Re: [boost] IS THERE A WAY TO FIND IF A CERTAIN TYPE HAS A CERTAIN MEMBER?

Subject: Re: [boost] IS THERE A WAY TO FIND IF A CERTAIN TYPE HAS A CERTAIN MEMBER?
From: Paul Fultz II (pfultz2_at_[hidden])
Date: 2016-01-14 20:28:01

With clang in Boost.Hana you can use `hana:overload_linearly`:

    template<class T>
    void optional_foo(T&& x)
    {
        boost::hana::overload_linearly(
            // Call foo if it exists
            [](auto&& x) -> decltype(x.foo(), void()) { x.foo(); },
            [](auto) { std::cout << "No foo" << std::endl; }
        )(x);
    }

However, if you need more portability(such as running on MSVC), you can use
`fit::conditional` as well:

http://fit.readthedocs.org/en/latest/conditional/index.html

Although its not yet a boost library, it is in the review queue and can work
for MSVC and C++11 compilers. With the Fit library you can also define the
function directly from the lambdas as well:

    FIT_STATIC_LAMBDA_FUNCTION(optional_foo) = fit::conditional(
        // Call foo if it exists
        [](auto&& x) -> decltype(x.foo(), void()) { x.foo(); },
        [](auto) { std::cout << "No foo" << std::endl; }
    );

This still requires generic lambdas. If you are stuck in a C++11 compiler, 
you
can write it using function objects as well:

    struct with_foo
    {
        template<class T>
        auto operator()(T&& obj) const -> decltype(x.foo(), void())
        {
            x.foo();
        }
    };

    struct without_foo
    {
        template<class T>
        void operator()(T&&) const
        {
            std::cout << "No foo" << std::endl;
        }
    };

    FIT_STATIC_FUNCTION(optional_foo) = fit::conditional(
        with_foo(),
        without_foo()
    );

If you are stuck in C++03 land, though, you will need to use Boost.TTI

Thanks,
Paul

On Thursday, January 14, 2016 at 1:43:09 PM UTC-6, Juan Dent wrote:
>
> > HI, 
> > 
> > 
> > I have read that via enable_if and other constructs, one can declare a 
> member of a type conditionally. 
> > 
> > What I need is a way to be able to test whether a certain type has a 
> certain method or member data, so that I can call it or use it accordingly. 
> > 
> > Does boost have something like this?? 
> > 
> > 
> > Regards, 
> > Juan 
>
>
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