In addition to Helpers for Generic C++ Code Iterating Over Sequences the C++17 Standard Library provides the following helpers for writing generic code:
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#include <iterator> // new helpers of C++17 are defined here template<class Container> void foo(Container& c) { // Generic way to obtain: auto data = std::data(c); // - data; auto empty = std::empty(c); // - flag of emptiness; auto size = std::size(c); // - size. } |
Function std::data() provides access to the underlying raw data of:
- containers that have a member function
data(); - instances of type type
std::initializer_list<T>; - built-in arrays.
Function std::empty() yields true if either container or an instance of type std::initializer_list<T> is empty. For built-in arrays the returned value is always false because built-int arrays cannot be zero-sized according to the standard.
Function std::size() returns a number of elements of:
- containers which are provides an iterator interface;
- built-in arrays.
Please note, that std::size() eliminates the need of use the conservative way to calculate the number of elements of built-in arrays, i.e.:
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#include <iterator> // for std::size() void foo() { int numbers[10]; // Conservative way to calculate the number of elements of built-in arrays constexpr auto size1 = sizeof(numbers) / sizeof(*numbers); // Modern way to calculate the number of elements of built-in arrays constexpr auto size2 = std::size(numbers); static_assert(size1 == size2); } |
Finally, please note, that being used with containers these functions just calls the corresponding member functions: data(), empty() and size(). Thus, for instance, std::size() does not work with instances of type std::forward_list<T> since this class doesn’t have the member function size() by design.
