In the following code, the X's move constructor (the constructor overloaded for rvalue reference) should be called for the copy-initialization `X y = static_cast<X&&>(x);', and two successive assertions should be passed. However, it seems that the implicitly defined copy constructor is called, and the second assertion fails. cryolite@thinblue:~/work/gcc_bug/move_on_tribially_copy_constructible$ g++-4.5.0 -v -save-temps -std=c++0x main.cpp Using built-in specs. COLLECT_GCC=g++-4.5.0 COLLECT_LTO_WRAPPER=/home/cryolite/local/libexec/gcc/i486-linux-gnu/4.5.0/lto-wrapper Target: i486-linux-gnu Configured with: ../gcc-4.5.0/configure --build=i486-linux-gnu --prefix=/home/cryolite/local --program-suffix=-4.5.0 --enable-version-specific-runtime-libs --enable-languages=c,c++ Thread model: posix gcc version 4.5.0 (GCC) COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++0x' '-shared-libgcc' '-mtune=i486' '-march=i486' /home/cryolite/local/libexec/gcc/i486-linux-gnu/4.5.0/cc1plus -E -quiet -v -D_GNU_SOURCE main.cpp -mtune=i486 -march=i486 -std=c++0x -fpch-preprocess -o main.ii ignoring duplicate directory "/home/cryolite/local/include" ignoring nonexistent directory "/home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/../../../../i486-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /home/cryolite/local/include /home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/include/c++ /home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/include/c++/i486-linux-gnu /home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/include/c++/backward /usr/local/include /home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/include /home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/include-fixed /usr/include End of search list. COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++0x' '-shared-libgcc' '-mtune=i486' '-march=i486' /home/cryolite/local/libexec/gcc/i486-linux-gnu/4.5.0/cc1plus -fpreprocessed main.ii -quiet -dumpbase main.cpp -mtune=i486 -march=i486 -auxbase main -std=c++0x -version -o main.s GNU C++ (GCC) version 4.5.0 (i486-linux-gnu) compiled by GNU C version 4.5.0, GMP version 4.3.1, MPFR version 2.4.1-p2, MPC version 0.8.1 GGC heuristics: --param ggc-min-expand=81 --param ggc-min-heapsize=95967 GNU C++ (GCC) version 4.5.0 (i486-linux-gnu) compiled by GNU C version 4.5.0, GMP version 4.3.1, MPFR version 2.4.1-p2, MPC version 0.8.1 GGC heuristics: --param ggc-min-expand=81 --param ggc-min-heapsize=95967 Compiler executable checksum: f3cfa96cf9dcd630b91f4110d3b8b2b3 COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++0x' '-shared-libgcc' '-mtune=i486' '-march=i486' as -V -Qy --32 -o main.o main.s GNU assembler version 2.20 (i486-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.20 COMPILER_PATH=/home/cryolite/local/libexec/gcc/i486-linux-gnu/4.5.0/:/home/cryolite/local/libexec/gcc/i486-linux-gnu/4.5.0/:/home/cryolite/local/libexec/gcc/i486-linux-gnu/:/home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/:/home/cryolite/local/lib/gcc/i486-linux-gnu/ LIBRARY_PATH=/home/cryolite/local/lib/:/home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/:/home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/../../../:/lib/:/usr/lib/ COLLECT_GCC_OPTIONS='-v' '-save-temps' '-std=c++0x' '-shared-libgcc' '-mtune=i486' '-march=i486' /home/cryolite/local/libexec/gcc/i486-linux-gnu/4.5.0/collect2 --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o /usr/lib/crti.o /home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/crtbegin.o -L/home/cryolite/local/lib -L/home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0 -L/home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/../../.. main.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/crtend.o /usr/lib/crtn.o cryolite@thinblue:~/work/gcc_bug/move_on_tribially_copy_constructible$ cat main.ii # 1 "main.cpp" # 1 "<built-in>" # 1 "<command-line>" # 1 "main.cpp" # 1 "/home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/include/c++/cassert" 1 3 # 43 "/home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/include/c++/cassert" 3 # 44 "/home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/include/c++/cassert" 3 # 1 "/usr/include/assert.h" 1 3 4 # 37 "/usr/include/assert.h" 3 4 # 1 "/usr/include/features.h" 1 3 4 # 313 "/usr/include/features.h" 3 4 # 1 "/usr/include/bits/predefs.h" 1 3 4 # 314 "/usr/include/features.h" 2 3 4 # 346 "/usr/include/features.h" 3 4 # 1 "/usr/include/sys/cdefs.h" 1 3 4 # 353 "/usr/include/sys/cdefs.h" 3 4 # 1 "/usr/include/bits/wordsize.h" 1 3 4 # 354 "/usr/include/sys/cdefs.h" 2 3 4 # 347 "/usr/include/features.h" 2 3 4 # 378 "/usr/include/features.h" 3 4 # 1 "/usr/include/gnu/stubs.h" 1 3 4 # 1 "/usr/include/bits/wordsize.h" 1 3 4 # 5 "/usr/include/gnu/stubs.h" 2 3 4 # 1 "/usr/include/gnu/stubs-32.h" 1 3 4 # 8 "/usr/include/gnu/stubs.h" 2 3 4 # 379 "/usr/include/features.h" 2 3 4 # 38 "/usr/include/assert.h" 2 3 4 # 68 "/usr/include/assert.h" 3 4 extern "C" { extern void __assert_fail (__const char *__assertion, __const char *__file, unsigned int __line, __const char *__function) throw () __attribute__ ((__noreturn__)); extern void __assert_perror_fail (int __errnum, __const char *__file, unsigned int __line, __const char *__function) throw () __attribute__ ((__noreturn__)); extern void __assert (const char *__assertion, const char *__file, int __line) throw () __attribute__ ((__noreturn__)); } # 45 "/home/cryolite/local/lib/gcc/i486-linux-gnu/4.5.0/include/c++/cassert" 2 3 # 2 "main.cpp" 2 struct X { X(int i) : i_(i) {} X(X &&x) : i_(x.i_) { x.i_ = 0; } int i_; }; int main() { X x(42); X y = static_cast<X&&>(x); ((y.i_ == 42) ? static_cast<void> (0) : __assert_fail ("y.i_ == 42", "main.cpp", 14, __PRETTY_FUNCTION__)); ((x.i_ == 0) ? static_cast<void> (0) : __assert_fail ("x.i_ == 0", "main.cpp", 15, __PRETTY_FUNCTION__)); } cryolite@thinblue:~/work/gcc_bug/move_on_tribially_copy_constructible$ ./a.out a.out: main.cpp:15: int main(): Assertion `x.i_ == 0' failed. Aborted
The rules say that for copy-initialization where the source type (X&&) is not the same as the destination type (X) a temporary is created. That temporary is copy-constructed from x, then y is move-constructed from the temporary. To avoid the temporary use direct-initialization; Y y( static_cast<X&&>(x) );
(In reply to comment #1) > Y y( static_cast<X&&>(x) ); ^ Oops. I meant X not Y
This is invalid then.
it might be a valid enhancement request, as I think the temporary is eligible for copy-elision (Jason?) but the compiler isn't required to elide it, so it is wrong to assert that a temporary won't be created
To be safe, let's reopen the bug. For the record, this works: #include <cassert> struct X { X(int i) : i_(i) {} X(X&& x) : i_(x.i_) { x.i_ = 0; } int i_; X(const X&) = delete; }; int main() { X x(42); X y = static_cast<X&&>(x); assert( y.i_ == 42 ); assert( x.i_ == 0 ); }
(In reply to comment #1) > The rules say that for copy-initialization where the source type (X&&) is not > the same as the destination type (X) a temporary is created. But the source type is X; there are no expressions of reference type. The problem is not the overload resolution, but that build_over_call looks at TYPE_HAS_COMPLEX_INIT_REF even though we're dealing with the move ctor, not the copy ctor.
Subject: Bug 44158 Author: jason Date: Mon May 17 19:53:45 2010 New Revision: 159508 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=159508 Log: PR c++/44158 * call.c (build_over_call): Don't do bitwise copy for move ctor. Modified: trunk/gcc/cp/ChangeLog trunk/gcc/cp/call.c trunk/gcc/testsuite/ChangeLog trunk/gcc/testsuite/g++.dg/cpp0x/rv-trivial-bug.C
Subject: Bug 44158 Author: jason Date: Wed May 19 15:49:01 2010 New Revision: 159577 URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=159577 Log: PR c++/44158 * call.c (build_over_call): Don't do bitwise copy for move ctor. Modified: branches/gcc-4_5-branch/gcc/cp/ChangeLog branches/gcc-4_5-branch/gcc/cp/call.c branches/gcc-4_5-branch/gcc/testsuite/ChangeLog branches/gcc-4_5-branch/gcc/testsuite/g++.dg/cpp0x/rv-trivial-bug.C
Fixed for 4.5.1.