How to let a template function accept anything you can construct some basic_string_view out

Question

I'm trying to write a simple template function which accepts all possible basic_string_view but i always get the compiler error "no matching overloaded function found".

I don't know the reason; explicitly converting to string_view by the caller works but i'd like to avoid that; or is intentionally made hard?

Are there deduction guidelines which prevent this?

And is there a easy way to implement this as a template?

Here (and on godbolt) is what i tried:

#include <string_view>
#include <string>

template <typename CharT, typename Traits> void func(std::basic_string_view<CharT, Traits> value)
//template <typename CharT> void func(std::basic_string_view<CharT> value)
//void func(std::string_view value)
{}

int main() {
    std::string s;
    std::string_view sv(s);
    char const cs[] = "";
    std::string_view csv(cs);

    std::wstring ws;
    std::wstring_view wsv(ws);
    wchar_t const wcs[] = L"";
    std::wstring_view wcsv(wcs);

    func(s);
    func(sv);
    func(cs);
    func(csv);

    func(ws);
    func(wsv);
    func(wcs);
    func(wcsv);
}

Here are the errors msvc, clang and gcc show:

error C2672: 'func': no matching overloaded function foundx64 msvc v19.latest #3
error C2783: 'void func(T)': could not deduce template argument for '<unnamed-symbol>'x64 msvc v19.latest #3
error: no matching function for call to 'func'x86-64 clang (trunk) #1
error: no matching function for call to 'func(std::string&)'x86-64 gcc (trunk) #2

EDIT:

Demo of a blend of Yakks c++20 answer with the addition of Jonathans raw character pointer support.

Answer 1

It's also possible to do this using just C++20 by checking whether the argument passed to the function is a range. The range can then be converted to a basic_string_view using its constructor overload that takes iterators.

I've also added an overload that can deal with char pointers since those aren't ranges.

#include <string_view>
#include <string>
#include <type_traits>
#include <ranges>


template <typename CharT, typename Traits>
void func(std::basic_string_view<CharT, Traits> value) {
    // ...
}

template<typename S> requires std::ranges::contiguous_range<S>
void func(const S& s) {
    func(std::basic_string_view{std::ranges::begin(s), std::ranges::end(s)});
}

template<typename S> requires (!std::ranges::range<S> && requires (S s) { std::basic_string_view<std::remove_cvref_t<decltype(s[0])>>(s); })
void func(const S& s) {
    func(std::basic_string_view<std::remove_cvref_t<decltype(s[0])>>(s));
}

Answer 2

It is in general impossible for the compiler to deduce the template arguments so a conversion can succeed. That is not how C++ templates were designed.

Given this:

char const cs[] = "";
func(cs);

The compiler would have to be able to answer

"What template arguments CharT, Traits must I deduce so there is an appropriate implicit conversion from const char[1] to std::basic_string_view<CharT,Traits>?"

Of course it cannot do that, at least not without somehow iterating over all types (which is infinite, countable set).

Unfortunately there are no deduction guides for template functions.

Answer 3

basic_string_view has a range version of CTAD in C++23, so in C++23, you can use the requires clause to constrain basic_string_view{s} to be well-formed, and deduce its type by borrowing the CTAD of basic_string_view in the function body

#include <string_view>

template<typename StrLike>
  requires requires (const StrLike& s) 
  { std::basic_string_view{s}; }
void func(const StrLike& s) {
  auto sv = std::basic_string_view{s};
  // use sv
}

Demo

Answer 4

@康桓瑋's answer is awesome. But you should avoid unnamed concepts. So:

#include <string_view>

template<typename ZLike, template<class...>class Z>
concept can_become = requires (const ZLike& s) {
  { Z{s} };
};

void func(const can_become<std::basic_string_view> auto& s) {
  auto sv = std::basic_string_view{s};
  // use sv
}

Live example.

This can also be solved using c++17 and earlier techniques:

template<class StrLike, std::enable_if_t<can_become_v<StrLike, std::basic_string_view>, bool> = true>
void func2(const StrLike& s) {
  auto sv = std::basic_string_view{s};
  // use sv
}

without concepts.

It isn't until c++23 that std::basic_string_view had a CTAD that was SFINAE friendly. Without it, you have to know the exact kind of basic_string_view your incoming type can convert to, and then use that specific type. operator basic_string_view<X,Y>() can only be found if you have the specific type you want to cast to available.

You can work around this by creating the range based constructors of basic_string_view, and manually deducing the type of the character (and possibly traits) involved.

A pure c++20 solution that doesn't support raw char pointers:

template<typename ZLike, template<class...>class Z>
concept can_range_become = requires (const ZLike& s) {
  { Z{std::ranges::begin(s), std::ranges::end(s)} };
};
template<typename T>
concept stringlike = can_range_become<T, std::basic_string_view> && std::ranges::contiguous_range<T>;

void func(const stringlike auto& s) {
  auto sv = std::basic_string_view{std::ranges::begin(s), std::ranges::end(s)};
  // use sv
}

Live example.