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Re: Global lambdas and ODR

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On Thursday, April 7, 2016 at 8:00:13 PM UTC-5, raskolnikov wrote:
> { edited by mod to shorten lines to ~70 characters. -mod }
> 
> Hi!
> 
> I was thinking about what some people including I are trying with
> transducers in C++14, which involves defining global lambdas [1] [2].
> 
> However, I am becoming afraid that this in fact violates the One
> Definition Rule. Not only is the object defined in multiple
translation
> units, it is even defined with different types!

You may want to look at this, which discusses these issues:

http://pfultz2.com/blog/2015/05/31/unique-address/

The difficult problem with lambdas is that they are not constexpr.
Otherwise,
you could just write:

template<class T>
constexpr auto global = T{};

template<class T>
const auto& static_const_var(const T&)
{
    return global<T>;
}

constexpr auto&& lambda = static_const_var([]{ ... });

In the Fit library[1], I would like to support the factory pattern for
lambdas, so you could write this:

auto lambda_factor()
{
    return []{ ... };
}

FIT_STATIC_FUNCTION(lambda) = fit::indirect(&lambda_factor, fit::apply);

The fit::indirect adaptor needs to be extended to support a custom
operator.

I don't know if any of those patterns will help you with your
transducers.

Paul

[1]: https://github.com/pfultz2/Fit


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Global lambdas and ODR raskolnikov <magnicida@googlemail.com> - 2016-04-07 19:56 -0600
  Re: Global lambdas and ODR pfultz2 <pfultz2@this.is.invalid> - 2016-04-11 19:30 -0600
    Re: Global lambdas and ODR Juan Pedro Bolivar Puente <raskolnikov@gnu.org> - 2016-04-12 06:49 -0600
    Re: Global lambdas and ODR Juan Pedro Bolivar Puente <raskolnikov@gnu.org> - 2016-04-12 06:49 -0600
      Re: Global lambdas and ODR pfultz2 <pfultz2@this.is.invalid> - 2016-04-12 12:53 -0600

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