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This article mainly explains "what is the definition and implementation method of C++ class". The explanation content in this article is simple and clear, easy to learn and understand. Please follow the idea of Xiaobian to study and learn "what is the definition and implementation method of C++ class" together.

1. Definition of class

As described in C++ Primer, a class definition is a C++ tool for converting abstractions into user-defined types. That is, the essence of a class is a user-defined type that combines numerical representations and methods for manipulating data into a neat package.

In actual development, implementing a class and writing a program that uses it is relatively complex and involves multiple steps.

Typically, we put the class definition in a header file and the implementation code in a source code file. Let's look at an example from C++ Primer: a class about stock trading.

The first is the definition of the class, written in the stock00.h file:

#ifndef STOCK00_H_#define STOCK00_H_#include class Stock { private: std::string company; long shares; double share_val; double total_val; void set_tot() {total_val = shares * share_val;} public: void accquire(const std::string &co, long n, double pr); void buy(long num, double price); void sell(long num, double price); void update(double price); void show();};#endif

The first is the keyword class, which means that we declare a class, the general class name we use the big hump nomenclature definition.

Second, in the definition of this class, we see two new keywords, private and public. These two keywords describe access control for class members. Programs that use class objects have direct access to the public part (public), but cannot access private members of the object.

To obtain private members, you can only use public member functions. For example, in the example above, we want to change share_val only through the public method sell or buy, and cannot directly change the value of the variable. Preventing programs from accessing data directly to modify data is called data hiding.

In addition to public and private, C++ also provides a third keyword called protected, which is related to class inheritance and we will discuss it in a later article.

The idea behind data hiding is that the ownership of member variables in a class should belong to the class itself, and when we need to read and modify data in the class, we should use the public methods provided by the class, rather than directly manipulating the class values. This is thought of as an object-oriented idea that data should only be accessed through methods provided by classes and should not be accessed directly.

C++ is designed based on this object-oriented idea, so the members of the class are private by default, we can omit the private keyword:

class World { float mass; char name[30];public: void tellall(); ...};

From the description of the class, it looks like a struct with member functions and data hiding, but in fact this is because C++ extends the struct to have the same characteristics as the class. In C, structs don't have so many features.

For example, we can design constructors for structs, add member functions to structs, and even structs can inherit, derive, and be polymorphic.

The only difference between them is that the default type of structs is public, whereas classes are private. So classes are usually used to implement objects, and structs are used only to store data structurally.

One more thing, the object-oriented concept of data hiding does not apply to all languages. For example, Python, the member variables in Python classes are public by default, and there is no private keyword.

II. Implementation of classes

After we have finished defining the class, we need to implement the functions in the class.

For example, we define a class in stock00.h:

#ifndef STOCK00_H_#define STOCK00_H_#include class Stock { private: std::string company; long shares; double share_val; double total_val; void set_tot() {total_val = shares * share_val;} public: void accquire(const std::string &co, long n, double pr); void buy(long num, double price); void sell(long num, double price); void update(double price); void show();};#endif1. Member Functions

In this definition, we only declare functions, and there is no logic to implement them.

We usually implement it in a cpp file with the same name, and when we implement it, we need to use the scope resolution operator to indicate the class to which the function belongs:

void Stock::update(double price) { ...}

This indicates that the update function belongs to the Stock class, which means that we can create functions of the same name belonging to other classes:

void Buffoon::update() { ...}

Second, we can access private-qualified member variables in member functions. For example, in the show function, we can do this:

void Stock::show() { std::cout

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