为什么编译器不喜欢所有与 "Iter" 关联的字符串？

Question

我正在实施一种行为设计模式。

编译器在与“Iter”相关的行中给出错误。我完全不明白为什么会这样，更不知道如何解决？

我以为可能哪里有笔误（我正在按照书上的例子做程序，代码是截图，需要重新输入）——但没有笔误，一切都是完全一样。

那么可能是什么问题？

我怀疑这是一些简单的事情，但由于我的知识水平低，我无法立即理解。

错误：

main.cpp:57:2: error: ‘Iter’ does not name a type
  Iter* createIterator() const;
  ^~~~
main.cpp:98:41: error: no ‘Iter* ContainerPerson::CreateIterator() const’ member function declared in class ‘ContainerPerson’
 Iter* ContainerPerson::CreateIterator() const {
                                         ^~~~~
main.cpp: In function ‘int main()’:
main.cpp:113:18: error: ‘class ContainerPerson’ has no member named ‘createIterator’
  Iter* it = list.createIterator();
                  ^~~~~~~~~~~~~~
main.cpp:118:10: error: invalid use of non-static member function ‘void Iter::first()’
  for(it->first; !it->isDoneBegin(); it->next()) {
      ~~~~^~~~~
main.cpp:69:7: note: declared here
  void first() {
       ^~~~~
main.cpp:124:10: error: invalid use of non-static member function ‘void Iter::first()’
  for(it->first; !it->isDoneBegin(); it->prev()) {
      ~~~~^~~~~
main.cpp:69:7: note: declared here
  void first() {
       ^~~~~

完整代码：

#include <iostream>
#include <string>
using namespace std;

class Person {
public: 
    string name;
    int age;
    string address;

    Person() {
        name = "";
        age = 0;
        address = "";
    }
    
    Person(string n, int a, string ad) {
        name = n;
        age = a;
        address = ad;
    }

    void prn() {
        cout << "Информация о человеке:";
        cout << "\n Фамилия: " << name;
        cout << "\t Возраст: " << age;
        cout << "\n Город: " << address;
    }
};

class ContainerPerson {
public:
    Person* masPerson;
    int currentpos;

    ContainerPerson(int size) {
        masPerson = new Person[size];
        currentpos = - 1;
    }

    ~ContainerPerson() {
        delete []masPerson;
    }

    void add(Person *obj) {
        int pos = ++currentpos;
        masPerson[pos].name = obj->name;
        masPerson[pos].age = obj->age;
        masPerson[pos].address = obj->address;
    }

    bool isEmpty() {
        return (currentpos == - 1);
    }

    friend class Iter;
    Iter* createIterator() const;
};

class Iter {
    const ContainerPerson* container;
    int index;

public:
    Iter(const ContainerPerson* con) {
        container = con;
    }
    
    void first() {
        index = 0;
    }

    void end() {
        index = container->currentpos;
    }

    void next() {
        index++;
    }

    void prev() {
        index--;
    }

    bool isDoneEnd() {
        return (index == container->currentpos + 1);
    }

    bool isDoneBegin() {
        return (index == -1);
    }

    Person currentItem() {
        return container->masPerson[index];
    }
};

Iter* ContainerPerson::CreateIterator() const {
    return new Iter(this);
}

int main() {
    setlocale(LC_ALL, "Russian");
    ContainerPerson list(7);
    list.add(new Person("Sur1", 26, "City1"));  
    list.add(new Person("Sur2", 33, "City2"));
    list.add(new Person("Sur3", 65, "City3"));
    list.add(new Person("Sur4", 43, "City4"));
    list.add(new Person("Sur5", 58, "City5"));
    list.add(new Person("Sur6", 47, "City6"));
    list.add(new Person("Sur7", 32, "City7"));
    
    Iter* it = list.createIterator();
    Person currentPerson;   
    

    cout << "\nOutput (beg-next):";
    for(it->first; !it->isDoneBegin(); it->next()) {
        currentPerson = it->currentItem();
        currentPerson.prn();
    }

    cout << "\nOutput (beg-prev):";
    for(it->first; !it->isDoneBegin(); it->prev()) {
        currentPerson = it->currentItem();
        currentPerson.prn();
    }

    return 0;
}

Answer 1

您需要在 class ContainerPerson 声明之外的前向声明 class Iter;。否则朋友 class 是 ContainerPerson::Iter 与您稍后声明的 Iter 无关。参见 Why does a C++ friend class need a forward declaration only in other namespaces?
Iter* ContainerPerson::CreateIterator() 定义中的拼写错误：应为 createIterator，小写 c。
for(it->first; ...)中的错别字；应该是 it->first().
您的迭代混淆了。他们应该是

    for(it->first(); !it->isDoneEnd(); it->next()) {
        // ...
    }
    for(it->end(); !it->isDoneBegin(); it->prev()) {
        // ...
    }

修复这些后，它可以干净地编译和运行：https://godbolt.org/z/3K7T7d14h

额外的错误供您稍后修复：你永远不会 delete 在 ContainerPerson::createIterator() 中分配的 Iter。那是内存泄漏。

Answer 2

main.cpp:57:2: error: ‘Iter’ does not name a type
  Iter* createIterator() const;
  ^~~~

虽然是朋友的声明class

friend class Iter;

出现在 class ContainerPerson 的定义中，但是名称 Iter 在 class ContainerPerson 的范围内不可见。

因此编译器针对该成员函数声明发出错误。

您需要在 class ContainerPerson 之前声明 class Iter。例如

class Iter;

class ContainerPerson
{
    //...
    friend class Iter;
    Iter* createIterator() const;
    //...
};

或者在函数声明中的 return 类型中使用详细的类型说明符，如

class ContainerPerson
{
    //...
    friend class Iter;
    class Iter* createIterator() const;
    //...
};


main.cpp:98:41: error: no ‘Iter* ContainerPerson::CreateIterator() const’ member function declared in class ‘ContainerPerson’
 Iter* ContainerPerson::CreateIterator() const {
                                         ^~~~~

你声明了成员函数

Iter* createIterator() const;
      ^^

不是成员函数

Iter* CreateIterator() const;
      ^^


main.cpp: In function ‘int main()’:
main.cpp:113:18: error: ‘class ContainerPerson’ has no member named ‘createIterator’
  Iter* it = list.createIterator();
                  ^~~~~~~~~~~~~~

此错误是第一个错误的结果。

至于这些错误

main.cpp:118:10: error: invalid use of non-static member function ‘void Iter::first()’
  for(it->first; !it->isDoneBegin(); it->next()) {
      ~~~~^~~~~

和

main.cpp:124:10: error: invalid use of non-static member function ‘void Iter::first()’
  for(it->first; !it->isDoneBegin(); it->prev()) {
      ~~~~^~~~~

那么你的意思好像是函数调用

it->first()

为什么编译器不喜欢所有与 "Iter" 关联的字符串？

Why doesn't the compiler like all the strings that are associated with "Iter"?

c++

class

scope

declaration

friend-class