C++11 用户定义的物理属性单位文本

c++11 user defined literals for units of physical properties

本文关键字：属性 单位 文本 用户 定义 C++11      更新时间：2023-10-16

我正在尝试学习如何使用c ++ 11用户定义的文字作为物理属性单位。问题是，如何避免混合这些单元。因此(8.0_kg + 8.0_km)-->给出错误。伙计们有什么想法吗？我是C ++的新手，要善良。

class Mass{
public:        
//Mass(){
//    cout << "only Mass units allowed in here" << endl;
//}
//~Mass();
long double getWeight(long double a);        
double car, house, cat;
private:
long double a;

};

long double Mass::getWeight(long double w) {
cout << "returning argument: " << w << 'n'<< endl;
return 0;
}

long double operator"" _km(long double d) { return d * 1000.0; }
long double operator"" _m (long double d) {return d;}
long double operator"" _cm(long double d) { return d / 100.0; }
long double operator"" _tonne(long double m) { return m * 1000.0 ; }
long double operator"" _kg(long double m) { return m ; }
long double operator"" _lb(long double m) { return m * 0.453592; }
long double getDistance(long double d){
long double starting_d = 61.0_kg;
long double total_d = d + starting_d;
cout << "the distance I have run is: " << total_d << endl;
return 0;
}
int main() {

cout <<  6.0_km << endl;
cout <<  6.0_km + 3.0_m << endl;
cout <<  6.0_km + 3.0_m + 15.0_cm << 'n' << endl;
cout <<  8.0_tonne << endl;
cout <<  8.0_km + 4.0_kg << endl;
cout <<  8.0_km + 4.0_kg + 21.0_lb << 'n' << endl;

long double distance = 5.45_km;
getDistance(distance);
Mass obj1;
obj1.getWeight(13.96_lb);
cout << "This is clearly wrong:  "<<  8.0_km + 4.0_kg << endl;
obj1.getWeight(10.96_km); // so is this

}

您需要定义自己的类型，因为您无法限制基元表示的内容。

您可以使用"标记模板">1来避免重复运算符等，并确保其类型安全。
这可以扩展，以便编译器distance * distance = area或speed * time = distance检查。

下面是一个简短的示例：

template<typename Kind>
struct Value
{
long double value;
Value& operator+= (Value v) { value += v.value; return *this; }
};
template <typename Kind>
Value<Kind> operator+ (Value<Kind> lhs, Value<Kind> rhs) { return lhs += rhs; }
// These types don't need definitions; we only need some unique type names.
struct M;
struct D;
using Mass = Value<M>;
using Distance = Value<D>;
Mass operator"" _kg(long double d) { return { d };}
Mass operator"" _lb(long double d) { return { d * 0.453592 };}
Distance operator"" _km(long double d) { return { d * 1000 };}
Distance operator"" _mile(long double d) { return { d * 1609 };}
int main()
{
// OK
Distance d = 1.2_km + 0.2_mile;
// OK
Mass m = 2.3_kg + 1.4_lb;      
// invalid operands to binary expression ('Distance' (aka 'Value<D>')
// and 'Mass' (aka 'Value<M>'))
Distance d2 = 2.4_km + 1.2_kg; // Nope
}

1)我不认为C++中有一个既定的术语，但它与Haskell所说的幻影类型非常相似。

创建表示不同单位数值的类。早在C++ 11之前，它就是这样做的。

但是，自定义文本可以使实例化更具可读性，因为它有助于保留数字和单位:)的通常顺序

见 http://en.cppreference.com/w/cpp/language/user_literal

class MassKg
{
double value;
// public c'tor, numeric operators, &c.
};
// ...
MassKg mass(5.0);
DistanceM distance(3.0);
auto c = mass * distance; // may yield an instance of TorqueKgM, or MomentumKgM, therefore
// explicit functions / methods are preferrable for mixed
// multiplication or division
auto mass2 = mass + MassKg(2.0); // yiels an instance of MassKg
auto invalid = mass + distance; // compile time error
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