https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81852
Bug ID: 81852
Summary: Feature request: __cpp_threadsafe_static_init
Product: gcc
Version: 8.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: daniel.kruegler at googlemail dot com
Target Milestone: ---
Since the recent update of "Feature-testing recommendations for C++",
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0096r4.html#recs.cpp11
the feature macro "__cpp_threadsafe_static_init" has been recommended for C++
language level c++11 on. This is extremely useful to have for code that has to
program defensively against the possibility that thread-safe static locale
initialization has been deactivated (via -fno-threadsafe-statics). With that
feature macro available, portable code can be written that always provides a
thread-safe static initialization, for example by means of std::call_once as
the following excerpt demonstrates:
Thingy& get_thing()
{
#ifndef __cpp_threadsafe_static_init
[...]
static std::once_flag flag;
std::call_once(flag, init::get);
return init::get();
#else
static Thingy result;
return result;
#endif
}
Current clang HEAD 6.0.0 (https://github.com/llvm-mirror/clang.git 43) has
already provided support for this.
A test-case would be that given the compiler flags
$ g++ prog.cc -Wall -Wextra -std=c++11 -pedantic
the following would be well-formed:
//<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
#ifndef __cpp_threadsafe_static_init
# error "__cpp_threadsafe_static_init not defined"
#endif
int main()
{
}
//<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
while given the compiler flags
$ g++ prog.cc -Wall -Wextra -std=c++11 -pedantic "-fno-threadsafe-statics"
the same code should be ill-formed:
prog.cc:2:3: error: #error "__cpp_threadsafe_static_init not defined"
# error "__cpp_threadsafe_static_init not defined"
^~~~~
