duck-detector.hpp File Reference

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Description

Metaprogramming helpers to check for specific properties of a type in question.

Building upon the "SFINAE" principle, it is possible to create metafunction templates, which answer some questions about a given type at compile time. A lot of generic predicates of this kind can be found in the <type_traits> library (standard since C++11). At times though, you want to ask more specific questions, like e.g. "does this type provide an operation quack() "? Because, if we can get a bool answer to such a question at compile time, we can use std::enable_if to pick a special implementation based on the test result. Together, these techniques allow to adopt a duck-typed programming style, where an arbitrary object is allowed to enter a given API function, provided this object supports some specific operations.

While C++ certainly isn't a dynamic language and does not provide any kind of run time introspection, doing such check-and branch at compile time allows to combine flexibility as known from dynamic languages with static type safety, which is compelling. We can generate similar implementations for types not further related by inheritance. Building on this, we're able to emulate some of the features enabled by type classes (or "concepts").

how the implementation works

Most of these trait templates rely on a creative use of function overloading. The C++ standard requires the compiler silently to drop any candidate of overload resolution which has gotten an invalid function signature as a result of instantiating a template (type). This rule allows us to set up kind of a "honey pot" for the compiler: we present two overloaded candidate functions with a different return type; by investigating the resulting return type we're able to figure out the overload actually picked by the compiler.

This header provides some pre-configured tests, available as macros. Each of them contains a template based on the described setup, containing a probe type expression at some point. The key is to build this probe expression in a way that it is valid if and only if the type in question exhibits a specific property.

• if the type should contain a nested type or typedef with a specific name, we simply use this nested type in the signature of the overloaded function
• if the type should contain a member with a specific name, we initialise a member pointer within a probe template with this member (if there isn't such a member, the probe template initialisation fails and the other function overload gets picked)
• as an extension to this approach, we can even declare a member function pointer with a specific function signature and then try to assign the named member. This allows even to determine if a member function of a type in question has the desired signature.

All these detection building blocks are written such as to provide a bool member ::value, which is in accordance to the conventions of modern C++ metaprogramming. I.e. you can directly use them within std::enable_if

some pitfalls to consider

Warning

The generated metafunctions all yield the false value by default. Effectively this means that an error in the test expression might go unnoticed; you'd be better off explicitly checking the detection result by an unit test.

There are several typical problems to care about

• none of these tests is able to detect any private members
• the name-only detectors will fail if the name is ambiguous
• a member can be both a variable or a function of that name
• function signatures need to match precisely, including const modifiers
• the generated metafunction (template) uses a type parameter 'TY', which could shadow or conflict with an type parameter in the enclosing scope
• some of the detectors require a complete type to work properly. They create a pointer-to-member or invoke sizeof(). In regular code, doing such on an incomplete type would provoke a compilation failure – however, here this code gets evaluated in a SFINAE context, which means, it will fail silently and thus produce a wrong detection result. This can be quite insidious when relying on the proper detection to pick the right implementation/specialisation; especially when instantiating mutually dependent templates, the distinction between "complete" and "incomplete" can be rather arbitrary while in the process of instantiation.
• the member and function checks rely on member pointers, which generally refer to the explicit static type. These checks won't see any inherited members / functions.
• obviously, all those checks are never able to detect anything depending on runtime types or RTTI

See also

util-foreach.hpp usage example

iter-explorer.hpp (example: is_StateCore<SRC>)

duck-detector-test.cpp

duck-detector-extension-test.cpp

Definition in file duck-detector.hpp.

#include "lib/meta/util.hpp"

Macros
#define	META_DETECT_EXTENSION_POINT(_FUN_)
	Detector for support of a free-function extension point. More...
#define	META_DETECT_FUNCTION(_RET_TYPE_, _FUN_NAME_, _ARGS_)
	Detector for a specific member function. More...
#define	META_DETECT_FUNCTION_ARGLESS(_FUN_)
	Detector for an argument-less member function with the given name. More...
#define	META_DETECT_FUNCTION_NAME(_FUN_NAME_)
	Detector for a member function with the given name. More...
#define	META_DETECT_MEMBER(_NAME_)
	Detector for a nested member (field or function). More...
#define	META_DETECT_NESTED(_TYPE_)
	Detector for a nested type. More...
#define	META_DETECT_OPERATOR_DEREF()
	Detector for a dereferentiation operator. More...
#define	META_DETECT_OPERATOR_INC()
	Detector for a prefix increment operator. More...

Macro Definition Documentation

META_DETECT_NESTED

#define META_DETECT_NESTED

(

_TYPE_

)

Value:

template<typename TY>                                      \
    class HasNested_##_TYPE_                                    \
      {                                                          \
                                                                  \
        template<class X>                                          \
        static Yes_t check(typename X::_TYPE_ *);                   \
        template<class>                                              \
        static No_t  check(...);                                      \
                                                                       \
      public:                                                           \
        static const bool value = (sizeof(Yes_t)==sizeof(check<TY>(0))); \
      };

Detector for a nested type.

Defines a metafunction (template), allowing to detect if a type TY in question has a nested type or typedef with the given name. To answer this question, instantiate resulting HasNested_XXX template with the type in question and check the static bool value field.

Warning

none of these checks can detect private members

Definition at line 111 of file duck-detector.hpp.

META_DETECT_MEMBER

#define META_DETECT_MEMBER

(

_NAME_

)

Value:

template<typename TY>                                   \
    class HasMember_##_NAME_                                 \
      {                                                       \
        template<typename X,                                   \
                 typename SEL = decltype(&X::_NAME_)>           \
        struct Probe                                             \
          { };                                                    \
                                                                   \
        template<class X>                                           \
        static Yes_t check(Probe<X> * );                             \
        template<class>                                               \
        static No_t  check(...);                                       \
                                                                        \
      public:                                                            \
        static const bool value = (sizeof(Yes_t)==sizeof(check<TY>(0)));  \
      };

Detector for a nested member (field or function).

Defines a metafunction (template), allowing to detect the presence of a member with the given name within a type in question.

Note

this check will likely fail if the name is ambiguous.

Warning

none of these checks can detect private members

Definition at line 134 of file duck-detector.hpp.

META_DETECT_FUNCTION

#define META_DETECT_FUNCTION	(		_RET_TYPE_,
			_FUN_NAME_,
			_ARGS_
	)

Value:

template<typename TY>                                   \
    class HasFunSig_##_FUN_NAME_                             \
      {                                                       \
        template<typename X, _RET_TYPE_ (X::*)_ARGS_>          \
        struct Probe                                            \
          { };                                                   \
                                                                  \
        template<class X>                                          \
        static Yes_t check(Probe<X, &X::_FUN_NAME_> * );            \
        template<class>                                              \
        static No_t  check(...);                                      \
                                                                       \
      public:                                                           \
        static const bool value = (sizeof(Yes_t)==sizeof(check<TY>(0))); \
      };

Detector for a specific member function.

Defines a metafunction (template), allowing to detect the presence of a member function with the specific signature, as defined by the parameters.

Note

this check is not sensitive to overloads, due to the explicitly given argument types

Definition at line 161 of file duck-detector.hpp.

META_DETECT_FUNCTION_NAME

#define META_DETECT_FUNCTION_NAME

(

_FUN_NAME_

)

Value:

template<typename TY>                                   \
    class HasFunName_##_FUN_NAME_                            \
      {                                                       \
        template<typename SEL>                                 \
        struct Probe;                                           \
        template<class C, typename RET, typename...ARGS>         \
        struct Probe<RET (C::*) (ARGS...)>                        \
          {                                                        \
            using Match = void;                                     \
          };                                                         \
        template<class C, typename RET, typename...ARGS>              \
        struct Probe<RET (C::*) (ARGS...)  const>                      \
          {                                                             \
            using Match = void;                                          \
          };                                                              \
                                                                           \
        template<class X>                                                   \
        static Yes_t check(typename Probe<decltype(&X::_FUN_NAME_)>::Match * ); \
        template<class>                                                       \
        static No_t  check(...);                                               \
                                                                                \
      public:                                                                    \
        static const bool value = (sizeof(Yes_t)==sizeof(check<TY>(0)));          \
      };

Detector for a member function with the given name.

Defines a metafunction (template), allowing to detect the presence of a member function with a specific name, but without imposing any additional constraints on arguments and return type. Yet a non-function member will not trigger this detector.

Note

this check will fail if there are overloads or similar ambiguity

Definition at line 186 of file duck-detector.hpp.

META_DETECT_FUNCTION_ARGLESS

#define META_DETECT_FUNCTION_ARGLESS

(

_FUN_

)

Value:

template<typename TY>                                   \
    class HasArglessFun_##_FUN_                              \
      {                                                       \
        template<typename X,                                   \
                 typename SEL = decltype(std::declval<X>()._FUN_())>\
        struct Probe                                             \
          { };                                                    \
                                                                   \
        template<class X>                                           \
        static Yes_t check(Probe<X> * );                             \
        template<class>                                               \
        static No_t  check(...);                                       \
                                                                        \
      public:                                                            \
        static const bool value = (sizeof(Yes_t)==sizeof(check<TY>(0)));  \
      };

Detector for an argument-less member function with the given name.

Defines a metafunction (template), allowing to detect a member function taking no arguments, and with arbitrary return type.

Remarks

the presence of overloads is irrelevant, since we explicitly from an invocation to that function (within decltype)

Definition at line 219 of file duck-detector.hpp.

META_DETECT_EXTENSION_POINT

#define META_DETECT_EXTENSION_POINT

(

_FUN_

)

Value:

template<typename TY>                                   \
    class HasExtensionPoint_##_FUN_                          \
      {                                                       \
        template<typename X,                                   \
                 typename SEL = decltype( _FUN_(std::declval<X>()))>\
        struct Probe                                             \
          { };                                                    \
                                                                   \
        template<class X>                                           \
        static Yes_t check(Probe<X> * );                             \
        template<class>                                               \
        static No_t  check(...);                                       \
                                                                        \
      public:                                                            \
        static const bool value = (sizeof(Yes_t)==sizeof(check<TY>(0)));  \
      };

Detector for support of a free-function extension point.

Defines a metafunction (template), allowing to probe if the type in question supports a specific extension point function. Typically such functions are injected by some type in a way to be picked up by ADL. The detection test works by forming an expression to invoke the extension point, passing the type given as template parameter as function argument. If this expression type checks, the extension point is assumed to be supported.

Warning

beware of implicit type conversions

Definition at line 248 of file duck-detector.hpp.

META_DETECT_OPERATOR_DEREF

#define META_DETECT_OPERATOR_DEREF

(

)

Value:

template<typename TY>                                   \
    class HasOperator_deref                                  \
      {                                                       \
        template<typename X, int i = sizeof(&X::operator*)>    \
        struct Probe                                            \
          { };                                                   \
                                                                  \
        template<class X>                                          \
        static Yes_t check(Probe<X> * );                            \
        template<class>                                              \
        static No_t  check(...);                                      \
                                                                       \
      public:                                                           \
        static const bool value = (sizeof(Yes_t)==sizeof(check<TY>(0))); \
      };

Detector for a dereferentiation operator.

Works like member detection

Definition at line 269 of file duck-detector.hpp.

META_DETECT_OPERATOR_INC

#define META_DETECT_OPERATOR_INC

(

)

Value:

template<typename TY>                              \
    class HasOperator_inc                               \
      {                                                  \
        template<typename X, X& (X::*)(void)>             \
        struct Probe_1                                     \
          { };                                              \
        template<typename X, int i = sizeof(&X::operator++)> \
        struct Probe_2                                        \
          { };                                                 \
                                                                \
        template<class X>                                        \
        static Yes_t check1(Probe_1<X, &X::operator++> * );       \
        template<class>                                            \
        static No_t  check1(...);                                   \
        template<class X>                                            \
        static Yes_t check2(Probe_2<X> * );                           \
        template<class>                                                \
        static No_t  check2(...);                                       \
                                                                         \
      public:                                                             \
        static const bool value = (sizeof(Yes_t)==sizeof(check1<TY>(0))    \
                                 ||sizeof(Yes_t)==sizeof(check2<TY>(0)));   \
      };

Detector for a prefix increment operator.

Note

there is a twist: because of the prefix and postfix version of increment, detection through member-check will fail when both are present (ambiguity). OTOH, when using function signature detection, the return type must match. The latter fails in the common situation, when the increment operator was mixed in through some base class. As a pragmatic solution, we do both kinds of tests; so either remove one of the operators (typically postfix), or add an forwarding override in the class to be checked

Definition at line 296 of file duck-detector.hpp.