https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61892
Bug ID: 61892
Summary: RVO not occurs with constructor with universal
reference arguments
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: tower120 at gmail dot com
I'm not sure that this is bug. But this is strange behavior, if you ask me.
LIVE: http://coliru.stacked-crooked.com/a/d11d50f611ed0cee
In the following code:
#include <iostream>
template<class ...ArgsIn>
struct C {
template<class ...Args>
C(Args&& ... args) {std::cout << "Ctr\n";} // elision occurs without
&&
~C(){std::cout << "Dstr\n";}
//C(C&&) { std::cout << "A move was made.\n"; } // with this and universal
references in ctr, rvo occurs
};
template<class ...Args>
auto f(Args ... args) {
int i = 1;
std::cout << "call "<<std::endl;
return C<>(i, i, i);
}
int main() {
std::cout << "Hello World!\n";
auto obj = f();
}
OUTPUT:
g++ -std=c++1y -O3 -Winline -Wextra -pthread -pedantic-errors main.cpp -lm &&
./a.out
Hello World!
call
Ctr
Ctr
Dstr
Ctr
Dstr
Dstr
=========================================================
1) If I have constructor with "universal" references I need to have ANY move
constructor, so rvo happened.
2) Moreover, when rvo not happens, it construct object with a wrong number of
arguments:
http://coliru.stacked-crooked.com/a/e3ce8882c68dbef2
=========================================================
P.S.
http://stackoverflow.com/questions/24925137/c-universal-reference-in-constructor-and-return-value-optimization-rvo/24925451#comment38729738_24925137
someone claims that with gcc 4.8.3 this problem not exists. I don't have that
compiler at hand, so I can't check that.
