Class 模板与数组或向量 arguments

Question

I need to write a class template definition with two template parameters (type, functor) and two template arguments (array/std::vector, int) that can execute the following code:

    const char* message= "Message";

    const transform<char, firstFunctor> first(message, lengthOfMessage);

    transform_view<int, secondFunctor> second(intArray, intSize);

    transform<double, thirdFunctor> third(doubleArray, doubleSize);

The type of the array/ vector has to match the type of the first template parameter.

I tried some variations like this:

   template <typename A, typename B>
   class transform
   {
   public:
    transform<A, B>(A[], B) {...};
   }

But I could not get the constructor's first parameter to match all of the three types.

Any advice is appreciated, thanks!

Answer 1

you wrote the definition of the constructor incorrectly.

transform<A, B>(A[], B) {...}; you will pass a vector, so why did you write A[] as a parameter type?

You need something like the following

#include <iostream>

template <typename T>
struct functor{
    void operator()(const T array [], size_t sze) {
        for (int i{}; i < sze; ++i) {
            std::cout << array[i] << " ";
        }
        std::cout << "\n";
    }
};

template<typename T, typename Function>
class transform {
    const T* array;
    size_t sze;
    Function functor{};
public:
    transform(const T array [], size_t sze):array{array}, sze{sze}{
        functor(array, sze);

    }
};

template< typename T, typename E>
using transform_view =  transform<T, E>;


int main()
{
    using firstFunctor = functor<char>;
    using secondFunctor = functor<int>;
    using thirdFunctor = functor<double>;
    const char *message = "Message";
    size_t lengthOfMessage = 7;

    int intArray[] = {1, 3};
    size_t intSize = 2;

    double doubleArray[] = {1.4, 3.2};
    size_t doubleSize = 2;

    //The given three lines
    const transform<char, firstFunctor> first(message, lengthOfMessage);

    transform_view<int, secondFunctor> second(intArray, intSize);

    transform<double, thirdFunctor> third(doubleArray, doubleSize);
}
The output
M e s s a g e
1 3
1.4 3.2

Answer 2

From what I understand, the problem is the first parameter being the array in the template. Try the following constructor:

transform(A* arr, B el){
    //now here is a tricky part, because there is a very big difference
    //between a char*, and an int/double/unsigned/float/... *.

}

If you have an array of the type A in the class and want to change it to the one passed:

private:
    A* my_array;

you can try sth like this:

if(dynamic_cast<char*>(arr)) //if arr is of type char* {

    if (this->my_array != nullptr) delete my_array; //not needed if in a basic constructor...
    size_t len = strlen(arr);
    my_array = new char [len + 1];
    strcpy(this->my_array, arr); //(destination, source)
    my_array[len] = '\0';
} 
else //if it is a numeric array
{
this->my_array = arr;
//redirecting the pointers in enough
}

Oh and if you are on Visual Studio, strcpy will work if you write

'#pragma warning (disable: 4996)' at the top of the file.

Otherwise it marks it as unsafe and suggests strncpy, strcpy_s, ...