[英]Trouble with member function pointer as template argument
由於F
類型不符合專業要求,因此下面的代碼中的注釋行將無法編譯。 有人可以解釋為什么嗎?
#include <memory>
#include <functional>
#include <map>
#include <tuple>
template <typename R, typename T, typename... Args>
std::function<R(Args...)> memoizeMemberFunction (R T::*f(Args...), const T* t) {
auto cache = std::make_shared<std::map<std::tuple<const T*, Args...>, R>>();
return ([f, cache](T* t, Args... args) {
const std::tuple<const T*, Args...> tuple(t, args...);
if (cache->find(tuple) == cache->end())
(*cache)[tuple] = (t->*f)(args...); // Insert 'tuple' as a new key in the map *cache.
return (*cache)[tuple];
});
}
template <typename Class, typename Fptr, Fptr> struct MemberFunctionMemoizer;
template <typename Class, typename R, typename... Args, R Class::*F(Args...)>
struct MemberFunctionMemoizer<Class, R (Class::*)(Args...) const, F> {
static std::function<R(Class*, Args...)>& get (const Class* p) {
static std::function<R (Args...)> memoizedF (memoizeMemberFunction(F, p));
return memoizedF;
}
};
struct FibonacciCalculator {
unsigned long calculate(unsigned num) const {
using F = MemberFunctionMemoizer<FibonacciCalculator,
unsigned long (FibonacciCalculator::*)(unsigned) const, &FibonacciCalculator::calculate>;
// return (num < 2) ? num : F::get(this)(num - 1) + F::get(this)(num - 2);
// Won't compile because F does not meet the specialization.
}
};
#include <iostream>
int main() {
FibonacciCalculator fib;
std::cout << fib.calculate(10) << '\n';
}
我在這里想念什么嗎? 如何獲得F
來滿足專業化要求? 我嘗試從圖片中刪除const限定詞,但是仍然存在相同的問題。
我也想保持使用成員函數指針作為模板參數的設計,即使通過使用非成員函數指針可以解決此特定問題。
您遇到的最大問題是,您正試圖記住簽名功能:
unsigned long calculate();
但是您正在使用num - 1
或num - 2
調用備忘錄緩存的返回函數F::get(this)
。
換句話說:記憶依賴於以下事實:“使用相同參數調用的函數會產生相同的返回值”,但是您要記憶的函數不帶任何參數(該參數本身沒有問題),但是您不應傳遞它任何參數。
您當前的FibonacciCalculator類無法使用記憶,因為它已被實現。
我已經實現了一種可能會實現您希望的效果; 它計算斐波那契數,並記住成員函數調用...注意: memoizeMemberFunction()
有一個額外的“噪聲”,但輸出表明它正在工作-您將在輸出中看到函數調用被使用兩次且僅被計算一旦。
#include <iostream>
#include <memory>
#include <functional>
#include <map>
#include <tuple>
template <typename R, typename T, typename... Args>
std::function<R(Args...)> memoizeMemberFunction (R (T::*f)(Args...), T* t) {
auto cache = std::make_shared<std::map<std::tuple<T*, Args...>, R>>();
return [f, t, cache](Args... args) {
const std::tuple<T*, Args...> tuple(t, args...);
if (cache->find(tuple) == cache->end()) {
(*cache)[tuple] = (t->*f)(args...); // Insert 'tuple' as a new key in the map *cache.
std::cout << "Computed f(";
int dummy[sizeof...(Args)] = { (std::cout << args << ", ", 0)... };
std::cout << ") = " << (*cache)[tuple] << std::endl;
}
std::cout << "Using f(";
int dummy[sizeof...(Args)] = { (std::cout << args << ", ", 0)... };
std::cout << ") = " << (*cache)[tuple] << std::endl;
return (*cache)[tuple];
};
}
struct FibonacciCalculator {
unsigned long num;
unsigned long calculate(unsigned long n = (unsigned long)-1) {
static auto memoizedF (memoizeMemberFunction(&FibonacciCalculator::calculate, this));
if( n==(unsigned long)-1 )
return memoizedF(num);
return (n < 2) ? n : memoizedF(n-1) + memoizedF(n-2);
}
};
int main() {
FibonacciCalculator fib{ 10 };
std::cout << fib.calculate() << '\n';
}
感謝dyp糾正了我在語法方面的艱苦努力,我完成了我打算要做的事情:
#include <memory>
#include <functional>
#include <map>
#include <tuple>
template <typename R, typename T, typename... Args>
std::function<R(Args...)> memoizeMemberFunction (R (T::*f)(Args...), T* t) {
auto cache = std::make_shared<std::map<std::tuple<T*, Args...>, R>>();
return ([f, cache, t](Args... args) {
const std::tuple<T*, Args...> tuple(t, args...);
if (cache->find(tuple) == cache->end())
(*cache)[tuple] = (t->*f)(args...); // Insert 'tuple' as a new key in the map *cache.
return (*cache)[tuple];
});
}
template <typename R, typename T, typename... Args>
std::function<R(Args...)> memoizeConstMemberFunction (R (T::*f)(Args...) const, const T* t) {
auto cache = std::make_shared<std::map<std::tuple<const T*, Args...>, R>>();
return ([f, cache, t](Args... args) {
const std::tuple<const T*, Args...> tuple(t, args...);
if (cache->find(tuple) == cache->end())
(*cache)[tuple] = (t->*f)(args...); // Insert 'tuple' as a new key in the map *cache.
return (*cache)[tuple];
});
}
template <typename Class, typename Fptr, Fptr> struct MemberFunctionMemoizer;
template <typename Class, typename Fptr, Fptr> struct ConstMemberFunctionMemoizer;
template <typename Class, typename R, typename... Args, R (Class::*F)(Args...) const>
struct ConstMemberFunctionMemoizer<Class, R (Class::*)(Args...) const, F> {
static std::function<R(Args...)>& get (const Class* p) {
static std::function<R (Args...)> memoizedF (memoizeConstMemberFunction(F, p));
return memoizedF;
}
};
template <typename Class, typename R, typename... Args, R (Class::*F)(Args...)>
struct MemberFunctionMemoizer<Class, R (Class::*)(Args...), F> {
static std::function<R(Args...)>& get (Class* p) {
static std::function<R (Args...)> memoizedF (memoizeMemberFunction(F, p));
return memoizedF;
}
};
// Testing
#include <iostream>
#include <vector>
struct FibonacciCalculator {
std::vector<unsigned long> computed;
unsigned long calculate (unsigned num) const {
using F = ConstMemberFunctionMemoizer<FibonacciCalculator, unsigned long (FibonacciCalculator::*)(unsigned) const, &FibonacciCalculator::calculate>; // 'decltype(&FibonacciCalculator::calculate)' can be used in place of 'unsigned long (FibonacciCalculator::*)(unsigned) const'.
return (num < 2) ? num : F::get(this)(num - 1) + F::get(this)(num - 2);
}
unsigned long calculateAndStore (unsigned num) {
using F = MemberFunctionMemoizer<FibonacciCalculator, unsigned long (FibonacciCalculator::*)(unsigned), &FibonacciCalculator::calculateAndStore>; // 'decltype(&FibonacciCalculator::calculateAndStore)' can be used in place of 'unsigned long (FibonacciCalculator::*)(unsigned)'.
const unsigned long result = (num < 2) ? num : F::get(this)(num - 1) + F::get(this)(num - 2);
computed.push_back(result);
return result;
}
};
int main() {
FibonacciCalculator fib;
for (unsigned i = 1; i < 20; i++)
std::cout << fib.calculate(i) << ' '; // 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181
std::cout << '\n';
for (unsigned i = 1; i < 20; i++)
std::cout << fib.calculateAndStore(i) << ' '; // 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181
std::cout << '\n';
for (unsigned long x : fib.computed)
std::cout << x << ' '; // 1 1 0 1 1 2 2 3 3 5 5 8 8 13 13 21 21 34 34 55 55 89 89 144 144 233 233 377 377 610 610 987 987 1597 1597 2584 2584 4181
std::cout << '\n';
}
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