Re: [boost] [Fit] Review

13 Mar 2016

      On Fri, 11 Mar 2016 19:41:22 +0000 (UTC)
paul Fultz  wrote:
...
...
On Friday, March 11, 2016 12:30 PM, Bjorn Reese
 wrote:
...
On 03/11/2016 01:50 AM, Paul Fultz II wrote:
...
library. I do, however, need to discuss some of the misperceived
issues   
with  
 using global function objects in the documentation. As the issues
raised in the review were:
The destruction order of global objects in different translation
units is undefined. This means that I cannot use a global fit
function in the destructor of one of my own global objects because
the former may have
...
been destroyed before the latter.
This is not a problem, because it is initialized at compile-time,
that means it is a literal type:
http://en.cppreference.com/w/cpp/concept/LiteralType
So besides the constructor not having side-effects, the destructor
must be trivial, as well. For example, this cannot compile:
struct foo_fn
{
    template
    auto operator()(Ts&&...) const
    {}
~foo_fn()
    {
        std::cout << "Hello";
    }
};
BOOST_FIT_STATIC_FUNCTION(foo) = foo_fn();
int main() {
        foo();
}
So with trivial destructors, the order of destruction doesn't matter.
Furthermore, on most compilers(that is everything but MSVC), the
function is the same across all translation units. So there is only
one function in the executable, not one per translation unit. Taking
the address of the function will yield the same address across all
translation units. This is the same way functions work.
The following program throws an exception and aborts in Clang 3.4 and
Gcc 4.8, both on Ubuntu 14.04

    #include <iostream>
    #include <stdexcept>
    #include 
    #include 

    class log_ {
    public:
      log_()
        : active_(true) {
        std::cout << "constructing log" << std::endl;
        // or some equally terrible global state changing
        // code such as a file open 
      }

      ~log_() {
        std::cout << "destructing log" << std::endl;
        active_ = false;
      }

      void operator()(const char* const msg) const {
        if (active_) {
          std::cout << msg << std::endl;
        } else {
          throw std::runtime_error{"not active"};
        }
      }
    private:
      bool active_;
    };

    FIT_STATIC_FUNCTION(log) = fit::static_{};

    struct one_ {
      ~one_() {
        log("destructing one");
      }
    };

    struct two_ {
      two_() {
        log("constructing two");
      }
    };

    const one_ one{};
    const two_ two{};

    int main() {
      return 0;
    }

Both Gcc and Clang print:

    constructing log
    constructing two
    destructing log
    terminate called after throwing an instance of 'std::runtime_error'
      what():  not active

The generated program conforms to the C++ standard because `log_` was
created as a function local static, and the call to `operator()` after
destruction can do _anything_ (undefined behavior). Also, `log_` can be
constructed before `one_` and `two_` (and therefore destructed after
both) as well, and still conform to the C++ specifications. Or at least
that is my understanding of static construction/destruction, which is
complex in C++.

Lee

Re: [boost] [Fit] Review

Lee Clagett