CODEWITHSUNDEEP
c++variadic-functions
user1324674
user1324674

Reputation: 384

How to use Variadic Function with strings, without using templates?

I have the following template function:

struct ms {
    template <typename... Args>
    void update(string& query, Args&... args);
};

template <typename... Args>
void ms::update(string& query, Args&... args)
{
    const int size = sizeof...(args);
    vector<string> vec = { args... };
    for (int i = 0; i < size; ++i) {
        cout << query << ": " << vec[i] << endl;
    }
}

However, I would like to eliminate the use of template and simply make this a member function that takes one or more string arguments. All of the documentation and examples on Variadic Functions I can find show use of character arrays and doing a while pointer != null to grab each value in the array.

If I do something like:

void update(string& query, string&... args);

how would I iterate over the args parameters, if there are any?

Upvotes: 1

Views: 953

Answers (1)

Yakk - Adam Nevraumont
Yakk - Adam Nevraumont

Reputation: 275385

This is an array_view<T>:

template<class T>
struct array_view;
template<class D>
struct array_view_base;

template<class T>
struct array_view_base<array_view<T>> {
  T* b=0; T* e=0;
  T* begin() const { return b; }
  T* end() const { return e; }
  T& operator[](std::size_t i)const{ return begin()[i]; }
  std::size_t size() const { return end()-begin(); }
  T& front() const { return *begin(); }
  T& back() const { return *(end()-1); }
  array_view<T> without_front( std::size_t N=1 ) const {
    N=(std::min)(N, size());
    return {begin()+N, end()};
  }
  array_view<T> without_back( std::size_t N=1 ) const {
    N=(std::min)(N, size());
    return {begin(), end()-N};
  }

  array_view_base( T* s, T* f ):b(s),e(f){}
  array_view_base( T* s, std::size_t sz ):array_view_base(s, s+sz) {}

  template<std::size_t N>
  array_view_base( T(&arr)[N] ):array_view_base(arr, N) {}
  template<class C,
    std::enable_if_t<!std::is_same<std::decay_t<C>, array_view<T>>{}, int> =0
  >
  array_view_base( C&& c ):array_view_base(c.data(), c.size()) {}
};

template<class T>
struct array_view:array_view_base<array_view<T>> {
  using array_view_base<array_view<T>>::array_view_base;
};
template<class T>
struct array_view<T const>:array_view_base<array_view<T const>> {
  using array_view_base<array_view<T const>>::array_view_base;
  array_view( std::initializer_list<T> il ):array_view( std::addressof(*il.begin()), il.size() ) {}
};

it works a bit like a gsl::span<T>. it is a contiguous range of T.

Unlike gsl::span<T>, array_view<T const> can be constructed from an initializer_list<T>.

With it your code should look like:

struct ms {
  void update(string const& query, array_view<string const> args);
};
void ms::update(string const& query, array_view<string const> args)
{
  for (int i = 0; i < args.size(); ++i) {
    cout << query << ": " << args[i] << endl;
  }
}

and you call it like:

ms{}.update( "Hello", {"one", "two", "three"} );

Live example.

As a worse example, simply take a std::vector<std::string>.

At point of call

ms{}.update( "Hello", {"one", "two", "three"} );

also works. Unlike my solution, this causes a memory allocation.

Upvotes: 1

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