[/ Copyright (C) 2009-2012 Lorenzo Caminiti ]
[/ Distributed under the Boost Software License, Version 1.0 ]
[/ (see accompanying file LICENSE_1_0.txt or a copy at ]
[/ http://www.boost.org/LICENSE_1_0.txt) ]
[/ Home at http://www.boost.org/libs/utility/identity_type ]

[library Boost.Utility/IdentityType
    [quickbook 1.5]
    [version 1.0.0]
    [copyright 2009-2012 Lorenzo Caminiti]
    [purpose wraps types with round parenthesis]
    [license
        Distributed under the Boost Software License, Version 1.0
        (see accompanying file LICENSE_1_0.txt or a copy at
        [@http://www.boost.org/LICENSE_1_0.txt])
    ]
    [authors [Caminiti <email>lorcaminiti@gmail.com</email>, Lorenzo]]
    [category Utilities]
]

This library allows to wrap types within round parenthesis so they can always be passed as macro parameters.

[import ../test/var_error.cpp]
[import ../test/var.cpp]
[import ../test/template.cpp]
[import ../test/abstract.cpp]
[import ../test/paren.cpp]

[section Motivation]

Consider the following macro which declares a variable named `var`[^['n]] with the specified [^['type]] (see also [@../../test/var_error.cpp =var_error.cpp=]):

[var_error]

The first macro invocation works correctly declaring a variable named `var1` of type `int`.
However, the second macro invocation fails generating a preprocessor error similar to the following:

[pre
    error: macro "VAR" passed 3 arguments, but takes just 2
]

That is because the `std::map` type passed as the first macro parameter contains a comma `,` not wrapped by round parenthesis `()`.
The preprocessor interprets that unwrapped comma as a separation between macro parameters concluding that a total of three (and not two) parameters are passed to the macro in the following order:

# `std::map<int`
# `char>`
# `2`

Note that, differently from the compiler, the preprocessor only recognizes round parenthesis `()`.
Angular `<>` and squared `[]` parenthesis are not recognized by the preprocessor when parsing macro parameters.

[endsect]

[section Solution]

In some cases, it might be possible to workaround this issue by avoiding to pass the type expression to the macro all together.
For example, in the case above a `typedef` could have been used to specify the type expression with the commas outside the macro (see also [@../../test/var.cpp =var.cpp=]):

[var_typedef]

When this is neither possible nor desired (e.g., see the function template `f` in the section below), this library header [headerref boost/utility/identity_type.hpp] defines a macro [macroref BOOST_IDENTITY_TYPE] which can be used to workaround the issue while keeping the type expression as one of the macro parameters (see also [@../../test/var.cpp =var.cpp=]).

[var_ok]

The [macroref BOOST_IDENTITY_TYPE] macro expands to an expression that evaluates (at compile-time) to the specified type.
The specified type is never split into multiple macro parameters because it is always wrapped by a set of extra round parenthesis `()`.
In fact, a total of two sets of round parenthesis must be used: The parenthesis to invoke the macro `BOOST_IDENTITY_TYPE(...)` plus the inner parenthesis to wrap the type passed to the macro `BOOST_IDENTITY_TYPE((...))`.

This macro works on any [@http://www.open-std.org/JTC1/SC22/WG21/docs/standards C++03] compiler (and it does not use [@http://en.wikipedia.org/wiki/Variadic_macro variadic macros]).
[footnote
Using variadic macros, it would be possible to require a single set of extra parenthesis `BOOST_IDENTITY_TYPE(`[^['type]]`)` instead of two `BOOST_IDENTITY_TYPE((`[^['type]]`))` but variadic macros are not part of C++03 (even if nowadays they are supported by most modern compilers and they are also part of C++11).
]
The authors originally developed and tested this library using GNU Compiler Collection (GCC) C++ 4.5.3 (with and without C++11 features enabled `-std=c++0x`) on Cygwin and Miscrosoft Visual C++ (MSVC) 8.0 on Windows 7.
See the library [@http://www.boost.org/development/tests/release/developer/utility-identity_type.html regressions test results] for more information on supported compilers and platforms.

[endsect]

[section Templates]

This macro must be prefixed by `typename` when used within templates.
For example, let's program a macro that declares a function parameter named `arg`[^['n]] with the specified [^['type]] (see also [@../../test/template.cpp =template.cpp=]):

[template_f_decl]
[template_f_call]

However, note that the template parameter `char` must be manually specified when invoking the function as in `f<char>(a)`.
In fact, when the [macroref BOOST_IDENTITY_TYPE] macro is used to wrap a function template parameter, the template parameter can no longer be automatically deduced by the compiler form the function call as `f(a)` would have done.
[footnote
This is because the implementation of [macroref BOOST_IDENTITY_TYPE] wraps the specified type within a meta-function.
]
(This limitation does not apply to class templates because class template parameters must always be explicitly specified.)
In other words, without using the [macroref BOOST_IDENTITY_TYPE] macro, C++ would normally be able to automatically deduce the function template parameter as shown below:

[template_g_decl]
[template_g_call]

[endsect]

[section Abstract Types]

On some compilers (e.g., GCC), using this macro on abstract types (i.e., classes with one or more pure virtual functions) generates a compiler error.
This can be avoided by manipulating the type adding and removing a reference to it.

Let's program a macro that performs a static assertion on a [@http://en.wikipedia.org/wiki/Template_metaprogramming Template Meta-Programming] (TMP) meta-function (similarly to Boost.MPL [@http://www.boost.org/doc/libs/1_36_0/libs/mpl/doc/refmanual/assert.html `BOOST_MPL_ASSERT`]).
The [macroref BOOST_IDENTITY_TYPE] macro can be used to pass a meta-function with multiple template parameters to the assert macro (so to handle the commas separating the template parameters).
In this case, if the meta-function is an abstract type, it needs to be manipulated adding and removing a reference to it (see also [@../../test/abstract.cpp =abstract.cpp=]):

[abstract]

[endsect]

[section Annex: Usage]

The [macroref BOOST_IDENTITY_TYPE] macro can be used either when calling a user-defined macro (as shown by the examples so far), or internally when implementing a user-defined macro (as shown below).
When [macroref BOOST_IDENTITY_TYPE] is used in the implementation of the user-defined macro, the caller of the user macro will have to specify the extra parenthesis (see also [@../../test/paren.cpp =paren.cpp=]):

[paren]

However, note that the caller will /always/ have to specify the extra parenthesis even when the macro parameters contain no comma:

[paren_always]

In some cases, using [macroref BOOST_IDENTITY_TYPE] in the implementation of the user-defined macro might provide the best syntax for the caller.
For example, this is the case for `BOOST_MPL_ASSERT` because the majority of template meta-programming expressions contain unwrapped commas so it is less confusing for the user to always specify the extra parenthesis `((...))` instead of using [macroref BOOST_IDENTITY_TYPE]:

    BOOST_MPL_ASSERT(( // Natural syntax.
        boost::mpl::and_<
              boost::is_const<T>
            , boost::is_reference<T>
        >
    ));

However, in other situations it might be preferable to not require the extra parenthesis in the common cases and handle commas as special cases using [macroref BOOST_IDENTITY_TYPE].
For example, this is the case for [@http://www.boost.org/libs/local_function `BOOST_LOCAL_FUNCTION`] for which always requiring the extra parenthesis `((...))` around the types would lead to an unnatural syntax for the local function signature:

    int BOOST_LOCAL_FUNCTION( ((int&)) x, ((int&)) y ) { // Unnatural syntax.
        return x + y;
    } BOOST_LOCAL_FUNCTION_NAME(add)

Instead requiring the user to specify [macroref BOOST_IDENTITY_TYPE] only when needed allows for the more natural syntax `BOOST_LOCAL_FUNCTION(int& x, int& y)` in the common cases when the parameter types contain no comma (while still allowing to specify parameter types with commas as special cases using `BOOST_LOCAL_FUNCTION(BOOST_IDENTITY_TYPE((std::map<int, char>))& x, int& y)`).

[endsect]

[section Annex: Implementation]

The implementation of this library macro is equivalent to the following:
[footnote
There is absolutely no guarantee that the macro is actually implemented using the code listed in this documentation.
The listed code is for explanatory purposes only.
]

    #include <boost/type_traits/function_traits.hpp>
    
    #define BOOST_IDENTITY_TYPE(parenthesized_type) \
        boost::function_traits<void parenthesized_type>::arg1_type

Essentially, the type is wrapped between round parenthesis `(std::map<int, char>)` so it can be passed as a single macro parameter even if it contains commas.
Then the parenthesized type is transformed into the type of a function returning `void` and with the specified type as the type of the first and only argument `void (std::map<int, char>)`.
Finally, the type of the first argument `arg1_type` is extracted at compile-time using the `function_traits` meta-function therefore obtaining the original type from the parenthesized type (effectively stripping the extra parenthesis from around the specified type).

[endsect]

[xinclude reference.xml]