我很困惑如何从另一个类实现数据成员访问。
我在三个不同的头文件中有三个类。
A.h
#include "B.h"
#include "C.h"
class A{
B *b;
friend void dataaccess_b_from_class_A(int a);
}
B.h
class B{
}
C.h
class C{
void dataaccess_b_from_class_A(int a);
}
void C::dataaccess_b_from_class_A(int a)
{
b = new B(); //I got error as symbol b could not be resolved.
}
我喜欢
dataaccess_b_from_class_A()
中的class C
方法访问B *b
中的数据Class A
。我在
class A
中放入了朋友函数,但是却得到了error as symbol b could not be resolved
。如何实现它?编辑1:
根据讨论“ gmas80”,
我所做的是
class B; // forward declaration
class A{
public:
A(){};
private:
static B* b; // since in the dataaccess_to_class_A you are using new
friend class C; // this make b and dataaccess_from_class_C accessible
};
class B{
public:
B(){ cout << "B" << endl; };
// add content
};
class C{
public: // you need this keyword if you want to call this function from outside
void dataaccess_to_class_A(A a);
};
void C::dataaccess_to_class_A(A a)
{
A::b = new B(); //Error as undefined reference to class A::b
cout << "C" << endl; // test if called
}
如果不包括static,则得到b
could not be resolved
。如果包含static,则得到
undefined reference
。谢谢
最佳答案
您在这么小的代码段中包含了很多错误!为什么不从简单开始?这是编译后的代码的修改后的单文件版本:
#include <iostream>
using namespace std;
class B; // forward declaration
class A{
public:
A(){};
private:
B* b; // since in the dataaccess_to_class_A you are using new
void dataaccess_from_class_C(){ cout << "A" << endl; }; // test if called
friend class C; // this make b and dataaccess_from_class_C accessible
};
class B{
public:
B(){ cout << "B" << endl; };
// add content
};
class C{
public: // you need this keyword if you want to call this function from outside
void dataaccess_to_class_A(A a);
};
void C::dataaccess_to_class_A(A a)
{
a.b = new B(); // this is a potentially memory leak if you will not delete in somehow
a.dataaccess_from_class_C();
cout << "C" << endl; // test if called
}
// it is better if you post runnable code
int main() {
C c;
A a;
c.dataaccess_to_class_A(a);
}
编辑1之后
现在,您可以开始将类移动到头文件中,但是您需要添加监护人以避免多个定义。
啊
#ifndef H_GUARDIAN_A
#define H_GUARDIAN_A
#include <iostream>
using namespace std;
class B; // forward declaration
class A
{
public:
A()
{};
private:
B* b; // since in the dataaccess_to_class_A you are using new
void dataaccess_from_class_C()
{
cout << "A" << endl;
}; // test if called
friend class C; // this make b and dataaccess_from_class_C accessible
};
class B
{
public:
B()
{
cout << "B" << endl;
};
// add content
};
class C
{
public: // you need this keyword if you want to call this function from outside
void dataaccess_to_class_A( A a );
};
void C::dataaccess_to_class_A( A a )
{
a.b = new B(); // this is a potentially memory leak if you will not delete in somehow
a.dataaccess_from_class_C();
cout << "C" << endl; // test if called
}
#endif
main.cpp
#include "a.h"
// it is better if you post runnable code
int main()
{
C c;
A a;
c.dataaccess_to_class_A( a );
}
这对您有意义吗?我简单地将类声明移到了另一个文件中。
编辑2
现在,我们将类定义分为三个不同的标题。
啊
#ifndef H_GUARDIAN_A
#define H_GUARDIAN_A
#include <iostream>
using namespace std;
class B;
class A
{
public:
A(): b(NULL){}; // initialize to NULL the b pointer
~A(); // new entry: destructor to eventually delete b (only member function declaration)
private:
B* b; // since in the dataaccess_to_class_A you are using new
void dataaccess_from_class_C()
{
cout << "A" << endl; // test if called
};
friend class C; // this make b and dataaccess_from_class_C accessible
};
#endif
a.cpp //新条目!它避免了循环依赖..析构函数在这里定义
#include "a.h"
#include "b.h"
A::~A() // destructor that eventually clean memory for b
{
if( b ) delete b;
}
身高
#ifndef H_GUARDIAN_B
#define H_GUARDIAN_B
#include "a.h"
class B
{
public:
B()
{
cout << "B" << endl;
};
// add content
};
#endif
。
#ifndef H_GUARDIAN_C
#define H_GUARDIAN_C
#include "b.h"
class C
{
public: // you need this keyword if you want to call this function from outside
void dataaccess_to_class_A( A a );
};
void C::dataaccess_to_class_A( A a )
{
a.b = new B(); // this is a potentially memory leak if you will not delete in somehow
a.dataaccess_from_class_C();
cout << "C" << endl; // test if called
}
#endif
main.cpp
#include "c.h"
// it is better if you post runnable code
int main()
{
C c;
A a;
c.dataaccess_to_class_A( a );
}
关于c++ - 来自C++中不同类的成员函数的数据访问,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/26600340/