boostorg/typeof
1
0
Fork 1
mirror of https://github.com/boostorg/typeof.git synced 2025-07-29 20:37:28 +02:00
Code Issues Packages Projects Releases Wiki Activity

removed tabs

Browse Source 
[SVN r31198]
This commit is contained in:
Arkadiy Vertleyb
2005-10-05 02:47:56 +00:00
parent 0f7dcbe618
commit 670d5d4fd4
1 changed files with 46 additions and 46 deletions
Download Patch File Download Diff File Expand all files Collapse all files

92
doc/typeof.qbk
View File

@ -4,7 +4,7 @@
[license
Distributed under the Boost Software License, Version 1.0.
(See accompanying file LICENSE_1_0.txt or copy at
<ulink url="http://www.boost.org/LICENSE_1_0.txt">
<ulink url="http://www.boost.org/LICENSE_1_0.txt">
http://www.boost.org/LICENSE_1_0.txt
</ulink>)
]
@ -22,21 +22,21 @@ by utilizing the C++ template argument deduction facility. Consider `std::pair`
In order to instantiate this class template and create a temporary object of this instantiation,
one has to supply both type parameters and value parameters:
std::pair<int, double>(5, 3.14159);
std::pair<int, double>(5, 3.14159);
To avoid this duplication, STL supplies the `std::make_pair` object generator.
When it is used, the types of template parameters are deduced from supplied function arguments:
std::make_pair(5, 3.14159);
std::make_pair(5, 3.14159);
For the temporary objects it is enough. However, when a named object needs to be allocated,
the problem appears again:
std::pair<int, double> p(5, 3.14159);
std::pair<int, double> p(5, 3.14159);
The object generator no longer helps:
std::pair<int, double> p = std::make_pair(5, 3.14159);
std::pair<int, double> p = std::make_pair(5, 3.14159);
It would be nice to deduce the type of the object (on the left) from the expression
it is initialized with (on the right), but the current C++ syntax does not allow for this.
@ -45,37 +45,37 @@ The above example demonstrates the essence of the problem but does not demonstra
Many libraries, especially expression template libraries, create objects of really complicated types,
and go a long way to hide this complexity behind object generators. Consider a nit Boost.Lambda functor:
_1 > 15 && _2 < 20
_1 > 15 && _2 < 20
If one wanted to allocate a named copy of such an innocently looking functor,
she would have to specify something like this:
lambda_functor<
lambda_functor_base<
logical_action<and_action>,
tuple<
lambda_functor<
lambda_functor_base<
relational_action<greater_action>,
tuple<
lambda_functor<placeholder<1> >,
int const
>
>
>,
lambda_functor<
lambda_functor_base<
relational_action<less_action>,
tuple<
lambda_functor<placeholder<2> >,
int const
>
>
>
>
>
>
f = _1 > 15 && _2 < 20;
lambda_functor<
lambda_functor_base<
logical_action<and_action>,
tuple<
lambda_functor<
lambda_functor_base<
relational_action<greater_action>,
tuple<
lambda_functor<placeholder<1> >,
int const
>
>
>,
lambda_functor<
lambda_functor_base<
relational_action<less_action>,
tuple<
lambda_functor<placeholder<2> >,
int const
>
>
>
>
>
>
f = _1 > 15 && _2 < 20;
Not exactly elegant. To solve this problem, the C++ standard committee is considering
@ -87,11 +87,11 @@ The `typeof` operator (or `decltype`, which is a slightly different flavor of `t
allows one to determine the type of an expression at compile time. Using `typeof`,
the above example can be simplified drastically:
typeof(_1 > 15 && _2 < 20) f = _1 > 15 && _2 < 20;
typeof(_1 > 15 && _2 < 20) f = _1 > 15 && _2 < 20;
Much better, but some duplication still exists. The `auto` type solves the rest of the problem:
auto f = _1 > 15 && _2 < 20;
auto f = _1 > 15 && _2 < 20;
[endsect]
@ -774,11 +774,11 @@ We get the following error message from VC7.1
[pre
error C2504: 'boost::type_of::'anonymous-namespace'::encode_type_impl<V,Type_Not_Registered_With_Typeof_System>' : base
class undefined
with
\[
V=boost::type_of::'anonymous-namespace'::encode_type_impl<boost::mpl::vector0<boost::mpl::na>,std::pair<X,Y<int,true>>>::V0,
Type_Not_Registered_With_Typeof_System=X
\]
with
\[
V=boost::type_of::'anonymous-namespace'::encode_type_impl<boost::mpl::vector0<boost::mpl::na>,std::pair<X,Y<int,true>>>::V0,
Type_Not_Registered_With_Typeof_System=X
\]
]
Inspecting this error message, we see that the compiler complains about `X`
@ -790,11 +790,11 @@ Recompiling, we get a new error message from VC7.1
[pre
error C2504: 'boost::type_of::'anonymous-namespace'::encode_type_impl<V,Type_Not_Registered_With_Typeof_System>' : base
class undefined
with
\[
V=boost::type_of::'anonymous-namespace'::encode_type_impl<boost::mpl::vector0<boost::mpl::na>,std::pair<X,Y<int,true>>>::V1,
Type_Not_Registered_With_Typeof_System=Y<int,true>
\]
with
\[
V=boost::type_of::'anonymous-namespace'::encode_type_impl<boost::mpl::vector0<boost::mpl::na>,std::pair<X,Y<int,true>>>::V1,
Type_Not_Registered_With_Typeof_System=Y<int,true>
\]
]
Inspecting this error message, we see that the compiler complains about `Y<int,true>`.
@ -841,8 +841,8 @@ as nested classes. Instead, instantiations can be registered:
[section:cont Contributed By:]
* Compliant compilers -- Arkadiy Vertleyb, Peder Holt
* MSVC 6.5, 7.0, 7.1 -- Igor Chesnokov, Peder Holt
* Compliant compilers -- Arkadiy Vertleyb, Peder Holt
* MSVC 6.5, 7.0, 7.1 -- Igor Chesnokov, Peder Holt
[endsect]