Templated is_in() function (check if array contains string) in C-CodePudding

I would like to do the following:

std::string b = "b";
is_in("a", { "a", "b", "c" });
is_in("d", { "a", "b", "c" });
is_in(b, { "a", "b", "c" }); // fails
is_in(b, std::array{ "a", "b", "c" });

using the templates

template<typename Element, typename Container>
bool is_in(const Element& e, const Container& c)
{
    // https://stackoverflow.com/questions/20303821/how-to-check-if-string-is-in-array-of-strings
    return std::find(std::begin(c), std::end(c), e) != std::end(c);
}

template<typename Element>
bool is_in(Element e, std::initializer_list<Element> l)
{
    // return std::find(std::begin(l), std::end(l), e) != std::end(l);
    return is_in<Element, std::initializer_list<Element>>(e, l);
}

but I get the following error (using GCC 9.3.0):

no matching function for call to ‘is_in(std::string&, <brace-enclosed initializer list>)’

Any big brain template fellas out there got suggestions?

CodePudding user response：

For is_in(b, { "a", "b", "c" });, template parameter Element is deduced as std::string on the 1st argument b, and deduced as const char* on the 2nd argument { "a", "b", "c" }; they don't match.

You can give two template parameters for is_in, e.g.

template<typename E1, typename E2>
bool is_in(E1 e, std::initializer_list<E2> l)
{
    // return std::find(std::begin(l), std::end(l), e) != std::end(l);
    return is_in<E1, std::initializer_list<E2>>(e, l);
}

Or use std::type_identity (since C 20; and it's quite easy to write one for pre-C 20) to exclude the 2nd function parameter from type deduction.

template<typename Element>
bool is_in(Element e, std::initializer_list<std::type_identity_t<Element>> l)
{
    // return std::find(std::begin(l), std::end(l), e) != std::end(l);
    return is_in<Element, std::initializer_list<Element>>(e, l);
}