c – 使用可变参数模板和lambda函数的二进制搜索
作者：互联网
想想这个,
struct Person {
std::string name;
Person (const std::string& n) : name(n) {}
std::string getName(int, char) const {return name;} // int, char play no role in this
// simple example, but let's suppose that they are needed.
} *Bob = new Person("Bob"), *Frank = new Person("Frank"), *Mark = new Person("Mark"),
*Tom = new Person("Tom"), *Zack = new Person("Zack");
const std::vector<Person*> people = {Bob, Frank, Mark, Tom, Zack};
因为人们按名称排序,我们可以进行二分查找以找到具有特定名称的人的元素.我希望这个看起来像是这样的
Person* person = binarySearch (people, "Tom",
[](Person* p, int n, char c) {return p->getName(n,c);},
[](const std::string& x, const std::string& y) {return x.compare(y) < 0;}, 5, 'a');
所以模板函数binarySearch可以一般使用.我得到了以下工作：
#include <iostream>
#include <string>
#include <vector>
#include <functional>
struct Person {
std::string name;
Person (const std::string& n) : name(n) {}
std::string getName(int, char) const {return name;} // int, char play no role in this
// simple example, but let's supposes that they are needed.
} *Bob = new Person("Bob"), *Frank = new Person("Frank"), *Mark = new Person("Mark"),
*Tom = new Person("Tom"), *Zack = new Person("Zack");
const std::vector<Person*> people = {Bob, Frank, Mark, Tom, Zack};
template <typename Container, typename Ret>
typename Container::value_type binarySearch (const Container& container, const Ret& value,
std::function<Ret(const typename Container::value_type&, int, char)> f,
std::function<bool(const Ret&, const Ret&)> comp,
typename Container::difference_type low, typename Container::difference_type high,
int n, char c) {
if (low > high)
std::cout << "Error! Not found!\n";
const typename Container::difference_type mid = (low + high) / 2;
const Ret& r = f(container[mid], n, c);
if (r == value)
return container[mid];
if (comp(r, value))
return binarySearch (container, value, f, comp, mid + 1, high, n, c);
return binarySearch (container, value, f, comp, low, mid - 1, n, c);
}
template <typename Container, typename Ret>
typename Container::value_type binarySearch (const Container& container, const Ret& value,
std::function<Ret(const typename Container::value_type&, int, char)> f,
std::function<bool(const Ret&, const Ret&)> comp, int n, char c) {
return binarySearch (container, value, f, comp, 0, container.size() - 1, n, c);
}
int main() {
const Person* person = binarySearch<std::vector<Person*>, std::string>
(people, "Tom", &Person::getName,
[](const std::string& x, const std::string& y) {return x.compare(y) < 0;}, 5, 'a');
std::cout << person->getName(5,'a') << '\n'; // Tom
}
但是现在由于我不理解的原因,我无法用Args替换特定的参数int,char …你可以继续放置Args … args和args ……在上面需要的地方代码,它不会编译.这有什么不对？如何在概括中执行这最后一步？或者整个方法应该改变？
这是我试过的：
template <typename Container, typename Ret, typename... Args>
typename Container::value_type binarySearch (const Container& container, const Ret& value,
std::function<Ret(const typename Container::value_type&, Args...)> f,
std::function<bool(const Ret&, const Ret&)> comp,
typename Container::difference_type low, typename Container::difference_type high,
Args... args) {
if (low > high)
std::cout << "Error! Not found!\n";
const typename Container::difference_type mid = (low + high) / 2;
const Ret& r = f(container[mid], args...);
if (r == value)
return container[mid];
if (comp(r, value))
return binarySearch (container, value, f, comp, mid + 1, high, args...);
return binarySearch (container, value, f, comp, low, mid - 1, args...);
}
template <typename Container, typename Ret, typename... Args>
typename Container::value_type binarySearch (const Container& container, const Ret& value,
std::function<Ret(const typename Container::value_type&, Args...)> f,
std::function<bool(const Ret&, const Ret&)> comp, Args... args) {
return binarySearch (container, value, f, comp, 0, container.size() - 1, args...);
}
int main() {
const Person* person = binarySearch<std::vector<Person*>, std::string> (people, "Tom",
&Person::getName,
[](const std::string& x, const std::string& y) {return x.compare(y) < 0;}, 5, 'a');
std::cout << person->getName(5,'a') << '\n';
}
GCC 4.9.2：
[Error] no matching function for call to 'binarySearch(std::vector<Person*>&, const char [4], main()::__lambda0, main()::__lambda1, int, char)'
template argument deduction/substitution failed:
[Note] 'main()::__lambda0' is not derived from 'std::function<std::basic_string<char>(Person* const&, Args ...)>'
更新：
在研究了Yakk的解决方案之后,我将我的解决方案改编为以下(使用更多的第一原则而不是std :: equal_range)：
#include <iostream>
#include <iterator>
template <typename Container, typename T, typename Comparator = std::less<T>>
typename Container::value_type binarySearchRandomAccessIterator (const Container& container, T&& value, Comparator&& compare, typename Container::difference_type low, typename Container::difference_type high) {
if (low > high)
{std::cout << "Error! Not found!\n"; return container[high];}
const typename Container::difference_type mid = (low + high) / 2;
const auto& t = compare.function(container[mid]); // Using 'const T& t' does not compile.
if (t == value)
return container[mid];
if (compare.comparator(t, value)) // 't' is less than 'value' according to compare.comparator, so search in the top half.
return binarySearchRandomAccessIterator (container, value, compare, mid + 1, high);
return binarySearchRandomAccessIterator (container, value, compare, low, mid - 1); // i.e. 'value' is less than 't' according to compare.comparator, so search in the bottom half.
}
template <typename ForwardIterator, typename T, typename Comparator = std::less<T>>
typename std::iterator_traits<ForwardIterator>::value_type binarySearchNonRandomAccessIterator (ForwardIterator first, ForwardIterator last, T&& value, Comparator&& compare) {
ForwardIterator it;
typename std::iterator_traits<ForwardIterator>::difference_type count, step;
count = std::distance(first, last);
while (count > 0) { // Binary search using iterators carried out.
it = first;
step = count / 2;
std::advance(it, step); // This is done in O(step) time since ForwardIterator is not a random-access iterator (else it is done in constant time). But the good news is that 'step' becomes half as small with each iteration of this loop.
const auto& t = compare.function(*it); // Using 'const T& t' does not compile.
if (compare.comparator(t, value)) { // 't' is less than 'value' according to compare.comparator, so search in the top half.
first = ++it; // Thus first will move to one past the half-way point, and we search from there.
count -= step + 1; // count is decreased by half plus 1.
}
else // 't' is greater than 'value' according to compare.comparator, so remain in the bottom half.
count = step; // 'count' and 'step' are both decreased by half.
}
if (compare.function(*first) != value)
std::cout << "Error! Not found!\n";
return *first;
}
template <typename Container, typename T, typename Comparator = std::less<T>> // Actually the version below could be used if Container has a random-access iterator. It would be with the same time complexity since std::advance has O(1) time complexity for random-access iterators.
typename std::enable_if<std::is_same<typename std::iterator_traits<typename Container::iterator>::iterator_category, std::random_access_iterator_tag>::value, typename Container::value_type>::type
binarySearch (const Container& container, T&& value, Comparator&& compare = {}) {
std::cout << "Calling binarySearchWithRandomAccessIterator...\n";
return binarySearchRandomAccessIterator (container, value, compare, 0, container.size() - 1);
}
// Overload used if Container does not have a random-access iterator.
template <typename Container, typename T, typename Comparator = std::less<T>>
typename std::enable_if<!std::is_same<typename std::iterator_traits<typename Container::iterator>::iterator_category, std::random_access_iterator_tag>::value, typename Container::value_type>::type
binarySearch (const Container& container, T&& value, Comparator&& compare = {}) {
std::cout << "Calling binarySearchNonRandomAccessIterator...\n";
return binarySearchNonRandomAccessIterator (std::begin(container), std::end(container), value, compare);
}
template <typename Function, typename Comparator>
struct FunctionAndComparator {
Function function;
Comparator comparator;
FunctionAndComparator (Function&& f, Comparator&& c) : function(std::forward<Function>(f)), comparator(std::forward<Comparator>(c)) {}
};
template <typename Function, typename Comparator = std::less<>>
FunctionAndComparator<std::decay_t<Function>, std::decay_t<Comparator>> functionAndComparator (Function&& f, Comparator&& c = {}) {
return {std::forward<Function>(f), std::forward<Comparator>(c)};
}
#include <string>
#include <vector>
#include <list>
struct Person {
std::string name;
Person (const std::string& n) : name(n) {}
std::string getName (int, char) const {return name;} // int, char play no role in this simple example, but let's supposes that they are needed.
} *Bob = new Person("Bob"), *Frank = new Person("Frank"), *Mark = new Person("Mark"), *Tom = new Person("Tom"), *Zack = new Person("Zack");
const std::vector<Person*> peopleVector = {Bob, Frank, Mark, Tom, Zack};
const std::list<Person*> peopleList = {Bob, Frank, Mark, Tom, Zack};
int main() {
Person* tom = binarySearch (peopleVector, "Tom", functionAndComparator([](const Person* p) {return p->getName(5,'a');}, [](const std::string& x, const std::string& y) {return x.compare(y) < 0;}));
if (tom) std::cout << tom->getName(5,'a') << " found.\n";
Person* bob = binarySearch (peopleVector, "Bob", functionAndComparator([](const Person* p) {return p->getName(3,'k');})); // The default comparator, std::less<std::string>, is actually the same as the comparator used above.
if (bob) std::cout << bob->getName(3,'k') << " found.\n";
Person* frank = binarySearch (peopleList, "Frank", functionAndComparator([](const Person* p) {return p->getName(8,'b');}));
if (frank) std::cout << frank->getName(8,'b') << " found.\n";
Person* zack = binarySearch (peopleList, "Zack", functionAndComparator([](const Person* p) {return p->getName(2,'c');}));
if (zack) std::cout << zack->getName(2,'c') << " found.\n";
Person* mark = binarySearch (peopleList, "Mark", functionAndComparator([](const Person* p) {return p->getName(6,'d');}));
if (mark) std::cout << mark->getName(6,'d') << " found.\n";
}
解决方法:
在我看来
Person* person = binarySearch (people, "Tom",
[](Person* p, int n, char c) {return p->getName(n,c);},
[](const std::string& x, const std::string& y) {return x.compare(y) < 0;}, 5, 'a');
是一种可怕的语法.你的binarySearch函数对于太多事情都是可以解决的.
但首先,出了什么问题：由于lambda不是std :: function,因此出现了模糊错误.它试图从lambda中推导出std :: function类型,并因为它们是不相关的类型而失败.从其他地方推断Args ……的能力无济于事.
您可以将std :: function参数包装在：
template<class T>struct tag{using type=T;};
template<class Tag>using type_t=typename Tag::type;
template<class T>using identity=type_t<tag<T>>;
同一性LT;的std ::功能<不管…> >并且您的代码将开始编译(因为Args …在其他地方推断).同一性LT;？>阻止该参数的模板类型推导,因此编译器不再尝试,而是从其他参数中推断出类型.
但是,这不是一个好的解决方案.
更好的解决方案是使f和c的类型为F和C – 根本不使它们成为std :: functions.这消除了无意义的类型擦除开销,并且消除了对身份的需要<？>
这仍然不是一个好的解决方案,因为你的模板函数做了很多事情,很少有.相反,将您的操作分解为更简单的问题,然后将它们组合在一起：
首先,我们已经有了std :: equal_range,这将是一个比你可能编写的更好的二进制搜索.编写一个返回单个元素并获取容器的函数似乎是合理的,因为使用迭代器很烦人.
为了解决这个问题,首先我们编写一些基于范围的样板：
namespace adl_aux {
using std::begin; using std::end;
template<class R>
auto adl_begin(R&&)->decltype(begin(std::declval<R>()));
template<class R>
auto adl_end(R&&)->decltype(end(std::declval<R>()));
}
template<class R>
using adl_begin = decltype(adl_aux::adl_begin(std::declval<R>));
template<class R>
using adl_end = decltype(adl_aux::adl_end(std::declval<R>));
template<class R>using iterator_t = adl_begin<R>;
template<class R>using value_t = std::remove_reference_t<decltype(*std::declval<iterator_t<R>>())>;
这允许我们支持std :: containers和数组以及第三方可迭代容器和范围. adl_ stuff为我们做了开始和结束的参数依赖查找. iterator_t和value_t对SFINAE友好地确定范围的值和迭代器类型.
现在,bin_search在该样板之上：
template<class R, class T, class F=std::less<T>>
value_t<R>* bin_search( R&& r, T&& t, F&& f={} ) {
using std::begin; using std::end;
auto range = std::equal_range( begin(r), end(r), std::forward<T>(t), std::forward<F>(f) );
if (range.first==range.second) return nullptr;
return std::addressof( *range.first ); // in case someone overloaded `&`
}
它返回一个指向排序f下的元素t的指针,假设R在它下面排序,如果它存在,否则为nullptr.
下一部分是你的订购混乱：
[](Person* p, int n, char c) {return p->getName(n,c);},
[](const std::string& x, const std::string& y) {return x.compare(y) < 0;}, 5, 'a'
首先,摆脱那个args …：
[](int n, char c){
return [n,c](Person* p) {return p->getName(n,c);}
}(5,'a'),
[](const std::string& x, const std::string& y) {return x.compare(y) < 0;}
如果你真的需要在一行上做,直接进行绑定.
接下来,我们想要order_by：
template<class F, class C>
struct order_by_t : private F, private C {
F const& f() const { return *this; }
C const& c() const { return *this; }
template<class T>
auto f(T&&t)const
->decltype( std::declval<F const&>()(std::declval<T>()) )
{
return f()(std::forward<T>(t));
}
template<class T, class... Unused> // Unused to force lower priority
auto f(T&&t, Unused&&... ) const
-> std::decay_t<T>
{ return std::forward<T>(t); }
template<class Lhs, class Rhs>
bool operator()(Lhs&& lhs, Rhs&& rhs) const {
return c()( f(std::forward<Lhs>(lhs)), f(std::forward<Rhs>(rhs)) );
}
template<class F0, class C0>
order_by_t( F0&& f_, C0&& c_ ):
F(std::forward<F0>(f_)), C(std::forward<C0>(c_))
{}
};
template<class C=std::less<>, class F>
auto order_by( F&& f, C&& c={} )
-> order_by_t<std::decay_t<F>, std::decay_t<C>>
{ return {std::forward<F>(f), std::forward<C>(c)}; }
order_by从域到范围的投影,以及可选地在该范围上的排序,并在域上产生排序.
order_by(
[](int n, char c){
return [n,c](Person const* p)
->decltype(p->getName(n,c)) // SFINAE enabled
{return p->getName(n,c);};
}(5,'a'),
[](const std::string& x, const std::string& y) {return x.compare(y) < 0;}
}
现在是符合您要求的Person const * s的排序.
然后我们将其提供给bin_search：
auto ordering = order_by(
[](int n, char c){
return [n,c](Person const* p)
->decltype(p->getName(n,c)) // SFINAE enabled
{return p->getName(n,c);}
}(5,'a'),
[](const std::string& x, const std::string& y) {return x.compare(y) < 0;}
);
Person*const* p = bin_search( people, "Tom", ordering );
现在,必须要注意使order_by一个“透明”的函数对象,它接受可以投影的东西(在投影下)而不能(它们直接传递给比较器).
这要求投影操作是SFINAE友好的(即,它“早期失败”).为此,我明确确定了它的返回类型. (下面我们看到这不是必需的,但它可能在更复杂的情况下).
有趣的是,你的[](const std :: string& x,const std :: string& y){return x.compare(y)< 0;}与运算符一致<在std :: string上,你可以删除它(并使order_by更简单).但是,我怀疑你的真实用例需要它,并且强化order_by是一个有用的功能. 最后,请注意这部分：
[](int n, char c){
return [n,c](Person const* p)
->decltype(p->getName(n,c)) // SFINAE enabled
{return p->getName(n,c);}
}(5,'a'),
是丑陋的,可以替换为：
[](Person const* p)
->decltype(p->getName(5,'a')) // SFINAE enabled
{return p->getName(5,'a');}
这不那么难看.另外,因为lambda的参数检查就足够了,我们可以删除SFINAE显式返回类型的东西：
[](Person const* p)
{return p->getName(5,'a');}
我们完成了. Simpler example：
auto ordering = order_by(
[](Person const* p)
{return p->getName(5,'a');}
);
Person*const* p = bin_search( people, "Tom", ordering );
甚至：
Person*const* p = bin_search( people, "Tom",
order_by( [](Person const* p) {return p->getName(5,'a');} )
);
看起来不那么难看,不是吗？
哦,并且：
using std::literals;
Person*const* p = bin_search( people, "Tom"s,
order_by( [](Person const* p) {return p->getName(5,'a');} )
);
可能会有更好的性能,因为它会避免在每次比较时重复构造一个std :: string(“Tom”).类似地,getName返回一个std :: string const& (如果可能的话)也可以提升性能. “投影lambda”可能必须有一个 – > decltype(auto)来实现第二次提升.
我上面用了一些C 14. std :: remove_reference_t<？> (和类似的)别名可以用typename std :: remove_reference<？> :: type替换,或者你可以编写自己的_t别名.使用decltype(auto)的建议可以用C 11中的decltype(返回表达式)替换.
order_by_t使用继承来存储F和C,因为它们很可能是空类,所以我想利用空基优化.
标签：variadic,c,c11,templates,binary 来源： https://codeday.me/bug/20190824/1708833.html