C++ error: Class members must be initialized in the initialization list, how to deal with it?

In C programming, if there is a member variable in the class definition that is not initialized in the initialization list, the compiler will report an error "Class members must be initialized in the initialization list." This means that when an object is created, the constructor of this member variable will not be called, and may cause instability in the program.

So, in C, when creating objects of a class, you must use the initialization list as much as possible. This article will introduce what an initialization list is and its usage, and how to solve the error that class members are not initialized in the initialization list.

What is an initialization list?

In C, members of a class can be basic data types, arrays, structures, classes, etc. When defining member variables in a class, you can choose to initialize them in the constructor body or use an initialization list (Constructor Initialization List).

The initialization list is a function header of the constructor. You can use the initialization list to initialize members before the constructor. It is a series of initialization operations separated by commas after the colon (:) before the constructor body. In the initialization list, you can initialize multiple member variables at the same time, or you can use the default constructor for initialization, thereby avoiding errors caused by class members not being initialized in the initialization list.

The syntax is as follows:

class SomeClass {
public:
    SomeClass(int a, int b, int c) : var1(a), var2(b), var3(c) {
        //构造函数体中的其它操作
    }
private:
    int var1;
    int var2;
    int var3;
};

In this example, SomeClass has three member variables: var1, var2, var3. They will be initialized before the constructor, and other operations in the constructor body It will be executed after initializing the list.

Usage of initialization list

1. Initialization of object members

If there are other objects in the class as class members, you can also use the initialization list to initialize these member. As shown below:

class Person {
public:
    Person(int _age, int _id) : age(_age), id(_id), car("BMW", "blue") {
    }
private:
    int age;
    int id;
    Car car;   //Car 是一个类类型成员
};

In this example, we use the initialization list to initialize the member variables age, id and car. Among them, car is a member variable of the Person class. It is an object of Car type and is declared with "Car car". When we create a Person object, we also need to initialize the car member. When using an initialization list, you need to use the constructor of the Car class to initialize the object.

2. Initialization of type conversion

When you need to convert one type to another type, you can use the initialization list:

class A {
public:
    A(int val) : a(val), b(val*1.2), c(val*1.5) {}
private:
    int a;
    double b;
    float c;
};

In In this example, an int type value is used for initialization, and type conversion can also be performed in the initialization list.

3. Initialization of read-only members

Sometimes, member variables in a class need to be read-only and can only be initialized when the object is created, not during construction Modifications within the function body. This can be achieved using an initialization list. As shown below:

class B {
public:
    B() : a(5), b(2), c(a*b) {}
private:
    const int a;
    const int b;
    const int c;
};

In this example, variables a, b and c are all read-only, and they all need to be initialized when the object is created. Since c is the product of a and b, and both a and b are read-only, they can only be operated on in the initialization list, and the expression assigned to it cannot be modified in the constructor body.

How to deal with the error "Class members must be initialized in the initialization list"?

If the class members are not initialized in the initialization list when creating an object of the class, the compiler will report an error "Class members must be initialized in the initialization list." In order to avoid this error, we can follow the following steps:

1. Determine uninitialized member variables.

In the compiler, the error message will indicate which member variables are not initialized in the initialization list. We can follow the prompts to find the corresponding member variables in the source code, such as:

class Test {
public:
    Test() {
        int a = 0;
        std::string b;
    }
};

In the Test class, there are two member variables a and b. In the constructor of the class, we did not initialize them in the initialization list, which is why the error was reported.

2. Add member variables to the initialization list.

Add uninitialized member variables to the initialization list of the constructor as needed. Modify the code of the constructor to:

class Test {
public:
    Test() : a(0), b("") {}
private:
    int a;
    std::string b;
};

In this example, we use the initialization list to initialize the member variables a and b, thereby avoiding the error "class members must be initialized in the initialization list" .

Notes

In C, using the initialization list can not only avoid errors when class members are not initialized in the initialization list, but also improve the efficiency of the program. If initialization is required in the constructor body, it may cause the constructor of the class member to be called multiple times at the end of the constructor, but using an initialization list will only call it once, thus improving the efficiency of the program.

In addition, when the member variable is of const type, if it is not initialized in the initialization list, the compiler will report an error. When using the initialization list, you should pay attention to the type of member variables and their initialization method.

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