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trick to print a type at compile-time

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Hicham Mouline

8 Sep 2010 8 Sep '10

7:08 a.m.

Hello, Is there a way to print the type returned by a metafunction typedef some_metafunction<T>::type result_type; during compile-time (probably via triggering a compilation error). I almost recall some sort of static assertion (mpl ?) to do this. regards,

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Emil Dotchevski

8 Sep 8 Sep

7:15 a.m.

On Wed, Sep 8, 2010 at 12:08 AM, Hicham Mouline wrote:

Hello,

Is there a way to print the type returned by a metafunction

typedef some_metafunction<T>::type result_type;

during compile-time (probably via triggering a compilation error).

I almost recall some sort of static assertion (mpl ?) to do this.

If all you want is an error you could do typedef typename some_metafunction<T>::result_type error[0] or something like that. Emil Dotchevski Reverge Studios, Inc. http://www.revergestudios.com/reblog/index.php?n=ReCode

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Hicham Mouline

12:24 p.m.

"Emil Dotchevski" wrote in message news:AANLkTi=3ZkQBNUPQsWy=oxcUdzDpm5XDk8b0qgFS0zoW@mail.gmail.com... On Wed, Sep 8, 2010 at 12:08 AM, Hicham Mouline wrote:

Hello,

Is there a way to print the type returned by a metafunction

typedef some_metafunction<T>::type result_type;

during compile-time (probably via triggering a compilation error).

I almost recall some sort of static assertion (mpl ?) to do this.

If all you want is an error you could do typedef typename some_metafunction<T>::result_type error[0] or something like that. Emil Dotchevski ---------------------- I'd like to print result_type, I don't know what it is. Shouldn't I make result_type a parameter to some template that just triggers an error printout when it is instantiated? would typedef typename some_metafunction<T>::result_type error[0] print what is result_type? thank you, regards,

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Eric Niebler

12:56 p.m.

On 9/8/2010 3:08 AM, Hicham Mouline wrote:

Hello,

Is there a way to print the type returned by a metafunction

typedef some_metafunction<T>::type result_type;

during compile-time (probably via triggering a compilation error). I almost recall some sort of static assertion (mpl ?) to do this.

boost::mpl::print in boost/mpl/print.hpp. Neither seem to be documented. -- Eric Niebler BoostPro Computing http://www.boostpro.com

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Dave Abrahams

1:58 p.m.

On Wed, Sep 8, 2010 at 8:56 AM, Eric Niebler wrote:

boost::mpl::print in boost/mpl/print.hpp. Neither seem to be documented.

mpl::print is mostly good for those cases where you need to *not* cause a compilation error, perhaps because you are trying to follow recursive instantiations. Otherwise, I'd go with an explicit error; you'll get less noise. -- Dave Abrahams BoostPro Computing http://www.boostpro.com

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Hicham Mouline

9 Sep 9 Sep

2:38 a.m.

-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Dave Abrahams Sent: 08 September 2010 14:58 To: boost-users@lists.boost.org Subject: Re: [Boost-users] trick to print a type at compile-time

On Wed, Sep 8, 2010 at 8:56 AM, Eric Niebler wrote:

...
boost::mpl::print in boost/mpl/print.hpp. Neither seem to be documented.

mpl::print is mostly good for those cases where you need to *not* cause a compilation error, perhaps because you are trying to follow recursive instantiations. Otherwise, I'd go with an explicit error; you'll get less noise.

I tried both ways: boost::mpl::print myvar; and template <typename T> struct print_error { }; typedef typename print_error::type my_print_error; my_print_error myvar; In both cases, the bit in the error in msvc2008 just shows: 1> with 1> [ 1> T=result_type 1> ] it doesn't print what that type actually is. regards,

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Steven Watanabe

2:55 a.m.

AMDG Hicham Mouline wrote:

I tried both ways:

boost::mpl::print myvar;

and

template <typename T> struct print_error { }; typedef typename print_error::type my_print_error; my_print_error myvar;

In both cases, the bit in the error in msvc2008 just shows:

1> with 1> [ 1> T=result_type 1> ]

it doesn't print what that type actually is.

MSVC does this for typedefs except when they are declared in a class template. Unfortunately, I don't know of any way to get MSVC to print the actual type in other cases. In Christ, Steven Watanabe

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Hicham Mouline

3:30 a.m.

-----Original Message----- From: boost-users-bounces@lists.boost.org [mailto:boost-users- bounces@lists.boost.org] On Behalf Of Steven Watanabe Sent: 09 September 2010 03:56 To: boost-users@lists.boost.org Subject: Re: [Boost-users] trick to print a type at compile-time

AMDG

Hicham Mouline wrote:

...
I tried both ways:

boost::mpl::print myvar;

and

template <typename T> struct print_error { }; typedef typename print_error::type my_print_error; my_print_error myvar;

In both cases, the bit in the error in msvc2008 just shows:

1> with 1> [ 1> T=result_type 1> ]

it doesn't print what that type actually is.

MSVC does this for typedefs except when they are declared in a class template. Unfortunately, I don't know of any way to get MSVC to print the actual type in other cases.

In Christ, Steven Watanabe

I've managed to change code a bit to allow for it to be printed at runtime instead: throw std::exception( typeid(result_type).name() ) the error I'm getting is related to a fusion::vector with 1 element only. I'll post another question about this now. Thanks,

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participants (5)

Dave Abrahams
Emil Dotchevski
Eric Niebler
Hicham Mouline
Steven Watanabe