// bslmf_forwardingtype.h -*-C++-*-
#ifndef INCLUDED_BSLMF_FORWARDINGTYPE
#define INCLUDED_BSLMF_FORWARDINGTYPE
#include <bsls_ident.h>
BSLS_IDENT("$Id: $")
//@PURPOSE: Provide a meta-function for determining an optimal forwarding type.
//
//@CLASSES:
// bslmf::ForwardingType: meta-function to determine optimal forwarding type
// bslmf::ForwardingTypeUtil: Namespace for forwarding functions
//
//@SEE_ALSO: bslmf_removecvq
//
//@DESCRIPTION: This component, while not deprecated, is largely **superseded**
// by the simpler 'bslmf_forwardingreftype'. Although there may be use cases
// for which 'bslmf_forwardingtype' is to be prefered, new users are encouraged
// to consider 'bslmf_forwardingreftype' and understand the differences between
// the two components (see {Comparison to 'bslmf_forwardingreftype'}).
//
// This component provides a meta function, 'bslmf::ForwardingType',
// determining the most efficient forwarding type for a given template type
// 't_TYPE'. The forwarding type is used to pass an argument from the client
// of a component through a chain of nested function calls to the ultimate
// consumer of the argument. This component also provides a utility class
// template, 'bslmf::ForwardingTypeUtil', supplying functions to most
// efficiently forward an argument to another function.
//
// For instance, basic types (e.g., fundamental types, pointer types, function
// references and pointers) can efficiently be passed by value down a chain of
// nested function calls. However a large object or one with a non-trivial
// copy constructor would be better passed by const reference, even if the
// ultimate consumer takes that argument by value.
//
// Another form of optimization is the early decay of arrays to pointers,
// preventing a proliferation of different template instantiations for every
// array size being used. Although the outermost function may still be
// instantiated on the full array type, intermediate functions are all
// instantiated on the same pointer type, regardless of array size. This
// decay also applies to reference-to-array types. The user can recover the
// original array type when forwarding to the final consumer by using
// 'bslmf::ForwardingTypeUtil<T>::forwardToTarget()' (see below).
//
// An argument 'v' of type 'T' can be passed as type 'ForwardingType<T>::Type'
// down an arbitrarily-long chain of function calls without ever calling
// 'std::forward'. However, in order to avoid an extra copy as well as to
// select the correct overload and instantiation of the eventual target
// function, it should be converted back to a type that more closely resembles
// the original 'T' by calling 'ForwardingTypeUtil<T>::forwardToTarget(v)'.
//
// This component is intended to be used when performance is of highest concern
// or when creating function wrappers that are intended to minimize
// perturbations on the interface of the functions that they wrap.
//
// Note that previous versions of this component forwarded const references to
// basic types by value instead of by const reference. This transformation was
// an attempt to avoid an extra dereference operation, but in real use cases
// the extra dereference happened anyway, on the call to the outermost
// forwarding function. Moreover, such a transformation is subtly broken
// because, in the rare case where the target function cares about the address
// of the reference (e.g., if it compares it to some known address), it would
// wind up with the address of a temporary copy, rather than the address of the
// original argument. Thus, the current component forwards references as
// references in all cases, including for basic types, except in the case of
// arrays and functions (that decay to pointers).
//
///Comparison to 'bslmf_forwardingreftype'
///---------------------------------------
// The components 'bslmf_forwardingtype' and 'bslmf_forwardingreftype' serve
// the same purpose but have small behavioral differences. In general, we
// recommend 'bslmf_forwardingreftype' (the new component) in most contexts.
//
// Most notably, 'bslmf::ForwardingType' (the older class) forwards fundamental
// and pointer types by value, where as 'bslmf::ForwardingRefType' will forward
// fundamental and pointer types by const-reference. For example,
// 'bslmf::ForwardingType<int>::Type' is 'int' where as
// 'bslmf::ForwardingRefType<int>::Type' is 'const int&'. This applies to
// fundamental types, pointer types (including member-pointer types), and enum
// types (which we'll collectively call "basic types"). Forwarding these basic
// types by value was a performance optimization (and in some rare
// circumstances was hack needed by older compilers), which predated the
// standardization of many of the places where 'bslmf::ForwardingType' was used
// (function and bind components in particular). The optimzation (potentially)
// being that passing an 'int' by value is more likely to be done through a
// register, where as passing by reference is more likely to require
// de-referencing memory. Forwarding the types by const-reference, as the
// newer 'bslmf::ForwardingRefType' does', is generally simpler and more in
// line with the modern C++ standard. Using 'bslmf::ForwardingRefType' avoids
// some awkward edge cases at the expense of a possible optimization in
// parameter passing.
//
///Usage
///-----
// In this section we show intended use of this component.
//
///Example 1: Direct Look at Metafunction Results
/// - - - - - - - - - - - - - - - - - - - - - - -
// In this example, we invoke 'ForwardingType' on a variety of types and look
// at the resulting 'Type' member:
//..
// struct MyType {};
// typedef MyType& MyTypeRef;
//
// void main()
// // Usage example.
// {
// typedef int T1;
// typedef int& T2;
// typedef const volatile double& T3;
// typedef const double & T4;
// typedef const float * & T5;
// typedef const float * const & T6;
// typedef MyType T7;
// typedef const MyType& T8;
// typedef MyType& T9;
// typedef MyType *T10;
// typedef int T11[];
// typedef int T12[3];
//
// typedef int EXP1;
// typedef int& EXP2;
// typedef const volatile double& EXP3;
// typedef const double & EXP4;
// typedef const float * & EXP5;
// typedef const float * const & EXP6;
// typedef const MyType& EXP7;
// typedef const MyType& EXP8;
// typedef MyType& EXP9;
// typedef MyType *EXP10;
// typedef int *EXP11;
// typedef int *EXP12;
//
// assert((bsl::is_same<bslmf::ForwardingType<T1>::Type, EXP1>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T2>::Type, EXP2>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T3>::Type, EXP3>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T4>::Type, EXP4>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T5>::Type, EXP5>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T6>::Type, EXP6>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T7>::Type, EXP7>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T8>::Type, EXP8>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T9>::Type, EXP9>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T10>::Type, EXP10>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T11>::Type, EXP11>::value));
// assert((bsl::is_same<bslmf::ForwardingType<T12>::Type, EXP12>::value));
// }
//..
//
///Example 2: A Logging Invocation Wrapper
///- - - - - - - - - - - - - - - - - - - -
// This example illustrates the use of 'ForwardingType' to efficiently
// implement a wrapper class that holds a function pointer and logs
// information about each call to the pointed-to-function through the wrapper.
// Suppose the pointed-to-function takes three arguments whose types are
// specified via template arguments, where the first argument is required to
// be convertible to 'int'.
//
// First we create a wrapper class that holds a function pointer of the
// desired type:
//..
// // Primary template is never defined.
// template <class PROTOTYPE>
// class LoggingWrapper;
//
// template <class RET, class ARG1, class ARG2, class ARG3>
// class LoggingWrapper<RET(ARG1, ARG2, ARG3)> {
// // Specialization of wrapper for specified function prototype.
//
// RET (*d_function_p)(ARG1, ARG2, ARG3);
//
// public:
// explicit LoggingWrapper(RET (*function_p)(ARG1, ARG2, ARG3))
// // Create a 'LoggingWrapper' object for the specified 'function_p'
// // function.
// : d_function_p(function_p) { }
//..
// Then, we declare an overload of the function-call operator that actually
// invokes the wrapped function. In order to avoid excessive copies of
// pass-by-value arguments, we use 'ForwardingType' to declare a more efficient
// intermediate argument type to forward to the wrapped function pointer:
//..
// RET operator()(typename bslmf::ForwardingType<ARG1>::Type a1,
// typename bslmf::ForwardingType<ARG2>::Type a2,
// typename bslmf::ForwardingType<ARG3>::Type a3) const;
// // Invoke the stored function pointer with the specified 'a1',
// // 'a2', and 'a3' arguments, logging the invocation and returning
// // the result of the function pointer invocation.
// };
//..
// Next, we define logging functions that simply count the number of
// invocations and number of returns from invocations (e.g., to count how may
// invocations completed without exiting via exceptions):
//..
// int invocations = 0, returns = 0;
// void logInvocation(int /* ignored */) { ++invocations; }
// // Log an invocation of the wrapped function.
// void logReturn(int /* ignored */) { ++returns; }
// // Log a return from the wrapped function.
//..
// Now, we implement 'operator()' to call the logging functions, either side of
// calling the logged function through the wrapped pointer. To reconstitute
// the arguments to the function as close as possible to the types they were
// passed in as, we call the 'forwardToTarget' member of 'ForwardingTypeUtil':
//..
// template <class RET, class ARG1, class ARG2, class ARG3>
// RET LoggingWrapper<RET(ARG1, ARG2, ARG3)>::operator()(
// typename bslmf::ForwardingType<ARG1>::Type a1,
// typename bslmf::ForwardingType<ARG2>::Type a2,
// typename bslmf::ForwardingType<ARG3>::Type a3) const {
// logInvocation(a1);
// RET r = d_function_p(
// bslmf::ForwardingTypeUtil<ARG1>::forwardToTarget(a1),
// bslmf::ForwardingTypeUtil<ARG2>::forwardToTarget(a2),
// bslmf::ForwardingTypeUtil<ARG3>::forwardToTarget(a3));
// logReturn(a1);
// return r;
// }
//..
// Then, in order to see this wrapper in action, we must define a function we
// wish to wrap. This function will take an argument of type 'ArgType' that
// holds an integer 'value' and keeps track of whether it has been directly
// constructed or copied from anther 'ArgType' object. If it has been copied,
// it keeps track of how many "generations" of copy were made:
//..
// class ArgType {
// int d_value;
// int d_copies;
// public:
// explicit ArgType(int v = 0) : d_value(v), d_copies(0) { }
// // Create an 'ArgType' object. Optionally specify 'v' as the
// // initial value of this 'ArgType' object, otherwise this object
// // will hold the value 0.
//
// ArgType(const ArgType& original)
// // Create an 'ArgType' object that is a copy of the specified
// // 'original'.
// : d_value(original.d_value)
// , d_copies(original.d_copies + 1)
// { }
//
// int copies() const { return d_copies; }
// // Return the number of copies that this object is from the
// // original.
//
// int value() const { return d_value; }
// // Return the value of this object.
// };
//
// int myFunc(const short& i, ArgType& x, ArgType y)
// // Assign the specified 'x' the value of the specified 'y' and return
// // the 'value()' of 'x'. Verify that the specified 'i' matches
// // 'y.copies()'. 'x' is passed by reference in order to demonstrate
// // forwarding of reference arguments.
// {
// assert(i == y.copies());
// x = y;
// return x.value();
// }
//..
// Finally, we create a instance of 'LoggingWrapper' to wrap 'myFunc', and we
// invoke it. Note that 'y' is copied into the second argument of 'operator()'
// and is copied again when 'myFunc' is invoked. However, it is *not* copied
// when 'operator()' calls 'invoke()' because the 'ForwardType' of 'ArgType' is
// 'const ArgType&', which does not create another copy. In C++11, if
// 'ArgType' had a move constructor, then the number of copies would be only 1,
// since the final forwarding would be a move instead of a copy.
//..
// void usageExample2()
// // Usage Example
// {
// ArgType x(0);
// ArgType y(99);
//
// LoggingWrapper<int(const short&, ArgType&, ArgType)> lw(myFunc);
// assert(0 == invocations && 0 == returns);
// lw(1, x, y); // Expect exactly one copy of 'y'
// assert(1 == invocations && 1 == returns);
// assert(99 == x.value());
// }
//..
#include <bslscm_version.h>
#include <bslmf_isarray.h>
#include <bslmf_isenum.h>
#include <bslmf_isfunction.h>
#include <bslmf_isfundamental.h>
#include <bslmf_ispointer.h>
#include <bslmf_ismemberpointer.h>
#include <bslmf_isreference.h>
#include <bslmf_isrvaluereference.h>
#include <bslmf_movableref.h>
#include <bslmf_removecv.h>
#include <bslmf_removereference.h>
#include <bsls_compilerfeatures.h>
#include <bsls_platform.h>
#include <stddef.h>
#if defined(BSLS_PLATFORM_CMP_IBM) && \
((BSLS_PLATFORM_CMP_VERSION < 0x0c10) || \
(BSLS_PLATFORM_CMP_VERSION == 0x0c10 && __xlC_ver__ < 0x00000013))
# define BSLMF_FORWARDINGTYPE_NO_INLINE
// THe IBM xlC compiler trips an ICE or infinite compile loop when certain
// functions are inlined for optimized builds. This is resolved by the August
// 2017 PTF release, 12.1.0.19.
#endif
namespace BloombergLP {
namespace bslmf {
// FORWARD DECLARATIONS
template <class t_TYPE>
class ForwardingTypeUtil;
template <class t_TYPE, int k_CATEGORY>
struct ForwardingType_Imp;
// =============================
// class ForwardingType_Category
// =============================
struct ForwardingType_Category {
// This component-private struct provides a namespace for thpe type
// dispatch category enumeration values.
// CONSTANTS
enum {
e_LVALUE_REF, // Lvalue reference
e_MOVABLE_REF, // Movable (rvalue) reference
e_FUNCTION, // Function or pointer to function
e_ARRAY, // Array
e_BASIC, // Built-in, pointer, or enum type
e_CLASS // Class, struct or union
};
};
// =============================
// class ForwardingType_Dispatch
// =============================
template <class t_TYPE>
class ForwardingType_Dispatch {
// This component-private class template is a metafunction whose 'value'
// member is the forwarding category for the specified 't_TYPE'.
// PRIVATE TYPES
typedef ForwardingType_Category Cat; // Abbreviation
public:
enum {
value = (MovableRefUtil::IsLvalueReference<t_TYPE>::value
? Cat::e_LVALUE_REF
: MovableRefUtil::IsMovableReference<t_TYPE>::value
? Cat::e_MOVABLE_REF
: bsl::is_function<t_TYPE>::value ? Cat::e_FUNCTION
: bsl::is_array<t_TYPE>::value ? Cat::e_ARRAY
: bsl::is_fundamental<t_TYPE>::value ? Cat::e_BASIC
: bsl::is_pointer<t_TYPE>::value ? Cat::e_BASIC
: bsl::is_member_pointer<t_TYPE>::value ? Cat::e_BASIC
: bsl::is_enum<t_TYPE>::value ? Cat::e_BASIC
: Cat::e_CLASS)
};
};
// ====================
// class ForwardingType
// ====================
template <class t_TYPE>
class ForwardingType
: private ForwardingType_Imp<t_TYPE, ForwardingType_Dispatch<t_TYPE>::value> {
// This template metafunction has a member 'Type' computed such that, for a
// specified 't_TYPE' parameter, a function with argument of 't_TYPE' can
// be called efficiently from another function (e.g., a wrapper) by
// declaring the corresponding parameter of the other wrapper as 'typename
// ForwardingType<t_TYPE>::Type'. The 'Type' member is computed to
// minimize the number of expensive copies while forwarding the arguments
// as faithfully as possible.
// PRIVATE TYPES
typedef ForwardingType_Imp<t_TYPE, ForwardingType_Dispatch<t_TYPE>::value>
Imp;
public:
// TYPES
typedef typename Imp::Type Type;
// The type that should be used to forward 't_TYPE' through a chain of
// function calls.
typedef typename Imp::TargetType TargetType;
// The closest type used to "reconstitute" 't_TYPE' from
// 'ForwardingType<t_TYPE>::Type'. This type may differ from 't_TYPE'
// through the addition of a reference.
};
// ========================
// class ForwardingTypeUtil
// ========================
template <class t_TYPE>
class ForwardingTypeUtil
: private ForwardingType_Imp<t_TYPE, ForwardingType_Dispatch<t_TYPE>::value> {
// Provide a namespace for the 'forwardToTarget' function.
// PRIVATE TYPES
typedef ForwardingType_Imp<t_TYPE, ForwardingType_Dispatch<t_TYPE>::value>
Imp;
public:
// TYPES
typedef typename Imp::TargetType TargetType;
// The closest type used to "reconstitute" 't_TYPE' from
// 'ForwardingType<t_TYPE>::Type'. This type may differ from 't_TYPE'
// through the addition of a reference.
// CLASS METHODS static TargetType forwardToTarget(typename
// ForwardingType<t_TYPE>::Type v);
using Imp::forwardToTarget;
// Return (for the specified 'v' parameter) 'std::forward<t_TYPE>(v)',
// where 'v' is assumed to originally have been an argument of 't_TYPE'
// after forwarding through an intermediate call chain. Specifically,
// if 't_TYPE' is an rvalue type, return an rvalue reference to 'v',
// otherwise return 'v' unchanged, thus converting an rvalue copy into
// an rvalue move when possible. For compilers that do not support
// rvalue references, return 'v' unchanged. This function is intended
// to be called to forward an argument to the final target function of
// a forwarding call chain. Note that this function is not intended
// for use with 't_TYPE' parameters of 'volatile'-qualified rvalue
// type, which are effectively unheard of in real code and have strange
// and hard-to-understand rules.
};
// =========================
// class ConstForwardingType
// =========================
#ifndef BDE_OMIT_DEPRECATED
template <class t_TYPE>
struct ConstForwardingType : public ForwardingType<t_TYPE> {
//!DEPRECATED!: Use 'ForwardingType' instead.
};
#endif // BDE_OMIT_DEPRECATED
// ============================================================================
// INLINE DEFINITIONS
// ============================================================================
// BDE_VERIFY pragma: push // Relax some bde_verify rules in the imp section
// BDE_VERIFY pragma: -CD01 // Member function defined in class definition
// ========================
// class ForwardingType_Imp
// ========================
// PRIMARY TEMPLATE HAS NO DEFINITION
// PARTIAL SPECIALIZATIONS
template <class t_TYPE>
struct ForwardingType_Imp<t_TYPE, ForwardingType_Category::e_LVALUE_REF> {
// lvalue reference is forwarded unmodified.
// TYPES
typedef t_TYPE Type;
typedef t_TYPE TargetType;
// CLASS METHODS
static TargetType forwardToTarget(Type v)
// Return the specified 'v' argument.
{
return v;
}
};
template <class t_TYPE>
struct ForwardingType_Imp<t_TYPE, ForwardingType_Category::e_MOVABLE_REF> {
// Rvalue reference is forwarded as a reference to const lvalue.
// TYPES
typedef typename MovableRefUtil::RemoveReference<t_TYPE>::type UnrefType;
typedef const UnrefType& Type;
typedef t_TYPE TargetType;
// CLASS METHODS
static TargetType forwardToTarget(Type v)
{
// Since rvalues are forwarded as *const* lvalues, we must cast away
// the constness before converting to an rvalue reference. If 't_TYPE'
// is a const reference, then the constness will be reinstated on
// return.
// We split this cast up into two lines because Visual Studio 2015 and
// early versions of Visual Studio 2017 create a temporary in the
// one-liner.
UnrefType& result = const_cast<UnrefType&>(v);
return MovableRefUtil::move(result);
}
};
template <class t_TYPE>
struct ForwardingType_Imp<t_TYPE, ForwardingType_Category::e_FUNCTION> {
// Function type and is forwarded as function reference.
// TYPES
typedef t_TYPE& Type;
typedef t_TYPE& TargetType;
// CLASS METHODS
static TargetType forwardToTarget(Type v)
// Return the specified 'v' argument.
{
return v;
}
};
template <class t_TYPE, size_t k_NUM_ELEMENTS>
struct ForwardingType_Imp<t_TYPE[k_NUM_ELEMENTS],
ForwardingType_Category::e_ARRAY> {
// Array of known size and reference to array of known size is forwarded as
// pointer to array element type.
// TYPES
typedef t_TYPE *Type;
typedef t_TYPE (&TargetType)[k_NUM_ELEMENTS];
// CLASS METHODS
static TargetType forwardToTarget(Type v)
// Return the specified 'v', cast to a reference to array.
{
return reinterpret_cast<TargetType>(*v);
}
};
template <class t_TYPE>
struct ForwardingType_Imp<t_TYPE[], ForwardingType_Category::e_ARRAY> {
// Array of unknown size and reference to array of unknown size is
// forwarded as pointer to array element type.
// TYPES
typedef t_TYPE *Type;
typedef t_TYPE (&TargetType)[];
// CLASS METHODS
static TargetType forwardToTarget(Type v)
// Return the specified 'v' argument cast to a reference to array of
// unknown size.
{
return reinterpret_cast<TargetType>(*v);
}
};
template <class t_TYPE>
struct ForwardingType_Imp<t_TYPE, ForwardingType_Category::e_BASIC> {
// Rvalue of basic type is forwarded with any cv-qualifier removed.
// TYPES
typedef typename bsl::remove_cv<t_TYPE>::type Type;
typedef t_TYPE TargetType;
// CLASS METHODS
static TargetType& forwardToTarget(Type& v)
// Return the specified 'v' argument with cv qualifiers added to match
// the specified 'UNREF_TYPE'. The return type is a reference to a
// (possibly modifiable) 't_TYPE' object in order to work around
// certain dpkg bugs where an rvalue is being forwarded to a function
// that expects an lvalue.
{
return v;
}
};
template <class t_TYPE>
struct ForwardingType_Imp<t_TYPE, ForwardingType_Category::e_CLASS> {
// Rvalue of user type (i.e., class or union) is forwarded as a const
// reference.
// TYPES
typedef ForwardingType_Imp Imp;
typedef const t_TYPE& Type;
#if defined(BSLS_COMPILERFEATURES_SUPPORT_RVALUE_REFERENCES)
typedef MovableRef<t_TYPE> TargetType;
static TargetType forwardToTarget(Type v)
// Return the specified 'v' argument cast to a modifiable movable
// reference.
{
// Since rvalues are forwarded as *const* lvalues, we must cast away
// the constness before converting to an rvalue reference. If 't_TYPE'
// is a const reference, then the constness will be reinstated on
// return.
# if defined(BSLS_PLATFORM_CMP_MSVC)
// We use a C-style cast because Visual Studio 2013, 2015, and early
// versions of Visual Studio 2017 create a temporary with various
// formulations using C++ casts.
return MovableRefUtil::move((t_TYPE&)(v));
#else
// However, other platforms are known to complain about casting away
// the 'const' qualifier in 'Type' (i.e., in 'const t_TYPE&') unless a
// 'const_cast' is explicitly used.
return MovableRefUtil::move(const_cast<t_TYPE&>(v));
# endif
}
#else
typedef const t_TYPE& TargetType;
static TargetType forwardToTarget(Type v)
{
return v;
}
#endif
};
} // close package namespace
} // close enterprise namespace
#ifndef BDE_OPENSOURCE_PUBLICATION // BACKWARD_COMPATIBILITY
// ============================================================================
// BACKWARD COMPATIBILITY
// ============================================================================
#ifdef bslmf_ForwardingType
#undef bslmf_ForwardingType
#endif
#define bslmf_ForwardingType bslmf::ForwardingType
// !DEPRECATED! This alias is defined for backward compatibility.
#endif // BDE_OPENSOURCE_PUBLICATION -- BACKWARD_COMPATIBILITY
#ifdef bslmf_ConstForwardingType
#undef bslmf_ConstForwardingType
#endif
// BDE_VERIFY pragma: -SLM01 // Allow non-standard macro to leak from header
#define bslmf_ConstForwardingType bslmf::ConstForwardingType
// !DEPRECATED! This alias is defined for backward compatibility.
// BDE_VERIFY pragma: pop // Restore bde_verify rules
#endif
// ----------------------------------------------------------------------------
// Copyright 2015 Bloomberg Finance L.P.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// ----------------------------- END-OF-FILE ----------------------------------