C++ syntax error: The constructor defined outside the class must be added with the class name as a qualifier. How should it be corrected?

C is a widely used object-oriented programming language. When defining the constructor of a class in C, if you want to place the definition of the constructor outside the class, you need to Add the class name as a qualifier to the function definition to specify which class this constructor belongs to. This is a basic rule of C syntax. If this rule is not followed when defining the constructor of a class, a compilation error will appear, prompting "Constructors defined outside the class must be qualified with the class name."

So, if you encounter this kind of compilation error, how should you correct it? A simple and effective way is to add the class name as a qualifier when defining the constructor of a class to ensure that the compiler can correctly handle the class to which the function belongs.

Next, this article will detail how to define a constructor outside a class and avoid syntax errors.

1. Declare the constructor in the class definition

First, declare the constructor in the class definition. When declaring a constructor, you do not need to write the function body, you only need to write the function name and parameters. For example:

class Person {
public:
    Person(int age); // 声明构造函数
private:
    int m_age;
};

This code declares a Person class, which contains a constructor whose parameter is an integer type age.

2. Define the constructor outside the class

Next, define the constructor outside the class. It should be noted that when defining a constructor, you must add the class name as a qualifier to specify which class this function belongs to. For example:

Person::Person(int age) { // 类名作为限定符
    m_age = age;
}

This code defines the constructor of the Person class. It should be noted that when defining a constructor, the class name must be added as a qualifier. If the qualifier is omitted, the compiler will prompt an error message: "Constructors defined outside the class must be qualified with the class name."

3. Define multiple constructors outside the class

If you need to define multiple constructors outside the class, you can use function overloading, before each function name Add different formal parameter lists to distinguish different constructors. For example:

Person::Person() { // 默认构造函数
    m_age = 0;
}

Person::Person(int age) { // 带参构造函数
    m_age = age;
}

This code defines two constructors, one is the default constructor and the other is the parameterized constructor. By adding different formal parameter lists before the function name, the compiler can correctly distinguish between the two constructor definitions.

It should be noted that the definition of the constructor declared within the class is the default definition. If the user does not declare or define it outside the class, the compiler will automatically generate the definition of a default constructor. For constructors without parameters Function, its constructor body is empty; for a constructor with parameters, the parameters will be initialized. If you need to declare within the class and define the constructor outside the class, you need to clear the default definition declared within the class. For example, declare the following within the class:

class Person {
public:
  Person()=default； // 显示声明默认构造函数
  Person(int age);
private:
  int m_age;
}

Define the default constructor outside the class:

Person::Person() { // 默认构造函数
    m_age = 0;
}

4. Summary

In C, constructors defined outside a class must be qualified with the class name. If you omit the class name qualifier, the compiler will prompt a syntax error. In order to avoid this error, you can declare the constructor first in the class definition, then define the constructor outside the class, and add the class name as a qualifier before the function name. If you need to define multiple constructors, you can use function overloading. Mastering these basic knowledge, you can become more proficient in using C language to write programs.

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