C++ syntax error: Inheritance syntax error, how to fix it?

In C programming, inheritance is an important concept, which allows us to define a class and derive (or inherit) this class from another class. When we use inheritance, the subclass automatically inherits all member variables and methods of its parent class so that we can reuse the code.

However, when using inheritance, you sometimes encounter problems with inheritance syntax errors. These errors may cause the code to fail to compile, or to cause errors at runtime. This article explains some common inheritance syntax errors and how to fix them.

1. The access permissions of the base class are incorrect
   In C, you can use the three keywords public, protected, and private to specify the access permissions of class members. When we inherit from a class, the member variables and methods of the derived class inherit the access rights of its base class. If the member variables or methods in the base class are declared protected or private, and when you want to access these member variables or methods in the subclass, an access permission error will occur.

Solution: Change the access rights of member variables or methods in the base class to public, or use the "using" keyword in the derived class to import the name of the base class.

2. Use wrong syntax to define derived classes
   In C, when defining a derived class, you need to use the colon ":" to specify the base class. If a colon is not used, or incorrect syntax is used when using inheritance, the compiler will issue an error.

For example, the following code uses incorrect syntax:

class B: A {
  //...
};

The above code does not specify access permissions after using a colon. It should be changed to:

class B : public A {
  //...
};

Solution: You must use correct syntax to define derived classes, that is, use a colon after the derived class name to specify the name and access rights of the base class.

3. The base class constructor is missing in the derived class
   In the derived class, if the base class constructor is not specified or the base class constructor is not called correctly, it will cause compilation mistake.

Solution: In the constructor of the derived class, use the constructor of the base class to initialize the member variables of the base class. This can be achieved by calling the constructor of the base class in the constructor initialization list of the derived class:

class B : public A {
public:
  B(int n) : A(n) { //调用 A 的构造函数
    //...
  }
};

4. Multiple inheritance syntax error
   In C, we can also use multiple inheritance to Derive a class from multiple classes. In multiple inheritance, we need to use commas "," to separate multiple base class names. If incorrect syntax is used when using multiple inheritance, the compiler will report an error.

For example, the following code uses incorrect syntax:

class B : public A1, A2 {
  //...
};

The above code does not specify access permissions and should be changed to:

class B : public A1, public A2 {
  //...
};

Solution: Must use The correct syntax for defining multiple inheritance is to separate multiple base class names with commas and specify access permissions before each base class name.

5. Naming conflicts in inheritance
   In inheritance, there may be the same member variable or method name in the base class and the derived class, which will lead to naming conflicts. When this happens, the compiler will throw an error.

Solution: You can use the scope resolution operator "::" to distinguish base class member variables and methods from derived class member variables and methods. For example:

class A {
public:
  int n;
};
class B : public A {
public:
  int n;
  void print() {
    cout << A::n << endl; //调用基类的 n 变量
    cout << n << endl;    //调用派生类的 n 变量
  }
};

This article introduces some common inheritance syntax errors and solutions. When writing C programs, correct inheritance syntax must be followed to ensure that the code compiles and runs correctly. If you encounter an error, you should check the code promptly and fix it.

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