C++ 11個物理屬性單位的用戶定義文字

我想學習如何使用C++ 11用戶定義的文字單位的物理屬性。問題是，我如何避免混合這些單位。那麼（8.0_kg + 8.0_km） - >會給出錯誤。任何想法傢伙？我是新來的C++，善良。C++ 11個物理屬性單位的用戶定義文字

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 


}

來源

2017-03-06 WOT

我會通過使它們類型（'struct' /'類'）而不是隻是'長雙' – NathanOliver

你可以看看[strong typedef]（http://stackoverflow.com/questions/28916627/strong -typedefs）爲您的質量/長度。 – Jarod42

創建表示不同單位的數值的類。那怎麼它已經因爲長期做前C++ 11

自定義文本可以使實例更易讀，雖然，因爲這有助於保護數和學分

的通常的順序見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

來源

2017-03-06 13:04:30 yeoman

對於你的例子，我只是離開這個http://en.cppreference.com/w/cpp/language/user_literal – jamek

@ Jarod42是更好的變量命名，謝謝^^ – yeoman

方式更好，我的意思是 - .-愚蠢autocorrect。 – yeoman

您需要定義您自己的類型，因爲您無法限制原語所代表的內容。

您可以使用「標記模板」避免重複操作員等，並保持類型安全。
這可以擴展，所以你得到例如編譯器檢查distance * distance = area或speed * time = distance。

這裏有一個簡單的例子：

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）我不認爲有在C++中建立了長期，但它是非常相似，哈斯克爾是指爲phantom types。

來源

2017-03-06 13:18:47 molbdnilo

C++ 11個物理屬性單位的用戶定義文字

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