Correct syntax for variadic template

Question

I'm trying to convert some ascii characters into another one. However I've been unable due to inexperience with variadic template syntax.

template<typename T> void despecialize(const T& original, T& totransform, const int& charnum) { if (static_cast<int>(totransform) == charnum) { totransform = original; } } template<typename T> void despecialize(const T& original, T& totransform, const int& charnum, const int& charother...) { despecialize(original, totransform, charnum); despecialize(original, totransform, charother...); //C3546 there are no parameter packs available to expand } std::string removePortugueseSpecialLetter(std::string& data) { std::string transformed; for (auto& c : data) { despecialize('a', c, 176, 131, 132, 133, 134); transformed += c; } return transformed; }

What should be the correct syntax?

Once fixed, you might get rid of recursion with(C++17): (despecialize(original, totransform, charother), ...);. — Jarod42
– Jarod42, Commented Mar 20, 2020 at 15:05

Jarod42 · Accepted Answer · 2020-03-20 15:07:20Z

In order to have a variadic template, you need a variadic template parameter. That looks like

template <typename... VariadicTypeParater> // or template <some_type... VariadicNonTypeParater>

Applying that to your function would give you

template<typename T, typename... Chars> void despecialize(const T& original, T& totransform, const int& charnum, const Chars&... charother) { despecialize(original, totransform, charnum); despecialize(original, totransform, charother...); }

Although this will allow any type to be passed for Chars instead of just int. To constraint that you need to add SFINAE using enable_if_t non-type template parameter like

template <typename T, typename... Chars, std::enable_if_t<(std::is_same_v<Chars, int> && ...), bool> = true> void despecialize(const T& original, T& totransform, const int& charnum, const Chars& charother...) { despecialize(original, totransform, charnum); despecialize(original, totransform, charother...); }

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