当我尝试在 QTCreator 中构建此代码时，我一直试图弄清楚为什么会出现错误。我发现了一些与此类似的其他帖子，但我认为我有一个类似但不同的问题。

在 main.cpp 中，我基本上有一些我正在创建的过滤器（一个带有纯虚函数的模板化结构），它返回给我一个 MyProducts 向量。我给过滤器一个规范（一个带有纯虚函数的模板结构）和一个 MyProduct 向量来搜索。

但是我在构建过程中不断收到此错误消息。

/Users/marcokok/Qt-workspace/Open_Closed_Principle/main.cpp:112: error: allocating an object of abstract class type 'Specification<MyProduct>'
/Users/marcokok/Qt-workspace/Open_Closed_Principle/main.cpp:112:36: error: allocating an object of abstract class type 'Specification<MyProduct>'
    AndSpecification<MyProduct> as(cs, ss);
                                   ^
/Users/marcokok/Qt-workspace/Open_Closed_Principle/main.cpp:29:18: note: unimplemented pure virtual method 'is_satisfied' in 'Specification'
    virtual bool is_satisfied(T* item) = 0;
                 ^

这是我的源代码

主程序

#include <iostream>
#include <vector>

using namespace std;

enum class ColorTable
{
    red,
    green,
    blue
};

enum class SizeTable
{
    small,
    medium,
    large
};

struct MyProduct
{
    string name;
    ColorTable color;
    SizeTable size;
};

template <typename T> struct Specification
{
    virtual bool is_satisfied(T* item) = 0;
};

template <typename T> struct Filter
{
    virtual vector<T*> filter(vector<T*> items, Specification<T>& spec) = 0;
};

struct BetterFilter : Filter<MyProduct>
{
    vector<MyProduct*> filter(vector<MyProduct *> items, Specification<MyProduct> &spec) override
    {
        vector<MyProduct*> result;

        for (auto& item : items)
        {
            if (spec.is_satisfied(item))
                result.push_back(item);
        }
        return result;
    }
};

struct ColorSpecification : Specification<MyProduct>
{
    ColorTable color;

    ColorSpecification(ColorTable color) : color(color)
    {}

    bool is_satisfied(MyProduct* item) override
    {
        if (item->color == color)
            return true;
        else
            return false;
    }
};

struct SizeSpecification : Specification<MyProduct>
{
    SizeTable size;

    SizeSpecification(SizeTable size) : size(size)
    {}

    bool is_satisfied(MyProduct* item) override
    {
        if (item->size == size)
            return true;
        else
            return false;
    }
};

template <typename T> struct AndSpecification : Specification<T>
{
    Specification<T>& first;
    Specification<T>& second;

    AndSpecification(Specification<T> first, Specification<T> second) : first(first) , second(second)
    {}

    bool is_satisfied(T* item) override
    {
        if (first.is_satisfied(item) && second.is_satisfied(item))
            return true;
        else
            return false;
    }

};
int main()
{
    MyProduct prod_1{"Apple", ColorTable::green, SizeTable::large};
    MyProduct prod_2{"Jeans", ColorTable::green, SizeTable::small};
    MyProduct prod_3{"Graphics Card", ColorTable::blue, SizeTable::large};

    vector<MyProduct*> items = {&prod_1, &prod_2, &prod_3};

    BetterFilter bf;
    ColorSpecification cs(ColorTable::green);
    SizeSpecification ss(SizeTable::large);
    AndSpecification<MyProduct> as(cs, ss);

    auto green_things = bf.filter(items, cs);
    for (auto& item : green_things)
        cout << item->name << " is green." << endl;

    auto large_things = bf.filter(items, ss);
    for (auto& item : large_things)
        cout << item->name << " is large.nn" << endl;

   // auto large_green_things = bf.filter(items, as);
    //for (auto& item : large_green_things)
     //   cout << item->name << " is large and green." << endl;
    return 0;
}

回答

main.cpp: In function 'int main()':
main.cpp:112:42: error: cannot allocate an object of abstract type 'Specification<MyProduct>'
  112 |     AndSpecification<MyProduct> as(cs, ss);
      |                                          ^
main.cpp:27:30: note:   because the following virtual functions are pure within 'Specification<MyProduct>':
   27 | template <typename T> struct Specification
      |                              ^~~~~~~~~~~~~
main.cpp:29:18: note:     'bool Specification<T>::is_satisfied(T*) [with T = MyProduct]'
   29 |     virtual bool is_satisfied(T* item) = 0;
      |                  ^~~~~~~~~~~~
main.cpp:112:33: warning: unused variable 'as' [-Wunused-variable]
  112 |     AndSpecification<MyProduct> as(cs, ss);
      |                                 ^~

template <typename T> struct AndSpecification : Specification<T>
{
    Specification<T>& first;
    Specification<T>& second;

    AndSpecification(Specification<T> first, Specification<T> second) : first(first) , second(second)
    {}
};

template <typename T> struct AndSpecification : Specification<T>
{
    Specification<T>& first;
    Specification<T>& second;

    AndSpecification(Specification<T> first, Specification<T> second) : first(first) , second(second)
    {}
};