Re: Quick C++ question for C++ experts :)
On 02/04/15 10:34 +0100, Ian Malone wrote: Thanks. Hadn't occurred to me the + operator here was a template as I'd never had to deal with basic_string. Still a bit puzzled as cplusplus.com says string is an instantiation of basic_string while cppreference.com says it's a typedef (which I guess doesn't count as explicit instantiation). cplusplus.com is notoriously inaccurate. It probably means string is a specialization of basic_string. This is going very off-topic for this list. -- devel mailing list devel@lists.fedoraproject.org https://admin.fedoraproject.org/mailman/listinfo/devel Fedora Code of Conduct: http://fedoraproject.org/code-of-conduct
Re: Quick C++ question for C++ experts :)
On 1 April 2015 at 16:09, Jonathan Wakely wrote: > On 01/04/15 15:16 +0100, Ian Malone wrote: >> >> Do you mind clarifying? I thought should provide that >> http://www.cplusplus.com/reference/string/string/operator+/ or is that >> what fno-implicit-templates is turning off? > > > Of course string provides it, but it's a template, so it needs to be > instantiated. The GCC manual documents -fno-implicit-templates like > so: > > Never emit code for non-inline templates that are instantiated > implicitly (i.e. by use); only emit code for explicit > instantiations. > > The invalid program in the OP uses operator+(), which would normally > instantiate the function template implicitly. But if you use > -fno-implicit-templates you are promising the compiler you will > provide explicit instantiations. The program above does not provide > them, so it is broken. > Thanks. Hadn't occurred to me the + operator here was a template as I'd never had to deal with basic_string. Still a bit puzzled as cplusplus.com says string is an instantiation of basic_string while cppreference.com says it's a typedef (which I guess doesn't count as explicit instantiation). -- imalone http://ibmalone.blogspot.co.uk -- devel mailing list devel@lists.fedoraproject.org https://admin.fedoraproject.org/mailman/listinfo/devel Fedora Code of Conduct: http://fedoraproject.org/code-of-conduct
Re: Quick C++ question for C++ experts :)
On 01/04/15 15:16 +0100, Ian Malone wrote: Do you mind clarifying? I thought should provide that http://www.cplusplus.com/reference/string/string/operator+/ or is that what fno-implicit-templates is turning off? Of course string provides it, but it's a template, so it needs to be instantiated. The GCC manual documents -fno-implicit-templates like so: Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations. The invalid program in the OP uses operator+(), which would normally instantiate the function template implicitly. But if you use -fno-implicit-templates you are promising the compiler you will provide explicit instantiations. The program above does not provide them, so it is broken. "If you lie to the compiler, it will get its revenge." -- Henry Spencer -- devel mailing list devel@lists.fedoraproject.org https://admin.fedoraproject.org/mailman/listinfo/devel Fedora Code of Conduct: http://fedoraproject.org/code-of-conduct
Re: Quick C++ question for C++ experts :)
On 1 April 2015 at 14:32, Jonathan Wakely wrote:
> On 28/03/15 16:45 -0300, Paulo César Pereira de Andrade wrote:
>>
>> 2015-03-28 16:06 GMT-03:00 Paulo César Pereira de Andrade
>> :
>>>
>>> Is this expected to not compile with -fno-implicit-templates?
>>>
>>> ---%<---
>>> $ cat test.cc
>>> #include
>>> std::string test(int i)
>>> {
>>> std::string t;
>>> std::string s = "(";
>>> t = "";
>>> for (int r = i; r; r>>=1) {
>>> if (r & 1)
>>> t = "1" + t;
>>> else
>>> t = "0" + t;
>>> }
>>> s += t;
>>> s += ")";
>>> return s;
>>> }
>>>
>>> int
>>> main(int argc, char *argv[])
>>> {
>>> std::string s = test(16);
>>> return 0;
>>> }
>>>
> As I stated in the bug report, the code is invalid, but used to work
> due to an undocumented "accidental" feature of libstdc++.so which
> happens to provide instantiations of the required operator+().
>
> If you use -fno-implicit-templates then it is your responsibility to
> instantiate all the templates you use. The program uses operator+()
> without instantiating it, so the program is wrong. (It also uses a
> number of other templates without instantiating them, which is also
> wrong).
>
Do you mind clarifying? I thought should provide that
http://www.cplusplus.com/reference/string/string/operator+/ or is that
what fno-implicit-templates is turning off?
--
imalone
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Re: Quick C++ question for C++ experts :)
On 28/03/15 16:45 -0300, Paulo César Pereira de Andrade wrote:
2015-03-28 16:06 GMT-03:00 Paulo César Pereira de Andrade
:
Is this expected to not compile with -fno-implicit-templates?
---%<---
$ cat test.cc
#include
std::string test(int i)
{
std::string t;
std::string s = "(";
t = "";
for (int r = i; r; r>>=1) {
if (r & 1)
t = "1" + t;
else
t = "0" + t;
}
s += t;
s += ")";
return s;
}
int
main(int argc, char *argv[])
{
std::string s = test(16);
return 0;
}
$ g++ -fno-implicit-templates test.cc
/tmp/ccai7t5T.o: In function `test(int)':
test.cc:(.text+0x9d): undefined reference to
`std::__cxx11::basic_string,
std::allocator > std::operator+,
std::allocator >(char const*, std::__cxx11::basic_string, std::allocator > const&)'
test.cc:(.text+0xd9): undefined reference to
`std::__cxx11::basic_string,
std::allocator > std::operator+,
std::allocator >(char const*, std::__cxx11::basic_string, std::allocator > const&)'
collect2: error: ld returned 1 exit status
---%<---
As I stated in the bug report, the code is invalid, but used to work
due to an undocumented "accidental" feature of libstdc++.so which
happens to provide instantiations of the required operator+().
If you use -fno-implicit-templates then it is your responsibility to
instantiate all the templates you use. The program uses operator+()
without instantiating it, so the program is wrong. (It also uses a
number of other templates without instantiating them, which is also
wrong).
I will open a gcc bug report. It must be a bug, because if using
a temporary to convert "1" or "0" to a std::string it works. Or,
explicit converting, e.g.:
-t = "1" + t;
+t = std::string("1") + t;
This just happens to work because it uses a different overload of
operator+ that is defined inline, so the compiler inlines the code and
doesn't require an instantiation.
Using -fno-implicit-templates in the package is probably a bug IMHO.
The justification in the package's readme is weak.
However, in order not to break such buggy programs which worked (by
accident) previously I have added the relevant instantiations to
libstdc++.
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Re: Quick C++ question for C++ experts :)
2015-03-28 16:06 GMT-03:00 Paulo César Pereira de Andrade
:
> Is this expected to not compile with -fno-implicit-templates?
>
> ---%<---
> $ cat test.cc
> #include
> std::string test(int i)
> {
> std::string t;
> std::string s = "(";
> t = "";
> for (int r = i; r; r>>=1) {
> if (r & 1)
> t = "1" + t;
> else
> t = "0" + t;
> }
> s += t;
> s += ")";
> return s;
> }
>
> int
> main(int argc, char *argv[])
> {
> std::string s = test(16);
> return 0;
> }
>
> $ g++ -fno-implicit-templates test.cc
> /tmp/ccai7t5T.o: In function `test(int)':
> test.cc:(.text+0x9d): undefined reference to
> `std::__cxx11::basic_string,
> std::allocator > std::operator+,
> std::allocator >(char const*, std::__cxx11::basic_string std::char_traits, std::allocator > const&)'
> test.cc:(.text+0xd9): undefined reference to
> `std::__cxx11::basic_string,
> std::allocator > std::operator+,
> std::allocator >(char const*, std::__cxx11::basic_string std::char_traits, std::allocator > const&)'
> collect2: error: ld returned 1 exit status
> ---%<---
I will open a gcc bug report. It must be a bug, because if using
a temporary to convert "1" or "0" to a std::string it works. Or,
explicit converting, e.g.:
-t = "1" + t;
+t = std::string("1") + t;
My C++ foo is not that great, thus asking before opening a
bug report.
Thanks,
Paulo
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Quick C++ question for C++ experts :)
Is this expected to not compile with -fno-implicit-templates?
---%<---
$ cat test.cc
#include
std::string test(int i)
{
std::string t;
std::string s = "(";
t = "";
for (int r = i; r; r>>=1) {
if (r & 1)
t = "1" + t;
else
t = "0" + t;
}
s += t;
s += ")";
return s;
}
int
main(int argc, char *argv[])
{
std::string s = test(16);
return 0;
}
$ g++ -fno-implicit-templates test.cc
/tmp/ccai7t5T.o: In function `test(int)':
test.cc:(.text+0x9d): undefined reference to
`std::__cxx11::basic_string,
std::allocator > std::operator+,
std::allocator >(char const*, std::__cxx11::basic_string, std::allocator > const&)'
test.cc:(.text+0xd9): undefined reference to
`std::__cxx11::basic_string,
std::allocator > std::operator+,
std::allocator >(char const*, std::__cxx11::basic_string, std::allocator > const&)'
collect2: error: ld returned 1 exit status
---%<---
I am trying to fix a package, but it is documented to
require -fno-implicit-templates and instantiate templates
in one of the sources, but it instantiates templates for
its own types.
Thanks,
Paulo
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