C++ syntax error: functions cannot be included in class definitions, how to deal with it?

C As an object-oriented programming language, the definition of a class is one of its core concepts. When writing classes, we often encounter some syntax errors, including errors that functions cannot be included in class definitions. So how do we deal with this syntax error?

1. Cause Analysis

In C language, a class definition can only contain member variables and member functions, and functions cannot be defined directly in the class definition. This is because the functions defined in the class definition are member functions and must be called through an instance of the class. The function defined in the class definition cannot determine the instance to which the function belongs and cannot be called.

Therefore, if a function is defined in a class definition, a syntax error will occur. The following is a common error example of defining a function in a class definition:

class MyClass {
private:
  int a;
  void func() { // 错误！在类定义中定义函数
    // ...
  }
public:
  // constructor, destructor, other member functions...
};

2. Solution

When a syntax error occurs when a function is included in a class definition, we can take the following steps Solution:

(1) Move the functions defined in the class definition outside the class definition. We can move the function definition after the class definition and define the member functions of the class. This will resolve the error. The modified code is as follows:

class MyClass {
private:
  int a;
public:
  void func(); // 在类定义中声明函数
  // constructor, destructor, other member functions...
};

void MyClass::func() { // 在类定义外定义函数
  // ...
}

(2) Define the function as a static function. In the class definition, the function can also be defined as a static function. The static function belongs to the entire class and is called directly by the class name. It does not require an object or instantiation. Use static functions to place function definitions in class definitions. If the function defined in the class definition does not need to access the member variables of the class, but only needs to implement some special functions, you can try to use static functions. The following is a modified code example:

class MyClass {
private:
  int a;
public:
  static void func(); // 静态函数
  // constructor, destructor, other member functions...
};

void MyClass::func() {
  // ...
}

(3) Use inline functions. An inline function is a special function that is usually used for functions that need to be called frequently. The function definition can be placed inside the class definition, and its execution efficiency is higher. Unlike the previous static functions, the definition of inline functions can be placed inside the class definition. The following is an example:

class MyClass {
private:
  int a;
public:
  inline void func() { // 内联函数，放在类定义内部
    // ...
  }
  // constructor, destructor, other member functions...
};

Through the above three methods, we can solve the syntax error of functions included in the class definition.

3. Summary

It is a common syntax error that a class definition cannot contain functions. The reason for this error is that a class definition can only contain member variables and member functions. . We can solve this error by moving the function definition outside the class definition, a static function, or an inline function. Only by properly correcting grammatical errors can you better use the C programming language and improve your programming skills.

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