我收到一个错误，当我声明一个类作为其他类的成员。错误：在声明朋友时必须使用类键 [英] I'm getting a error when I declare a class as a member of other class. error : a class-key must be used when declaring a friend

问题描述

我是一个C ++菜鸟，我正在试验boost序列化，我想看看它是否工作，当一个类被声明为另一个类的成员。但是当我编译我的代码，我得到了大量的错误。我试图声明基于结构，但没有改变的错误。我的代码：

  #include< iostream> 
 #include< fstream> 
 #include< boost / archive / text_iarchive.hpp> 
 #include< boost / archive / text_oarchive.hpp> 
 
 class baseds {}; 
 class superior {}; 
 
基于类{
 private：
 friend class boost :: serialization :: access; 
 public：
 int a; 
 int b; 
 int c; 
 baseds（）{} 
〜baseds（）{} 
 template< class Archive> 
 void serialize（Archive& ar，const unsigned int version）
 {
 ar&一个; 
 ar& b; 
 ar& C; 
} 
}; 
 
 class superior {
 private：
 friend class boost :: serialization :: access; 
 public：
 int x; 
 int y 
 baseds lag; 
 superior（）{} 
〜superior（）{} 
 template< class Archive> 
 void serialize（Archive& ar，const unsigned int version）
 {
 ar& X; 
 ar& y; 
 ar&落后; 
} 
}; 
 
 int main（）{
 
 superior myData，myData2; 
 myData.x = 10; 
 myData.y = 20; 
 myData.lag.a = 1; 
 myData.lag.b = 2; 
 myData.lag.c = 3; 
 
 ofstream ofs（stages.txt）; 
 {
 boost :: serialization :: archive one（ofs）; 
 one<< myData; 
} 
 
 ifstream ifs（steps.txt）; 
 {
 boost :: serialization :: archive two（ifs）; 
 two>> myData2; 
} 
 std :: cout<<\\\
<< myData2.x; 
 std :: cout<<\\\
<< myData2.y; 
 std :: cout<<\\\
<< myData2.lag.a; 
 std :: cout<<\\\
<< myData2.lag.b; 
 std :: cout<<\\\
<< myData2.lag.c; 
 
 return 0; 
 
} 
  

错误：

  tier2.cpp：10：错误：在声明朋友时必须使用类键
 tier2.cpp：29：error：class-当声明一个朋友
 tier2.cpp：32：error：expected`;'beforeint
 tier2.cpp：在成员函数`void superior :: serialize（Archive& '：
 tier2.cpp：38：error：`x'undeclared（首先使用此函数）
 tier2.cpp：38：error：（每个未声明的标识符仅对每个函数报告一次， 。）
 tier2.cpp：39：error：`y'undeclared（首先使用此函数）
 tier2.cpp：40：error：`lag'undeclared（首先使用此函数）
 tier2.cpp：在函数`int main（）'：
 tier2.cpp：47：error：'class superior'没有成员名为'x'
 tier2.cpp：48：error：'class上级没有名为'y'的成员
 tier2.cpp：49：error：'class superior'没有名为'lag'的成员
 tier2.cpp：50：error：'class superior'has no成员名为'lag'
 tier2.cpp：51：error：'class superior'没有成员名为'lag'
 tier2.cpp：53：error：`ofstream'undeclared（首先使用此函数） 
 tier2.cpp：53：error：expected`;'beforeofs
 tier2.cpp：55：error：`archive'不是`boost :: serialization'的成员
 tier2.cpp：55：error：expected`;'beforeone
 tier2.cpp：56：error：`one'undeclared（首先使用此函数）
 tier2.cpp：57：error： 
 tier2.cpp：59：error：`ifstream'undeclared（首先使用此函数）
 tier2.cpp：59：error：expected`;'beforeifs 
 tier2.cpp：61：error：`archive'不是`boost :: serialization'的成员
 tier2.cpp：61：error：expected`;'beforeones
 tier2.cpp：62：error：`ones'undeclared（首先使用这个函数）
 tier2.cpp：63：error：expected`;'before'} token 
 tier2.cpp：64：错误：'class superior'没有成员命名为'x'
 tier2.cpp：65：error：'class superior'没有成员命名为'y'
 tier2.cpp：66：error：'class上级'没有名为'lag'的成员
 tier2.cpp：67：error：'class superior'没有名为'lag'的成员
 tier2.cpp：68：error：'class superior'没有成员名为'lag'
 tier2.cpp：71：2：warning：文件末尾没有换行符
  

解决方案

您正在定义 baseds 和 superior p>

在gcc上会得到正确的错误信息：

  main.cpp ：9：7：错误：重定义'baseds'
类为基础{
 ^ 
 main.cpp：6：7：注意：以前的定义在这里
 class baseds { ; 
 ^ 
 main.cpp：27：7：error：redefinition of'superior'
 class superior {
 ^ 
 main.cpp：7：7：以前的定义是这里
 class superior {}; 
  

看起来你的编译器不能识别重定义并丢失，产生错误的错误信息。

如果您打算提供两个类的前瞻性声明，您需要丢失大括号：

  class baseds; 
 class superior; 
 
基于类{
 / * ... * / 
}; 
  

但是你不使用 superior 对于 baseds 的定义，您可以完全忽略前向声明 - 对于 superior 的定义，您需要 baseds 的定义，因为您有该类型的成员，因此向前声明是不够的。

I am a C++ rookie and I was experimenting with boost serialization and I wanted to see if it works when a class is declared as a member of another class. But when I compile my code I get loads of errors. I tried declaring baseds as a struct but no change in errors. My code :

#include <iostream>
#include <fstream>
#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>

class baseds{};
class superior{};

class baseds {
private:
    friend class boost::serialization::access;
public:
    int a;
    int b;
    int c;
    baseds(){}
    ~baseds(){}
    template <class Archive>
    void serialize(Archive & ar, const unsigned int version)
    {
        ar & a;
        ar & b;
        ar & c;
    }
};

class superior  {
private:
    friend class boost::serialization::access;
public:
    int x;
    int y;
    baseds lag;
    superior(){}
    ~superior(){}
    template <class Archive>
    void serialize(Archive & ar, const unsigned int version)
    {
        ar & x;
        ar & y;
        ar & lag;
    }
};

int main()  {

    superior myData,myData2;
    myData.x=10;
    myData.y=20;
    myData.lag.a=1;
    myData.lag.b=2;
    myData.lag.c=3;

    ofstream ofs("steps.txt");
    {
        boost::serialization::archive one(ofs);
        one << myData;
    }

    ifstream ifs("steps.txt");
    {
        boost::serialization::archive two(ifs);
        two >> myData2;
    }
    std::cout<<"\n"<<myData2.x;
    std::cout<<"\n"<<myData2.y;
    std::cout<<"\n"<<myData2.lag.a;
    std::cout<<"\n"<<myData2.lag.b;
    std::cout<<"\n"<<myData2.lag.c;

    return 0;

}

errors:

tier2.cpp:10: error: a class-key must be used when declaring a friend
tier2.cpp:29: error: a class-key must be used when declaring a friend
tier2.cpp:32: error: expected `;' before "int"
tier2.cpp: In member function `void superior::serialize(Archive&, unsigned int)':
tier2.cpp:38: error: `x' undeclared (first use this function)
tier2.cpp:38: error: (Each undeclared identifier is reported only once for each function it appears in.)
tier2.cpp:39: error: `y' undeclared (first use this function)
tier2.cpp:40: error: `lag' undeclared (first use this function)
tier2.cpp: In function `int main()':
tier2.cpp:47: error: 'class superior' has no member named 'x'
tier2.cpp:48: error: 'class superior' has no member named 'y'
tier2.cpp:49: error: 'class superior' has no member named 'lag'
tier2.cpp:50: error: 'class superior' has no member named 'lag'
tier2.cpp:51: error: 'class superior' has no member named 'lag'
tier2.cpp:53: error: `ofstream' undeclared (first use this function)
tier2.cpp:53: error: expected `;' before "ofs"
tier2.cpp:55: error: `archive' is not a member of `boost::serialization'
tier2.cpp:55: error: expected `;' before "one"
tier2.cpp:56: error: `one' undeclared (first use this function)
tier2.cpp:57: error: expected `;' before '}' token
tier2.cpp:59: error: `ifstream' undeclared (first use this function)
tier2.cpp:59: error: expected `;' before "ifs"
tier2.cpp:61: error: `archive' is not a member of `boost::serialization'
tier2.cpp:61: error: expected `;' before "ones"
tier2.cpp:62: error: `ones' undeclared (first use this function)
tier2.cpp:63: error: expected `;' before '}' token
tier2.cpp:64: error: 'class superior' has no member named 'x'
tier2.cpp:65: error: 'class superior' has no member named 'y'
tier2.cpp:66: error: 'class superior' has no member named 'lag'
tier2.cpp:67: error: 'class superior' has no member named 'lag'
tier2.cpp:68: error: 'class superior' has no member named 'lag'
tier2.cpp:71:2: warning: no newline at end of file

解决方案

You are defining baseds and superior twice.

On gcc you would get the proper error message:

main.cpp:9:7: error: redefinition of 'baseds'
class baseds {
      ^
main.cpp:6:7: note: previous definition is here
class baseds{};
      ^
main.cpp:27:7: error: redefinition of 'superior'
class superior  {
      ^
main.cpp:7:7: note: previous definition is here
class superior{};

Seemingly your compiler does not recognize the redefinition and gets lost, producing the poor error message.

If you meant to provide a forward declaration of both classes, you need to lose the braces:

class baseds;
class superior;

class baseds {
  /* ... */
};

But sice you don't use superior for the definition of baseds at all, you can leave out the forward declarations completely - for the definition of superior you need the definition of baseds, since you have a member of that type and therefore a forward declaration is not sufficient.

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