYPE>
class Member {
public:
static const constexpr fixed_string name__ { NAME };
public:
template<fixed_string TARGET_NAME>
const TYPE& get() const requires NameMatches<TARGET_NAME, TYPE::name__> const {
return member_;
}
protected:
TYPE member_;
};
template<typename ... MEMBERS>
class Container: public MEMBERS... {
};
The error messages are:
../src/test-concepts.cpp:43:35: error: class template argument deduction failed:
43 | const TYPE& get() const requires NameMatches<TARGET_NAME, TYPE::name__> const {
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/test-concepts.cpp:43:35: error: no matching function for call to ‘fixed_string(fixed_string<...auto...>)’
../src/test-concepts.cpp:8:12: note: candidate: ‘template<long unsigned int N> fixed_string(const char*)-> fixed_string<N>’
8 | constexpr fixed_string(char const* s) :
| ^~~~~~~~~~~~
../src/test-concepts.cpp:8:12: note: template argument deduction/substitution failed:
../src/test-concepts.cpp:43:35: note: couldn’t deduce template parameter ‘N’
43 | const TYPE& get() const requires NameMatches<TARGET_NAME, TYPE::name__> const {
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/test-concepts.cpp:5:8: note: candidate: ‘template<long unsigned int N> fixed_string(fixed_string<N>)-> fixed_string<N>’
5 | struct fixed_string {
| ^~~~~~~~~~~~
../src/test-concepts.cpp:5:8: note: template argument deduction/substitution failed:
../src/test-concepts.cpp:43:35: note: mismatched types ‘fixed_string<N>’ and ‘fixed_string<...auto...>’
43 | const TYPE& get() const requires NameMatches<TARGET_NAME, TYPE::name__> const {
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/test-concepts.cpp:28:1: note: candidate: ‘template<long unsigned int N> fixed_string(const char (&)[N])-> fixed_string<(N - 1)>’
28 | fixed_string(char const (&)[N]) -> fixed_string<N - 1>;
| ^~~~~~~~~~~~
../src/test-concepts.cpp:28:1: note: template argument deduction/substitution failed:
../src/test-concepts.cpp:43:35: note: mismatched types ‘const char [N]’ and ‘fixed_string<...auto...>’
43 | const TYPE& get() const requires NameMatches<TARGET_NAME, TYPE::name__> const {
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/test-concepts.cpp:43: confused by earlier errors, bailing out
This is part of a project using templates that overloads a given function (get in this case), for which there is only one suitable candidate that fulfils the constraint. The value that the constraint operates on is a name - a string literal - not a const string variable, which is passed as a non-type parameter to an instantiation of the template: something like:
Container<Member<"fred", std::string>, Member<"bert", int>, Member<"alfie", bool>> some_values;
I want to be able to retrieve a value using something like
int result = some_values.get<"bert">();
I have had difficulty finding much information about the "<...auto...>" specialisation of the template. I presume this is an internal representation used by gcc for constant, non-type values.
The error messages point me to the lack of a suitable overloaded constructor for fixed_string. What should this be?

Answer 1

The problem I see is that fixed_string is a template class, not a class.
So, when you define a concept as
template<fixed_string TARGET_NAME, fixed_string THIS_NAME>
concept NameMatches = (TARGET_NAME.compare(THIS_NAME));
you have that fixed_string TARGET_NAME (fixed_string THIS_NAME also) doesn't works because fixed_string isn't a type. I mean: fixed_string<5> is a type, not fixed_string.
I know that you have a deduction guide that, given the literal string, deduce the template parameter for the fixed_string, but remain the problem that the concept should works with elements of different types (fixed_string of different lengths).
I suppose you can solve the problem with auto
template <auto TARGET_NAME, auto THIS_NAME>
concept NameMatches = (TARGET_NAME.compare(THIS_NAME));
but, when you declare Member, you have the same problem: fixed_string NAME doesn't works because fixed_sting (without a length) isn't a type.
template<fixed_string NAME, typename TYPE>
class Member
Unfortunately, if you use auto
template <auto NAME, typename TYPE>
class Member
defining a Member with a literal string argument (Member<"bert", int>, by example), nothing bring "bert" to a fixed_string.
Suggestion: what about a fixed_string without a template argument?