为什么会告诉我string不明确，应该怎么改

#pragma once
class Library
{
    Publication* publications[100]; //所有馆藏，假设馆藏上限为 100
    int total; //实际馆藏数
public:
    Library();
    void add(Publication& p); //添加新馆藏（此函数需完成）
    void show();// //输出所有馆藏的相应信息
};
//此函数可用于输出所有馆藏的相应信息
// 如： C++ zhengli
// Thames 2010/1/1
// DuZhe 2011/5/1 第 10 期
//注意不同种类出版物会输出不同种类的信息。
Library::Library():total(0)
{
    fill(publications, publications+100, nullptr);
}
void Library::add(Publication& p)
{
    publications[total++] = &p;
}
void Library::show()
{
    for (int i = 0; i < total; i++)
        publications[i]->print();
    cout << endl;
}

#pragma once
class Publication
{
public:
    virtual ~Publication() {};
    virtual void print() = 0;
};

#pragma once
class Book :public Publication
{
private:
    string name;
    string authorName;
public:
    Book();
    Book(string _n, string _an = "Unknown");
    ~Book();
    void print();
};
Book::Book() :name("Unknown"), authorName("Unknown"){}
Book::Book(string _n, string _an ) :name(_n), authorName(_an){}
Book::~Book(){}
void Book::print()
{
    cout << name << ' ' << authorName << endl;
}

#pragma once
class Newspaper :public Publication
{
private:
    string name;
    string publishTime;
public:
    Newspaper();
    Newspaper(string _n, string _pt = "Unknown");
    ~Newspaper();
    void print();
};
Newspaper::Newspaper() :name("Unknown"), publishTime("Unknown") {}
Newspaper::Newspaper(string _n, string _pt ) :name(_n), publishTime(_pt) {}
Newspaper::~Newspaper() {}
void Newspaper::print()
{
    cout << name << ' ' << publishTime << endl;
}

#pragma once
class Magzine :public Publication
{
private:
    string name;
    string publishTime;
    string number;
public:
    Magzine();
    Magzine(string _n, string _pt = "Unknown", string _nm = "Unknown");
    ~Magzine();
    void print();
};
Magzine::Magzine() :name("Unknown"), publishTime("Unknown"), number("Unknown") {};
Magzine(string _n, string _pt , string _nm )
    :name(_n), publishTime(_pt), number(_nm)
{}
Magzine::~Magzine(){}
void Magzine::print()
{
    cout << name << ' ' << publishTime << ' ' << number << endl;
}

#include
#include
#include
#include
using namespace std;
#include"Publication.h"
#include"Book.h"
#include"Newspaper.h"
#include"Magzine.h"
#include"Library.h"

int main()
{

}

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不知道你这个问题是否已经解决, 如果还没有解决的话:

• 你可以参考下这个问题的回答, 看看是否对你有帮助, 链接: https://ask.csdn.net/questions/670715
• 这篇博客你也可以参考下：给定一个String字符串，输出每个字符的个数（用数组解决）
• 除此之外, 这篇博客: 模拟实现string类中的 测试 部分也许能够解决你的问题, 你可以仔细阅读以下内容或者直接跳转源博客中阅读:
  

  void TestString1()
  {
  	String s1;
  	String s2("hello world");
  	String s3(s2);
  	s1 = s3;
  	cout << s1 << endl;
  	cout << s2 << endl;
  	cout << s3 << endl;
  }
  void TestString2()
  {
  	String s1("hello");
  	s1.PushBack(' ');
  	s1.PushBack('1');
  	s1.Append("31");
  	s1 += '4';
  	cout << s1 << endl;
  	cout << s1.Size() << endl;
  	cout << s1.Capacity() << endl;
  	// 利用迭代器打印string中的元素
  	auto it = s1.Begin();
  	while (it != s1.End())
  	{
  		cout << *it++;
  	}
  	cout << endl;
  	String s2("hello world!!!");
  	s1.Swap(s2);
  	cout << s1 << endl;
  	cout << s2 << endl;
  }
  
  void TestString3()
  {
  	String s("hello");
  	cout << s << endl;
  	cout << s.Size() << endl;
  	cout << s.Capacity() << endl;
  	s.Resize(10, 'a');
  	cout << s << endl;
  	cout << s.Size() << endl;
  	cout << s.Capacity() << endl;
  	s.Resize(20);
  	cout << s << endl;
  	cout << s.Size() << endl;
  	cout << s.Capacity() << endl;
  	s.Resize(5);
  	cout << s << endl;
  	cout << s.Size() << endl;
  	cout << s.Capacity() << endl;
  	s.Reserve(50);
  	cout << s << endl;
  	cout << s.Size() << endl;
  	cout << s.Capacity() << endl;
  }
  
  void TestString4()
  {
  	String s1("abcdefg");
  	String s2 = s1;
  	cout << s1 << endl;
  	s1.Insert(2, 'h');
  	cout << s1 << endl;
  	s1.Insert(2, "123");
  	cout << s1 << endl;
  	s2.Erase(2, 5);
  	cout << s2 << endl;
  }
  
  void TestString5()
  {
  	String s1 = "abcdef";
  	String s2 = "abcdh";
  	cout << (s1 < s2) << endl;
  	cout << (s1 > s2) << endl;
  	cout << (s1 == s2) << endl;
  	cout << (s1 != s2) << endl;
  	cout << (s1 >= s2) << endl;
  	cout << (s1 <= s2) << endl;
  }
  
  int main()
  {
  	cout << "Test1: 默认成员函数" << endl;
  	TestString1();
  	cout  << endl;
  	cout << "Test2: Iterators + Modifiers" << endl;
  	TestString2();
  	cout << endl;
  	cout << "Test3: Capacity" << endl;
  	TestString3();
  	cout << endl;
  	cout << "Test4: Modifiers" << endl;
  	TestString4();
  	cout << endl;
  	cout << "Test5: 比较" << endl;
  	TestString5();
  	cout << endl;
  
  	return 0;
  }
  

  

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