問題描述

我正在嘗試解決一個編程問題，該問題由一個包含多個參數的對象(稱為圖)組成.每個參數(Parameter 類)可以是以下幾種類型之一:int、double、complex、string - 僅舉幾例.

I am trying to solve a programming problem that consists of an object (call it Diagram), that contains several parameters. Each parameter (the Parameter class) can be one of several types: int, double, complex, string - to name a few.

所以我的第一反應是將我的 Diagram 類定義為具有模板參數的向量，看起來像這樣.

So my first instinct was to define my Diagram class as having a vector of template parameters, which would look like this.

class Diagram
{
private:
    std::vector<Parameter<T> > v;
};

這不能編譯，我明白為什么.因此，基于此頁面上的建議 如何聲明屬于類中任何類型對象的數據成員，我將代碼修改為:

This doesn't compile, and I understand why. So, based on the recommendations on this page How to declare data members that are objects of any type in a class, I modified my code to look like:

class ParameterBase
{
public:
    virtual void setValue() = 0;
    virtual ~ParameterBase() { }
};


template <typename T>
class Parameter : public ParameterBase
{
public:
    void setValue() // I want this to be 
                    // void setValue(const T & val)
    {
        // I want this to be 
        // value = val;
    }

private:
    T value;
};

class Diagram
{
public:
    std::vector<ParameterBase *> v;
    int type;
};

我無法弄清楚如何使用適當的模板化參數調用 setValue 函數.在 ParameterBase 抽象基類中不可能有模板化參數.非常感謝任何幫助.

I'm having trouble figuring out how to call the setValue function with an appropriate templated parameter. It is not possible to have a templated parameter in the ParameterBase abstract base class. Any help is greatly appreciated.

附言我沒有使用 boost::any 的靈活性.

P.S. I don't have the flexibility to use boost::any.

推薦答案

你們已經很接近了.我添加了一些，因為它們很方便

You got very close. I added a few bits because they're handy

class ParameterBase
{
public:
    virtual ~ParameterBase() {}
    template<class T> const T& get() const; //to be implimented after Parameter
    template<class T, class U> void setValue(const U& rhs); //to be implimented after Parameter
};

template <typename T>
class Parameter : public ParameterBase
{
public:
    Parameter(const T& rhs) :value(rhs) {}
    const T& get() const {return value;}
    void setValue(const T& rhs) {value=rhs;}    
private:
    T value;
};

//Here's the trick: dynamic_cast rather than virtual
template<class T> const T& ParameterBase::get() const
{ return dynamic_cast<const Parameter<T>&>(*this).get(); }
template<class T, class U> void ParameterBase::setValue(const U& rhs)
{ return dynamic_cast<Parameter<T>&>(*this).setValue(rhs); }

class Diagram
{
public:
    std::vector<ParameterBase*> v;
    int type;
};

然后圖表可以做這樣的事情:

Diagram can then do stuff like these:

Parameter<std::string> p1("Hello");
v.push_back(&p1);
std::cout << v[0]->get<std::string>(); //read the string
v[0]->set<std::string>("BANANA"); //set the string to something else
v[0]->get<int>(); //throws a std::bad_cast exception

看起來您的意圖是將擁有資源的指針存儲在向量中.如果是這樣，請注意使 Diagram 具有正確的析構函數，并使其不可復制構造，不可復制賦值.

It looks like your intent is to store resource-owning pointers in the vector. If so, be careful to make Diagram have the correct destructor, and make it non-copy-constructable, and non-copy-assignable.

這篇關于帶有模板成員變量的 C++ 類的文章就介紹到這了，希望我們推薦的答案對大家有所幫助，也希望大家多多支持html5模板網！