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/*
* Copyright 2012 Facebook, Inc.
*
* 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.
*/
#ifndef FOLLY_BASE_FOREACH_H_
#define FOLLY_BASE_FOREACH_H_
/*
* Iterim macros (until we have C++0x range-based for) that simplify
* writing loops of the form
*
* for (Container<data>::iterator i = c.begin(); i != c.end(); ++i) statement
*
* Just replace the above with:
*
* FOR_EACH (i, c) statement
*
* and everything is taken care of.
*
* The implementation is a bit convoluted to make sure the container is
* only evaluated once (however, keep in mind that c.end() is evaluated
* at every pass through the loop). To ensure the container is not
* evaluated multiple times, the macro defines one do-nothing if
* statement to inject the Boolean variable FOR_EACH_state1, and then a
* for statement that is executed only once, which defines the variable
* FOR_EACH_state2 holding a reference to the container being
* iterated. The workhorse is the last loop, which uses the just defined
* reference FOR_EACH_state2.
*
* The state variables are nested so they don't interfere; you can use
* FOR_EACH multiple times in the same scope, either at the same level or
* nested.
*
* In optimized builds g++ eliminates the extra gymnastics entirely and
* generates code 100% identical to the handwritten loop.
*
* This will not work with temporary containers. Consider BOOST_FOREACH
* if you need that.
*/
#include <boost/type_traits/remove_cv.hpp>
namespace folly { namespace detail {
/*
* Simple template for obtaining the unqualified type given a generic
* type T. For example, if T is const int,
* typeof(remove_cv_from_expression(T())) yields int. Due to a bug in
* g++, you need to actually use
* typeof(remove_cv_from_expression(T())) instead of typename
* boost::remove_cv<T>::type. Note that the function
* remove_cv_from_expression is never defined - use it only inside
* typeof.
*/
template <class T> typename boost::remove_cv<T>::type
remove_cv_from_expression(T value);
}}
/*
* Use a "reference reference" (auto&&) to take advantage of reference
* collapsing rules, if available. In this case, FOR_EACH* will work with
* temporary containers.
*/
#define FB_AUTO_RR(x, y) auto&& x = y
/*
* The first AUTO should be replaced by decltype((c)) &
* FOR_EACH_state2, but bugs in gcc prevent that from functioning
* properly. The second pair of parens in decltype is actually
* required, see
* cpp-next.com/archive/2011/04/appearing-and-disappearing-consts-in-c/
*/
#define FOR_EACH(i, c) \
if (bool FOR_EACH_state1 = false) {} else \
for (auto & FOR_EACH_state2 = (c); \
!FOR_EACH_state1; FOR_EACH_state1 = true) \
for (auto i = FOR_EACH_state2.begin(); \
i != FOR_EACH_state2.end(); ++i)
/*
* Similar to FOR_EACH, but iterates the container backwards by
* using rbegin() and rend().
*/
#define FOR_EACH_R(i, c) \
if (bool FOR_EACH_R_state1 = false) {} else \
for (auto & FOR_EACH_R_state2 = (c); \
!FOR_EACH_R_state1; FOR_EACH_R_state1 = true) \
for (auto i = FOR_EACH_R_state2.rbegin(); \
i != FOR_EACH_R_state2.rend(); ++i)
/*
* Similar to FOR_EACH but also allows client to specify a 'count' variable
* to track the current iteration in the loop (starting at zero).
* Similar to python's enumerate() function. For example:
* string commaSeparatedValues = "VALUES: ";
* FOR_EACH_ENUMERATE(ii, value, columns) { // don't want comma at the end!
* commaSeparatedValues += (ii == 0) ? *value : string(",") + *value;
* }
*/
#define FOR_EACH_ENUMERATE(count, i, c) \
if (bool FOR_EACH_state1 = false) {} else \
for (auto & FOR_EACH_state2 = (c); \
!FOR_EACH_state1; FOR_EACH_state1 = true) \
if (size_t FOR_EACH_privateCount = 0) {} else \
if (const size_t& count = FOR_EACH_privateCount) {} else \
for (auto i = FOR_EACH_state2.begin(); \
i != FOR_EACH_state2.end(); ++FOR_EACH_privateCount, ++i)
/**
* Similar to FOR_EACH, but gives the user the key and value for each entry in
* the container, instead of just the iterator to the entry. For example:
* map<string, string> testMap;
* FOR_EACH_KV(key, value, testMap) {
* cout << key << " " << value;
* }
*/
#define FOR_EACH_KV(k, v, c) \
if (unsigned int FOR_EACH_state1 = 0) {} else \
for (FB_AUTO_RR(FOR_EACH_state2, (c)); \
!FOR_EACH_state1; FOR_EACH_state1 = 1) \
for (auto FOR_EACH_state3 = FOR_EACH_state2.begin(); \
FOR_EACH_state3 != FOR_EACH_state2.end(); \
FOR_EACH_state1 == 2 \
? ((FOR_EACH_state1 = 0), ++FOR_EACH_state3) \
: (FOR_EACH_state3 = FOR_EACH_state2.end())) \
for (auto &k = FOR_EACH_state3->first; \
!FOR_EACH_state1; ++FOR_EACH_state1) \
for (auto &v = FOR_EACH_state3->second; \
!FOR_EACH_state1; ++FOR_EACH_state1)
namespace folly { namespace detail {
// Boost 1.48 lacks has_less, we emulate a subset of it here.
template <typename T, typename U>
class HasLess {
struct BiggerThanChar { char unused[2]; };
template <typename C, typename D> static char test(decltype(C() < D())*);
template <typename, typename> static BiggerThanChar test(...);
public:
enum { value = sizeof(test<T, U>(0)) == 1 };
};
/**
* notThereYet helps the FOR_EACH_RANGE macro by opportunistically
* using "<" instead of "!=" whenever available when checking for loop
* termination. This makes e.g. examples such as FOR_EACH_RANGE (i,
* 10, 5) execute zero iterations instead of looping virtually
* forever. At the same time, some iterator types define "!=" but not
* "<". The notThereYet function will dispatch differently for those.
*
* Below is the correct implementation of notThereYet. It is disabled
* because of a bug in Boost 1.46: The filesystem::path::iterator
* defines operator< (via boost::iterator_facade), but that in turn
* uses distance_to which is undefined for that particular
* iterator. So HasLess (defined above) identifies
* boost::filesystem::path as properly comparable with <, but in fact
* attempting to do so will yield a compile-time error.
*
* The else branch (active) contains a conservative
* implementation.
*/
#if 0
template <class T, class U>
typename std::enable_if<HasLess<T, U>::value, bool>::type
notThereYet(T& iter, const U& end) {
return iter < end;
}
template <class T, class U>
typename std::enable_if<!HasLess<T, U>::value, bool>::type
notThereYet(T& iter, const U& end) {
return iter != end;
}
#else
template <class T, class U>
typename std::enable_if<
(std::is_arithmetic<T>::value && std::is_arithmetic<U>::value) ||
(std::is_pointer<T>::value && std::is_pointer<U>::value),
bool>::type
notThereYet(T& iter, const U& end) {
return iter < end;
}
template <class T, class U>
typename std::enable_if<
!(
(std::is_arithmetic<T>::value && std::is_arithmetic<U>::value) ||
(std::is_pointer<T>::value && std::is_pointer<U>::value)
),
bool>::type
notThereYet(T& iter, const U& end) {
return iter != end;
}
#endif
/**
* downTo is similar to notThereYet, but in reverse - it helps the
* FOR_EACH_RANGE_R macro.
*/
template <class T, class U>
typename std::enable_if<HasLess<U, T>::value, bool>::type
downTo(T& iter, const U& begin) {
return begin < iter--;
}
template <class T, class U>
typename std::enable_if<!HasLess<U, T>::value, bool>::type
downTo(T& iter, const U& begin) {
if (iter == begin) return false;
--iter;
return true;
}
} }
/*
* Iteration with given limits. end is assumed to be reachable from
* begin. end is evaluated every pass through the loop.
*
* NOTE: The type of the loop variable should be the common type of "begin"
* and "end". e.g. If "begin" is "int" but "end" is "long", we want "i"
* to be "long". This is done by getting the type of (true ? begin : end)
*/
#define FOR_EACH_RANGE(i, begin, end) \
for (auto i = (true ? (begin) : (end)); \
::folly::detail::notThereYet(i, (end)); \
++i)
/*
* Iteration with given limits. begin is assumed to be reachable from
* end by successive decrements. begin is evaluated every pass through
* the loop.
*
* NOTE: The type of the loop variable should be the common type of "begin"
* and "end". e.g. If "begin" is "int" but "end" is "long", we want "i"
* to be "long". This is done by getting the type of (false ? begin : end)
*/
#define FOR_EACH_RANGE_R(i, begin, end) \
for (auto i = (false ? (begin) : (end)); ::folly::detail::downTo(i, (begin));)
#endif
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