[SOLVED] How to enable the C/C++ "Conditional with Omitted Operand" (aka Elvis Operator "?:") in GCC

Issue

In the talk Non-conforming C++ from CppCon2019 it is introduced the "href="https://gcc.gnu.org/onlinedocs/gcc/Conditionals.html#Conditionals" rel="nofollow noreferrer">Elvis Operator" in C++, which is a non-standard extension supported by many compilers.

It works by omitting the middle operand of an ?: expression:

std::shared_ptr<foo> read();
std::shared_ptr<foo> default_value();

auto read_or_default()
{
    return read() ?: default_value();
}

This is exactly the sample taken from slide 11 of the presentation.

However when I build it on GCC 7.4.0 on Ubuntu 18.04, or with GCC 8.2.0 on MinGW, using the -std=gnu++14 flag, I get this error:

error: lvalue required as unary '&' operand
     return read() ?: default_value();
                                    ^

In the talk it is said that this extension is present in GCC at least since version 4.1.2.

So what's wrong?

Solution

It is not the basic "conditionals with omitted operand" feature that fails. It's the combination with std::shared:ptr that's triggering a bug in GCC fixed in GCC 9.1.

The below code works since GCC 4.1.2:

int read();
int default_value();

int read_or_default()
{
    return read() ?: default_value();
}

Answered By - Pibben

Answer Checked By - Willingham (WPSolving Volunteer)