如何定义运算符，以便将用户定义类型数组转换为原始类型数组?

2022-01-24 00:00:00 containers implicit-conversion c++

我给出以下代码来说明我的问题:

I give the following code to illustrate my question:

#include <vector>

struct Complex
{
     int a, b, c;

     Complex() : a(3), b(4), c(10) {}

     operator int() const {  return a+b+c; }
};

int main()
{
   Complex abc;
   int value = (abc);
   Complex def;
   def.a = 20;
   int value2 = (def);

   std::vector<Complex> ar;
   ar.push_back(abc);
   ar.push_back(def);

   std::vector<int> ar2;
   ar2.push_back(abc);
   ar2.push_back(def);

   std::vector<int> ar3;
   ar3 = (ar);
}

由于表达式 ar3 = (ar)，这不会编译.我已经声明了一个转换运算符，以便 Complex 类可以在需要 int 的地方使用.我是否也可以将 Complex 对象数组分配给 int 数组?

This won't compile, due to the expression ar3 = (ar). I have declared a conversion operator so that the Complex class can be used in where int is expected. Can I also make it work for assigning an array of Complex objects to an array of int?

我尝试为 Complex 数组声明一个非成员转换运算符，但这是不允许的:

I tried to declare a non-member conversion operator for array of Complex, but that's not allowed:

void std::vector<int> operator = (std::vector<Complex> complexArray)
{
    std::vector<int> abc;
    for(int i=0; i<complexArray.size(); i++)
     abc.push_back(complexArray[i]);
    return abc;
}

推荐答案

忘记自动隐式转换(至少对于标准库容器).但是，如果您愿意接受下面示例中的显式转换

Forget about automatic implicit conversion (at least for the Standard Library containers). But if you are willing to accept an explicit conversion like in the below example

 const std::vector<int> vi {1, 2, 3, 4, 5};
 const std::vector<double> vd = container_cast(vi);

然后执行 container_cast() 实用程序.请注意，它不仅可以在不同元素类型的同一模板容器的实例之间进行转换(即 std::vector 到 std::vector), 也可以在不同的容器之间(例如 std::vector 到 std::list).

then the implementation of the container_cast() utility follows. Note that it can cast not only between instantiations of the same template container for different element types (i.e. std::vector<int> to std::vector<double>), but also between different containers (e.g. std::vector to std::list).

#include <iostream>
#include <vector>
#include <list>

template<class SourceContainer>
class ContainerConverter
{
    const SourceContainer& s_;
public:
    explicit ContainerConverter(const SourceContainer& s) : s_(s) {}

    template<class TargetContainer>
    operator TargetContainer() const
    {
        return TargetContainer(s_.begin(), s_.end());
    }
};

template<class C>
ContainerConverter<C> container_cast(const C& c)
{
    return ContainerConverter<C>(c);
}

template<class C>
void printContainer(const C& c)
{
    std::cout << "{ ";
    for( auto x : c )
        std::cout << x << ' ';
    std::cout << "}" << std::endl;
}

int main()
{
    const std::vector<double> vd {2.2, 7.7, 5.5, 1.1, -4.4};
    printContainer(vd);

    const std::vector<int> vi = container_cast(vd);
    printContainer(vi);

    const std::list<float> lf = container_cast(vd);
    printContainer(lf);
    return 0;
}

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