这是我正在尝试的事情的进一步发展我最近的问题。
我想创建一个
bool(double, double)constexpr如何在 C++ 中执行与以下工作 Python 代码等效的操作：
#! /usr/bin/env python3
class FixPrecCompare:
'''Compares two floats to the given precision'''
def __init__(self, numDigits: int) -> None:
self.factor = 10 ** numDigits
def __call__(self, a: float, b: float) -> bool:
return int(a * self.factor) == int(b * self.factor)
class AreClose:
'''Same as Python's inbuilt math.isclose [which should be called areclose :-)]'''
def __init__(self, absTol = 0, relTol = 1e-09) -> None:
self.absTol = absTol; self.relTol = relTol
def __call__(self, a: float, b: float) -> bool:
diff = abs(a - b)
return diff <= self.absTol or \
diff <= abs(self.relTol * a) or \
diff <= abs(self.relTol * b)
# Now for signature compatibility (which is not a matter in Python)
# I make a function which internally uses the functor
def makeApproxCompare(FunctorType: type, *args):
functor = FunctorType(*args)
def fn(a: float, b: float) -> bool:
return functor(a, b)
return fn
approxCompare2 = makeApproxCompare(FixPrecCompare, 2)
approxCompare3 = makeApproxCompare(FixPrecCompare, 3)
areClose = makeApproxCompare(AreClose, 1e-3)
a, b = 4.561, 4.569
print(f"Compare {a:.3f}, {b:.3f} to 2 decimals: {approxCompare2(a, b)}")
print(f"Compare {a:.3f}, {b:.3f} to 3 decimals: {approxCompare3(a, b)}")
a, b = 2.7099, 2.7101
print(f"Compare {a} to {b} to 3 decimals: {approxCompare3(a, b)}")
print(f"Compare {a} to {b} using areClose: {areClose(a, b)}")
我的尝试：
typedef bool(*SuccessTester)(double, double);
#include <cmath>
#include <iostream>
using namespace std;
struct FixPrecCompare
{
FixPrecCompare(int numDigits): factor(pow(10, numDigits)) {}
bool operator()(double a, double b) { return int(a * factor) == int(b * factor); }
private:
double factor;
};
struct AreClose
{
AreClose(double absTol, double relTol): abs_tol{absTol}, rel_tol{relTol} {}
bool operator()(double a, double b)
{
double diff = fabs(a - b);
return diff <= abs_tol ||
diff <= fabs(rel_tol * a) ||
diff <= fabs(rel_tol * b);
}
private:
double abs_tol, rel_tol;
};
template<typename Functor>
struct Helper
{
static Functor fr;
static bool func(double a, double b) { return fr(a, b); }
};
template<typename Functor, typename ... ArgTypes>
SuccessTester makeSuccessTester(ArgTypes ... args)
{
Helper<Functor> h;
h.fr = Functor{args ...};
return &h.func;
}
int main()
{
SuccessTester fp;
cout << boolalpha;
double a, b;
a = 4.561; b = 4.569;
fp = makeSuccessTester<FixPrecCompare>(2);
cout << "Compare " << a << ", " << b << " to 2 decimals: " << fp(a, b) << endl;
fp = makeSuccessTester<FixPrecCompare>(3);
cout << "Compare " << a << ", " << b << " to 3 decimals: " << fp(a, b) << endl;
a = 2.7099; b = 2.7101;
cout << "Compare " << a << ", " << b << " to 3 decimals: " << fp(a, b) << endl;
fp = makeSuccessTester<AreClose>(1e-3, 1e-10);
cout << "Compare " << a << ", " << b << " using areClose: " << fp(a, b) << endl;
}
但是在 GCC 上这给出了：
/usr/bin/ld: /tmp/ccEkJxbk.o: warning: relocation against `_ZN6HelperI14FixPrecCompareE2frE' in read-only section `.text._ZN6HelperI14FixPrecCompareE4funcEdd[_ZN6HelperI14FixPrecCompareE4funcEdd]'
/usr/bin/ld: /tmp/ccEkJxbk.o: in function `bool (*makeSuccessTester<FixPrecCompare, int>(int))(double, double)':
<src>:(.text._Z17makeSuccessTesterI14FixPrecCompareJiEEPFbddEDpT0_[_Z17makeSuccessTesterI14FixPrecCompareJiEEPFbddEDpT0_]+0x38): undefined reference to `Helper<FixPrecCompare>::fr'
/usr/bin/ld: /tmp/ccEkJxbk.o: in function `bool (*makeSuccessTester<AreClose, double, double>(double, double))(double, double)':
<src>:(.text._Z17makeSuccessTesterI8AreCloseJddEEPFbddEDpT0_[_Z17makeSuccessTesterI8AreCloseJddEEPFbddEDpT0_]+0x4e): undefined reference to `Helper<AreClose>::fr'
/usr/bin/ld: <src>:(.text._Z17makeSuccessTesterI8AreCloseJddEEPFbddEDpT0_[_Z17makeSuccessTesterI8AreCloseJddEEPFbddEDpT0_]+0x55): undefined reference to `Helper<AreClose>::fr'
/usr/bin/ld: /tmp/ccEkJxbk.o: in function `Helper<FixPrecCompare>::func(double, double)':
<src>:(.text._ZN6HelperI14FixPrecCompareE4funcEdd[_ZN6HelperI14FixPrecCompareE4funcEdd]+0x2b): undefined reference to `Helper<FixPrecCompare>::fr'
/usr/bin/ld: /tmp/ccEkJxbk.o: in function `Helper<AreClose>::func(double, double)':
<src>:(.text._ZN6HelperI8AreCloseE4funcEdd[_ZN6HelperI8AreCloseE4funcEdd]+0x2b): undefined reference to `Helper<AreClose>::fr'
/usr/bin/ld: warning: creating DT_TEXTREL in a PIE
collect2: error: ld returned 1 exit status
Clang 更好一些：
<src>:40:7: warning: instantiation of variable 'Helper<FixPrecCompare>::fr' required here, but no definition is available [-Wundefined-var-template]
h.fr = Functor{args ...};
^
<src>:52:10: note: in instantiation of function template specialization 'makeSuccessTester<FixPrecCompare, int>' requested here
fp = makeSuccessTester<FixPrecCompare>(2);
^
<src>:32:20: note: forward declaration of template entity is here
static Functor fr;
^
<src>:40:7: note: add an explicit instantiation declaration to suppress this warning if 'Helper<FixPrecCompare>::fr' is explicitly instantiated in another translation unit
h.fr = Functor{args ...};
^
<src>:40:7: warning: instantiation of variable 'Helper<AreClose>::fr' required here, but no definition is available [-Wundefined-var-template]
h.fr = Functor{args ...};
^
<src>:59:10: note: in instantiation of function template specialization 'makeSuccessTester<AreClose, double, double>' requested here
fp = makeSuccessTester<AreClose>(1e-3, 1e-10);
^
<src>:32:20: note: forward declaration of template entity is here
static Functor fr;
^
<src>:40:7: note: add an explicit instantiation declaration to suppress this warning if 'Helper<AreClose>::fr' is explicitly instantiated in another translation unit
h.fr = Functor{args ...};
^
2 warnings generated.
/usr/bin/ld: /tmp/<src>-ce7af1.o: in function `bool (*makeSuccessTester<FixPrecCompare, int>(int))(double, double)':
<src>:(.text._Z17makeSuccessTesterI14FixPrecCompareJiEEPFbddEDpT0_[_Z17makeSuccessTesterI14FixPrecCompareJiEEPFbddEDpT0_]+0x1a): undefined reference to `Helper<FixPrecCompare>::fr'
/usr/bin/ld: /tmp/<src>-ce7af1.o: in function `bool (*makeSuccessTester<AreClose, double, double>(double, double))(double, double)':
<src>:(.text._Z17makeSuccessTesterI8AreCloseJddEEPFbddEDpT0_[_Z17makeSuccessTesterI8AreCloseJddEEPFbddEDpT0_]+0x28): undefined reference to `Helper<AreClose>::fr'
/usr/bin/ld: /tmp/<src>-ce7af1.o: in function `Helper<FixPrecCompare>::func(double, double)':
<src>:(.text._ZN6HelperI14FixPrecCompareE4funcEdd[_ZN6HelperI14FixPrecCompareE4funcEdd]+0x1f): undefined reference to `Helper<FixPrecCompare>::fr'
/usr/bin/ld: /tmp/<src>-ce7af1.o: in function `Helper<AreClose>::func(double, double)':
<src>:(.text._ZN6HelperI8AreCloseE4funcEdd[_ZN6HelperI8AreCloseE4funcEdd]+0x1f): undefined reference to `Helper<AreClose>::fr'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
但我不确定如何修复代码以使其按预期工作。请帮忙！谢谢！
您的方法的主要问题是您试图同时为同一个静态成员分配多个不同的值。
Helper<FixPrecCompare>::frFixPrecCompare(2)FixPrecCompare(3)std::functiondouble#include <cmath>
#include <iostream>
using namespace std;
typedef bool(*SuccessTester)(double, double);
template<int numDigits>
struct FixPrecCompare
{
static bool compare(double a, double b) { return int(a * factor) == int(b * factor); }
private:
static constexpr double factor = pow(10, numDigits);
};
template<double abs_tol, double rel_tol>
struct AreClose
{
static bool compare(double a, double b)
{
double diff = fabs(a - b);
return diff <= abs_tol ||
diff <= fabs(rel_tol * a) ||
diff <= fabs(rel_tol * b);
}
};
template<typename Functor>
SuccessTester makeSuccessTester()
{
return &Functor::compare;
}
int main()
{
SuccessTester fp;
cout << boolalpha;
double a, b;
a = 4.561; b = 4.569;
fp = makeSuccessTester<FixPrecCompare<2>>();
cout << "Compare " << a << ", " << b << " to 2 decimals: " << fp(a, b) << endl;
fp = makeSuccessTester<FixPrecCompare<3>>();
cout << "Compare " << a << ", " << b << " to 3 decimals: " << fp(a, b) << endl;
a = 2.7099; b = 2.7101;
cout << "Compare " << a << ", " << b << " to 3 decimals: " << fp(a, b) << endl;
fp = makeSuccessTester<AreClose<1e-3, 1e-10>>();
cout << "Compare " << a << ", " << b << " using areClose: " << fp(a, b) << endl;
}
#include <cmath>
#include <iostream>
#include <functional>
using namespace std;
using SuccessTester = std::function<bool(double, double)>;
struct FixPrecCompare
{
FixPrecCompare(int numDigits): factor(pow(10, numDigits)) {}
bool operator()(double a, double b) { return int(a * factor) == int(b * factor); }
private:
double factor;
};
struct AreClose
{
AreClose(double absTol, double relTol): abs_tol{absTol}, rel_tol{relTol} {}
bool operator()(double a, double b)
{
double diff = fabs(a - b);
return diff <= abs_tol ||
diff <= fabs(rel_tol * a) ||
diff <= fabs(rel_tol * b);
}
private:
double abs_tol, rel_tol;
};
template<typename Functor, typename ... ArgTypes>
SuccessTester makeSuccessTester(ArgTypes ... args)
{
return Functor{args ...};
}
int main()
{
SuccessTester fp;
cout << boolalpha;
double a, b;
a = 4.561; b = 4.569;
fp = makeSuccessTester<FixPrecCompare>(2);
cout << "Compare " << a << ", " << b << " to 2 decimals: " << fp(a, b) << endl;
fp = makeSuccessTester<FixPrecCompare>(3);
cout << "Compare " << a << ", " << b << " to 3 decimals: " << fp(a, b) << endl;
a = 2.7099; b = 2.7101;
cout << "Compare " << a << ", " << b << " to 3 decimals: " << fp(a, b) << endl;
fp = makeSuccessTester<AreClose>(1e-3, 1e-10);
cout << "Compare " << a << ", " << b << " using areClose: " << fp(a, b) << endl;
}