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[SG10] Missing feature macro for threadsafe function local statics

From: Daniel Krügler <daniel.kruegler_at_[hidden]>
Date: Tue, 21 Feb 2017 21:28:33 +0100
Thread-safe initialization of static local function variables is part
of C++11, via



(a) at least some implementations still do not support them, or
(b) provide means (outside of the spec) that allow to disable this
functionality (gcc: --fno-threadsafe-statics, Visual Studio 2015:

The compilers I am aware of both don't provide a vendor specific
feature macro to detect this situation.

I would therefore strongly vote for a feature macro that describes the
presence of that functionality, because user code can usually defend
against this missing feature by means of std::call_once. Here is
therefore a minimalistic example code hopefully suitable for a future
revision of p0096 (The example also shows my current feature macro
name suggestion as "__cpp_threadsafe_static_init"):

Thingy& get_thing()
#ifndef __cpp_threadsafe_static_init
    static std::once_flag flag;
    std::call_once(flag, init::get);
    return init::get();
    static Thingy result;
    return result;


- Daniel

Received on 2017-02-21 21:28:34