Howto use a fold expression with a specific type?

Question

I've a function to check if a std::string contains a substring. I pass the strings around as std::string_view, such that no copy takes place.

bool containsSubstr(std::string_view str, std::string_view substr)
{
    return str.find(substr) != std::string::npos;
}

I now want to make a function, using the new C++17 fold expressions, to check if a string contains several substrings. Again, I want to pass them by std::string_views.

How can I do that?

template<typename... Substrs>
bool containsAllSubstr(std::string_view str, Substrs... substrs)
{
    return  (containsSubstr(str, substrs) && ...);
}

As far as I know, the above version will take the substring's as the type with which they came in. So a std::string would be copied. How can I fix the type to std::string_view? Something like:

template<> // does not compile
bool containsAllSubstr(std::string_view str, std::string_view... substrs)
{
    return  (containsSubstr(str, substrs) && ...);
}

Barry · Accepted Answer · 2018-03-19T13:17:02.737

10

You can't have a function parameter pack of a specific type. But this is fine (once you add const&):

template <typename... Substrs>
bool containsAllSubstr(std::string_view str, Substrs const&... substrs)
{
    return  (containsSubstr(str, substrs) && ...);
}

That won't make any copies, and just won't compile if you pass in something not convertible to string_view. If you want to make it SFINAE-friendly, could just add a condition:

template <typename... Substrs,
    std::enable_if_t<(std::is_convertible_v<Substrs const&, std::string_view> && ...), int> = 0>
bool containsAllSubstr(std::string_view str, Substrs const&... substrs)
{
    return  (containsSubstr(str, substrs) && ...);
}

Or, if you're open to a slight change in syntax, you could take an array:

template <size_t N>
bool containsAllSubstr(std::string_view str, std::string_view (&substrs)[N]);

but then you don't actually have a pack immediately. But then, you could just write a loop. Or if you don't need compile-time size at all:

bool containsAllSubstr(std::string_view str, std::initializer_list<std::string_view> substrs);

edited Mar 19 '18 at 13:17

answered Mar 19 '18 at 13:08

Barry

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`std::initializer_list< std::string_view >`? – Yakk - Adam Nevraumont Mar 19 '18 at 13:10
@Yakk Or that, if you don't actually need the compile-time size (which... you probably don't?) – Barry Mar 19 '18 at 13:13
In my code base, I'd actually use a `my_ns::span< std::string_view >`, and my `span` has a `std::initializer_list< remove_const >` ctor. It works well, and it means I can pass in arrays, `{}` enclosed list (including empty), vectors, or whatever. – Yakk - Adam Nevraumont Mar 19 '18 at 13:49
@Yakk Does that... work? Wouldn't the `initializer_list` get destroyed and leave you with dangling pointers? – Barry Mar 19 '18 at 14:22
It lasts until the end of the full expression the initializer list is created in. Which is longer than the function call. Might need a `const&` in there to ensure it works. – Yakk - Adam Nevraumont Mar 19 '18 at 14:24
@Yakk Yeah, I think the `const&` is necessary. – Barry Mar 19 '18 at 14:28

Howto use a fold expression with a specific type?

1 Answers1