| |
| [/ 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] |
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