g++ 7.3.0 & 8.2.0: weird std::string_view behavior when using in-line if-else

g++ 7.3.0 & 8.2.0: weird std::string_view behavior when using in-line if-else

[Tmplt]

Hello,

today some code of mine using std::string_view broke out of seemingly
nowhere; the following program is a minimal recreation of the odd
behavior I'm seeing:

	$ cat string_view_test.cpp
	#include <string_view>
	#include <string>
	#include <iostream>

	int main()
	{
		std::string str = "Victory of Eagles";
		std::string_view sv = true ? str : "";
		std::cout << sv << "\n";
	}

The above program, when compiled with g++ v7.3.0 or v8.2.0 via

	$ g++ -std=c++17 string_view_test.cpp

prints "of Eagles" when run where "Victory of Eagles" is expected.
Omitting the in-lined if-else, and instead just

	std::string_view sv = str;

solves the issue, but I have no idea why. What's up with this behavior?

Cheers.

Re: g++ 7.3.0 & 8.2.0: weird std::string_view behavior when using in-line if-else

[Marc Glisse]

On Fri, 5 Oct 2018, Tmplt wrote:

> today some code of mine using std::string_view broke out of seemingly
> nowhere; the following program is a minimal recreation of the odd
> behavior I'm seeing:
>
> 	$ cat string_view_test.cpp
> 	#include <string_view>
> 	#include <string>
> 	#include <iostream>
>
> 	int main()
> 	{
> 		std::string str = "Victory of Eagles";
> 		std::string_view sv = true ? str : "";

What do you think is the type of true?str:"" ? It is going to be 
std::string (prvalue), you would need both arms to be the same type to get 
an lvalue. Your string_view thus refers to a temporary copy of str, which 
gets destructed at the end of the line.

> 		std::cout << sv << "\n";
> 	}
>
> The above program, when compiled with g++ v7.3.0 or v8.2.0 via
>
> 	$ g++ -std=c++17 string_view_test.cpp
>
> prints "of Eagles" when run where "Victory of Eagles" is expected.
> Omitting the in-lined if-else, and instead just
>
> 	std::string_view sv = str;
>
> solves the issue, but I have no idea why. What's up with this behavior?

-- 
Marc Glisse