基于类型的条件相等运算符/函数

Question

如何根据指定的类型为类实现部分比较运算符（或函数），以匹配成员变量的子集？给出以下示例：

struct A { int a; bool operator==(A rhs) const { return a==rhs.a; } };
struct B { int b; bool operator==(B rhs) const { return b==rhs.b; } };
struct C { int c; bool operator==(C rhs) const { return c==rhs.c; } };
struct D { int d; bool operator==(D rhs) const { return d==rhs.d; } };

class X
{
public:
    X(int a=0, int b=0, int c=0, int d=0)
    : _a{a}, _b{b}, _c{c}, _d{d}
    {}

    A _a;
    B _b;
    C _c;
    D _d;
};

我想添加支持，以便用户可以根据X成员的子集比较两个X个实例;例如：

X x1 (1,2,3,4);
X x2 (1,1,2,3);

match<A,B,C,D>( x1, x2 ); /* should return x1._a==x2._a && ... && x1._d==x2._d */
match<A,B,C>( x1, x2 );   /* should return x1._a==x2._a && ... x1._c==x2._c */
match<A,B>( x1, x2 );     /* should return x1._a==x2._a && x1._b==x2._b */
match<A>( x1, x2 );       /* should return x1._a==x2._a */
match<A,D>( x1, x2 );     /* should return x1._a==x2._a && x1._d==x2._d */

以下但是失败

template<typename T>
bool match(X x1, X x2) { return false; }

template<>
bool match<A>(X x1, X x2) { return x1._a == x2._a; }

template<>
bool match<B>(X x1, X x2) { return x1._b == x2._b; }

template<>
bool match<C>(X x1, X x2) { return x1._c == x2._c; }

template<>
bool match<D>(X x1, X x2) { return x1._d == x2._d; }

template<typename T, typename... Args>
bool match(X x1, X x2)
{ return match<T>(x1, x2) && match<Args...>(x1, x2); }

带错误信息（*）

vard.cc: In function ‘int main()’:
vard.cc:49:35: error: call of overloaded ‘match(X&, X&)’ is ambiguous
     std::cout << match<A>( x1, x2 ) << "\n" ;
                                   ^
vard.cc:25:6: note: candidate: bool match(X, X) [with T = A]
 bool match<A>(X x1, X x2) { return x1._a == x2._a; }
      ^
vard.cc:37:6: note: candidate: bool match(X, X) [with T = A; Args = {}]
 bool match(X x1, X x2)
      ^
vard.cc: In instantiation of ‘bool match(X, X) [with T = A; Args = {B, C, D}]’:
vard.cc:46:41:   required from here
vard.cc:38:18: error: call of overloaded ‘match(X&, X&)’ is ambiguous
 { return match<T>(x1, x2) && match<Args...>(x1, x2); }
                  ^
vard.cc:25:6: note: candidate: bool match(X, X) [with T = A]
 bool match<A>(X x1, X x2) { return x1._a == x2._a; }
      ^
vard.cc:37:6: note: candidate: bool match(X, X) [with T = A; Args = {}]
 bool match(X x1, X x2)
      ^
vard.cc: In instantiation of ‘bool match(X, X) [with T = A; Args = {B, C}]’:
vard.cc:47:39:   required from here
vard.cc:38:18: error: call of overloaded ‘match(X&, X&)’ is ambiguous
 { return match<T>(x1, x2) && match<Args...>(x1, x2); }
                  ^
vard.cc:25:6: note: candidate: bool match(X, X) [with T = A]
 bool match<A>(X x1, X x2) { return x1._a == x2._a; }
      ^
vard.cc:37:6: note: candidate: bool match(X, X) [with T = A; Args = {}]
 bool match(X x1, X x2)
      ^
vard.cc: In instantiation of ‘bool match(X, X) [with T = A; Args = {B}]’:
vard.cc:48:37:   required from here
vard.cc:38:18: error: call of overloaded ‘match(X&, X&)’ is ambiguous
 { return match<T>(x1, x2) && match<Args...>(x1, x2); }
                  ^
vard.cc:25:6: note: candidate: bool match(X, X) [with T = A]
 bool match<A>(X x1, X x2) { return x1._a == x2._a; }
      ^
vard.cc:37:6: note: candidate: bool match(X, X) [with T = A; Args = {}]
 bool match(X x1, X x2)
      ^
vard.cc:38:44: error: call of overloaded ‘match(X&, X&)’ is ambiguous
 { return match<T>(x1, x2) && match<Args...>(x1, x2); }
                                            ^
vard.cc:28:6: note: candidate: bool match(X, X) [with T = B]
 bool match<B>(X x1, X x2) { return x1._b == x2._b; }
      ^
vard.cc:37:6: note: candidate: bool match(X, X) [with T = B; Args = {}]
 bool match(X x1, X x2)
      ^

为什么这些电话含糊不清？什么是正确，明确的实施？这个功能可以合并到类的相等运算符中吗？

（*）编译的测试程序只是上面代码的串联;

#include <iostream>

struct A { int a; bool operator==(A rhs) const { return a==rhs.a; } };
struct B { int b; bool operator==(B rhs) const { return b==rhs.b; } };
struct C { int c; bool operator==(C rhs) const { return c==rhs.c; } };
struct D { int d; bool operator==(D rhs) const { return d==rhs.d; } };

class X
{
public:
    X(int a=0, int b=0, int c=0, int d=0)
    : _a{a}, _b{b}, _c{c}, _d{d}
    {}

    A _a;
    B _b;
    C _c;
    D _d;
};

template<typename T>
bool match(X x1, X x2) { return false; }

template<>
bool match<A>(X x1, X x2) { return x1._a == x2._a; }

template<>
bool match<B>(X x1, X x2) { return x1._b == x2._b; }

template<>
bool match<C>(X x1, X x2) { return x1._c == x2._c; }

template<>
bool match<D>(X x1, X x2) { return x1._d == x2._d; }

template<typename T, typename... Args>
bool match(X x1, X x2)
{ return match<T>(x1, x2) && match<Args...>(x1, x2); }

int main()
{
    X x1 (1,2,3,4);
    X x2 (0,1,2,3);
    X x3 (3,3,3,3);

    std::cout << match<A,B,C,D>( x1, x2 ) << "\n" ;
    std::cout << match<A,B,C>( x1, x2 ) << "\n" ;
    std::cout << match<A,B>( x1, x2 ) << "\n" ;
    std::cout << match<A>( x1, x2 ) << "\n" ;

    return 0;
}

用clang ++ 3.7.0编译的

（g ++（GCC）5.3.1给出了几乎相同的错误。）

Answer 1

这是一个想法，使用std::tuple来提供所有实际的产品操作。我们只需暴露类成员：

struct A { int a; bool operator==(A rhs) const { return a==rhs.a; } };
struct B { int b; bool operator==(B rhs) const { return b==rhs.b; } };
struct C { int c; bool operator==(C rhs) const { return c==rhs.c; } };
struct D { int d; bool operator==(D rhs) const { return d==rhs.d; } };

class X
{
    template <typename> friend struct Get;

public:
    X(int a=0, int b=0, int c=0, int d=0)
    : _a{a}, _b{b}, _c{c}, _d{d}
    {}

    A _a;
    B _b;
    C _c;
    D _d;
};

template <typename> struct Get;

template <> struct Get<A> { static const A & get(const X & x) { return x._a; } };
template <> struct Get<B> { static const B & get(const X & x) { return x._b; } };
template <> struct Get<C> { static const C & get(const X & x) { return x._c; } };
template <> struct Get<D> { static const D & get(const X & x) { return x._d; } };

#include <tuple>

template <typename ...Args> bool Match(const X & lhs, const X & rhs)
{
    return std::tie(Get<Args>::get(lhs)...) == std::tie(Get<Args>::get(rhs)...);
}

用法：

#include <iostream>

int main()
{
    X x1 (1,2,3,4);
    X x2 (1,1,2,3);

    std::cout << Match<A, A, A>(x1, x2) << "\n";
    std::cout << Match<A, D>(x1, x2) << "\n";
}

Answer 2

如果您可以使用C ++ 14，使用tie非常容易。

首先，我们向X添加class X { public: X(int a=0, int b=0, int c=0, int d=0) : _a{a}, _b{b}, _c{c}, _d{d} {} A _a; B _b; C _c; D _d; std::tuple<A,B,C,D> tie () { return std::tie(_a,_b,_c,_d); } }; 方法以绑定所有成员：

match

然后我们可以从该元组中提取我们为template<typename... Args> bool match(X x1, X x2) { return std::make_tuple(std::get<Args>(x1.tie())...) == std::make_tuple(std::get<Args>(x2.tie())...); } 传递的类型并进行比较：

xAxis = d3.svg.axis()
    .scale(xScale)
    .tickValues(calculateTickValue)
    .tickFormat(getTickFormat);

var getTickFormat = function(date){
    var diff = date.format("DD") != previousDate.format("DD");                    
    if (diff){
        previousDate = date;
        return date.format("MMM Do, HH:mm");
    }
    return date.format(tickFormat);
}

var calculateTickValue = function(){
    var startTime = scope.segment.startTime;
    var endTime = scope.segment.endTime;
    var tickValues = [];
    var difference = moment.duration(endTime.diff(startTime));
    var maximumTicks = calculateMaximumTicks(); 
    if (difference.asHours() >= maximumTicks){
       tickValues = calculateHoursTicks(startTime, endTime, maximumTicks);
       tickFormat = "HH:mm";
    }
    else {
        tickValues = caclulateMinutesTicks(startTime, endTime, maximumTicks);
        tickFormat = "HH:mm";
    }

    previousDate = startTime;
    return tickValues;
}