What does ?: mean in C++?

Question

I found this piece of code:

std::string(avdInfo_getSystemImagePath(m_avd)
                                ?: avdInfo_getSystemInitImagePath(m_avd))

I only found information about the conditional operator: http://www.cplusplus.com/articles/1AUq5Di1/

That is, ? and : are separated. But what does it mean when they are together? both avdInfo_getSystemImagePath and avdInfo_getSystemInitImagePath return char*

Answer 1

It's a GCC extension.

x ?: y

is the same as

x ? x : y

Except that x will only be evaluated once.