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
<paulo.cesar.pereira.de.andrade@xxxxxxxxx>:
Is this expected to not compile with -fno-implicit-templates?
---%<---
$ cat test.cc
#include <string>
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<char, std::char_traits<char>,
std::allocator<char> > std::operator+<char, std::char_traits<char>,
std::allocator<char> >(char const*, std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > const&)'
test.cc:(.text+0xd9): undefined reference to
`std::__cxx11::basic_string<char, std::char_traits<char>,
std::allocator<char> > std::operator+<char, std::char_traits<char>,
std::allocator<char> >(char const*, std::__cxx11::basic_string<char,
std::char_traits<char>, std::allocator<char> > 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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