http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58063
Daniel Krügler <daniel.kruegler at googlemail dot com> changed:
What |Removed |Added
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CC| |daniel.kruegler@googlemail.
| |com
--- Comment #1 from Daniel Krügler <daniel.kruegler at googlemail dot com> ---
The behavior looks indeed odd to me (and I can confirm it for gcc 4.9 as well).
I suspect at the moment that it is somehow related to the very specific
definition of std::cout, because when I try to mimic the problem for a model
type like the following, I cannot produce this effect:
//---------------------------------------------
#include <iostream>
struct my_ostream
{
my_ostream(){}
virtual ~my_ostream() {}
operator void*() const { return const_cast<my_ostream*>(this); }
bool operator!() const { return false; }
private:
my_ostream(const my_ostream&);
my_ostream& operator=(const my_ostream&);
} my_cout;
my_ostream& operator<<(my_ostream& os, const char* s)
{
std::cout << s;
return os;
}
void f(bool x = !(std::cout << "hi!\n")) {
std::cout << x << '\n';
}
void f2(bool x = !(my_cout << "hi!\n")) {
std::cout << x << '\n';
}
int main() {
f();
std::cout << "------------------\n";
f2();
}
//---------------------------------------------
gives the output:
<quote>
hi!
hi!
0
------------------
hi!
0
</quote>
Looking at the generate assembly (mingw 64 but), I see the following relevant
lines:
0x004017AD lea 0x7a86d(%rip),%rdx # 0x47c021
<std::piecewise_construct+1>
0x004017B4 mov 0x7e4a5(%rip),%rcx # 0x47fc60 <.refptr._ZSt4cout>
0x004017BB callq 0x4624c0 <std::operator<< <std::char_traits<char>
>(std::basic_ostream<char, std::char_traits<char> >&, char const*)>
0x004017C0 mov %rax,%rbx
0x004017C3 lea 0x7a857(%rip),%rdx # 0x47c021
<std::piecewise_construct+1>
0x004017CA mov 0x7e48f(%rip),%rcx # 0x47fc60 <.refptr._ZSt4cout>
0x004017D1 callq 0x4624c0 <std::operator<< <std::char_traits<char>
>(std::basic_ostream<char, std::char_traits<char> >&, char const*)>
0x004017D6 mov (%rax),%rax
0x004017D9 sub $0x18,%rax
0x004017DD mov (%rax),%rax
0x004017E0 add %rbx,%rax
0x004017E3 mov %rax,%rcx
0x004017E6 callq 0x433f60 <std::basic_ios<char, std::char_traits<char>
>::operator!() const>
0x004017EB movzbl %al,%eax
0x004017EE mov %eax,%ecx
