I'm walking through the examples of the help page of the awesome boost::hana library and am not able to get the introspection example working correctly.
This code is intended to check at compile time whether an object has a specific member function or not and then uses this member function or does something default.
So I declared these two types:
struct WithoutToString
{ };
struct WithToString
{
std::string toString()
{
return "implements toString()";
}
};
This is the 1st version of the check using hana::is_valid
:
auto has_toString = hana::is_valid([] (auto&& obj) -> decltype(obj.toString()) { });
template <typename T>
std::string optionalToString1(T const& obj)
{
return hana::if_(has_toString(obj),
[] (auto& x) { return x.toString(); },
[] (auto& x) { return "toString not defined"; }
)(obj);
}
This is the 2nd version of the check using hana::sfinae
:
template <typename T>
std::string optionalToString2(T const& obj)
{
auto maybeToString = hana::sfinae([](auto&& x) -> decltype(x.toString())
{
return x.toString();
});
return maybeToString(obj).value_or("toString not defined");
}
Using both versions like this...
int main()
{
WithToString obj;
std::cout << optionalToString1(obj);
std::cout << optionalToString2(obj);
return 0;
}
...always shows "toString not defined" instead of "implements toString()".
Note: checking obj
with static_assert(has_toString(obj), "Does not implement toString().");
shows the right behaviour.
Am I missing something? Or is it a compiler (clang 5.0.1) or library (boost 1.66) issue?
Thank you.
Your optionalToStringX
functions take T const&
. The toString
isn't a const-qualified member function, so it isn't applicable.
#include <boost/hana.hpp>
#include <string>
namespace hana = boost::hana;
struct WithoutToString { };
struct WithToString { std::string toString() const { return "implements toString()"; } };
namespace v1 {
//This is the 1st version of the check using hana::is_valid:
auto has_toString = hana::is_valid([] (auto&& obj) -> decltype(obj.toString()) { });
template <typename T>
std::string optionalToString1(T const& obj)
{
return hana::if_(has_toString(obj),
[] (auto&& x) { return std::forward<decltype(x)>(x).toString(); },
[] (auto&&) { return "toString not defined"; }
)(obj);
}
}
namespace v2 {
//This is the 2nd version of the check using hana::sfinae:
template <typename T>
std::string optionalToString2(T const& obj)
{
auto maybeToString = hana::sfinae([](auto&& x) -> decltype(x.toString())
{
return x.toString();
});
return maybeToString(obj).value_or("toString not defined");
}
}
#include <iostream>
int main()
{
WithToString with;
WithoutToString without;
std::cout << std::boolalpha << v1::has_toString(without) << std::endl;
std::cout << std::boolalpha << v1::has_toString(with) << std::endl;
std::cout << v1::optionalToString1(without) << std::endl;
std::cout << v1::optionalToString1(with) << std::endl;
std::cout << v2::optionalToString2(without) << std::endl;
std::cout << v2::optionalToString2(with) << std::endl;
}
Prints
true
false
implements toString()
toString not defined
implements toString()
toString not defined
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