Is it legal to construct std::string_view from empty vector<char>?

user_700825 0
Viewed 154

I have read some questions and documentation, and I guess answer is yes, since string_view will never touch the pointed to stuff, but I am a still bit confused if this is legal:

std::vector<char> v;
std::string_view sv(v.data(), v.size());

note:

  1. I know that constructor that just uses v.data() and does strlen will crash, I am asking about this particular way of constructing std::string_view.
  2. I know that constructed string_view is "evil", e.g. doing std::cout << sv.data() is UB, but I think std::cout << sv; should work

Full example:

#include <vector>
#include <iostream>
#include <string_view>

std::vector<char> v;
auto get_sv() {
std::string_view sv(v.data(), v.size());
return sv;
} 

int main() {
    std::cout << "|" << get_sv() << "|" << std::endl;

}
string-view c++20 c++
1 Answers

This looks perfectly legal to me.

According to basic_string_view( const CharT* s, size_type count );

  1. Constructs a view of the first count characters of the character array starting with the element pointed by s.

and std::vector::data() states:

Returns pointer to the underlying array serving as element storage. The pointer is such that range [data(); data() + size()) is always a valid range, even if the container is empty (data() is not dereferenceable in that case).

and

Notes
If size() is 0, data() may or may not return a null pointer.

So depending on the implementation, the underlying char array exists at the time of printing, or the string_view was built with a nullptr. In either case it is a valid range.

user_17415420
Related