7. What are the properties of C (object-oriented features)

Encapsulation, inheritance and multiple state.

In object-oriented programming languages, encapsulation is a feature that uses reusable components to construct software systems. It not only supports the reusability of the system, but also helps improve the scalability of the system; message passing can achieve Sending a common message while calling different methods; encapsulation is a technique to achieve information hiding, and its purpose is to separate the definition and implementation of a class.

#8. When do you need to use "constant references"?

If you want to use references to improve the efficiency of the program and protect the data passed to the function from being changed in the function, you should use constant references. Constant reference declaration method: const type identifier & reference name = target variable name;

int a

const int &ra=a;

ra=1; //Error

a=1; //Correct

Example 2 string foo();

void bar(string & s);

Then the following expression The formula will be illegal:

bar(foo( ));

bar("hello world");

The reason is that foo() and "hello world" string A temporary object will be generated, and in C, these temporary objects are of const type. Therefore, the above expression is trying to convert a const type object to a non-const type, which is illegal. Reference parameters should be defined as const if they can be defined as const

9. What is ASSERT() used for

Answer: ASSERT() is a macro often used when debugging a program. When the program is running, it evaluates the expression in the brackets. If the expression is FALSE (0),

Program An error will be reported and execution will be terminated. If the expression is not 0, continue executing the following statements. This macro usually determines whether obviously illegal data appears in the program. If so, it terminates the program to avoid serious consequences and also facilitates the search for errors. For example, the variable n should not be 0 in the program. If it is 0, it may cause an error.

10. achieve more Stateful method?

11. Three basic characteristics of object-oriented and simple Narrate it?

③ Polymorphism: It is a technology that sets a parent object to be equal to one or more of its child objects. After assignment, the parent object can operate in different ways according to the characteristics of the child objects currently assigned to it. To put it simply, it is one sentence: It is allowed to assign a pointer of a subclass type to a pointer of a parent class type.

Supplementary question: What is the role of polymorphism?

There are two main ones:

1. Hide implementation details so that the code can be modularized; extend code modules to achieve code reuse;

2. Interface reuse: for classes When inheriting and deriving, ensure the correct call when using an attribute of an instance of any class in the family.

12## Overload (overload) and rewrite (overried), some books are also called "cover ”)?

13What is the role of polymorphism?

14, empty class in C, default class member function:

class
{
  public:
    Empty();                         // 缺省构造函数
    Empty(const Empty&);             // 拷贝构造函数
    ~Empty();                        // 析构函数
    Empty& operator=(const Empty&);  // 赋值运算符
    Empty* operator&();              // 取值运算符
    const Empty* operator&() const;  // 取值运算符const
};

15. What are the methods of inter-process communication?

16Four necessary conditions for deadlock?

17 What is the difference between static members and non-static members of a class?

What is a shallow copy? What is a deep copy?

Shallow copy means that the source object and the copy object share an entity, only the referenced variables are different (different names). Changes to any one object will affect the other object.

Deep copy means that the source object and the copy object are independent of each other, and changes to any one object will not affect the other object.

Generally speaking, a shallow copy is to copy the pointer of that object. A deep copy copies that object.

19##、Windows Several ways to synchronize programming threads? (Important)

Atomic lock, critical section (segment), event, mutex (body), semaphore, waitable timer

20What is "quote"? What issues should we pay attention to when declaring and using "reference"?

21. What is the difference between a "reference" and a pointer?

22. What is the difference between association, aggregation (Aggregation) and composition (Composition)?

##23.For Three basic characteristics of objects and briefly describe them? 1. Encapsulation: Abstract objective things into classes, and each class implements protection (private, protected, public) for its own data and methods

2. Inheritance: There are three implementation forms of generalized inheritance: implementation inheritance (referring to the ability to use the properties and methods of the base class without additional coding), visual inheritance (the child form uses the appearance and implementation code of the parent form), and interface Inheritance (only properties and methods are used, implementation lags to subclass implementation). The first two (class inheritance) and the latter (object combination => interface inheritance and pure virtual function) constitute two ways of function reuse.

3. Polymorphism: It is a technique of setting a parent object to be equal to one or more of its child objects. After assignment, the parent object can be configured differently according to the characteristics of the child objects currently assigned to it. way to operate. To put it simply, it is one sentence: It is allowed to assign a pointer of a subclass type to a pointer of a parent class type.

24 What is the role of polymorphism? There are two main ones: 1. Hiding implementation details so that the code can be modularized; extending code modules to achieve code reuse; 2. Interface reuse: for classes to use inheritance and When deriving, ensure the correct call when using an attribute of an instance of any class in the family.

25 Why do we need to call a function compiled by the C compiler in a C program? Add extern "C" statement?

//extern is a keyword in C/C language that indicates the scope of functions and global variables. This keyword tells the compiler that the functions and variables it declares can be used in this module or other modules

// extern "C is a connection declaration. When compiling, it tells the compiler that the following code should be compiled and connected in a C-style way. The purpose is to realize mixed programming of C, C and other languages.

26.What is the difference between a "reference" and a pointer?

After a pointer points to an object through a pointer variable, it operates indirectly on the variable it points to. Using pointers in programs makes the program less readable; and the reference itself is the target variable. Alias, the operation on the reference is the operation on the target variable. In addition, it is the difference between passing ref and pointer to the function mentioned above.

23.The connection and difference between New delete and malloc free?

Answer: Both perform dynamic memory operations on the heap. Use The malloc function needs to specify the number of bytes of memory allocation and cannot initialize the object. new will automatically call the object's constructor. delete will call the object's destructor, while free will not call the object's destructor.

24 Characteristics of overloaded member functions:
(1) Same scope (In the same class);
(2) The function name is the same;
(3) The parameters are different;
(4) The virtual keyword is optional.

25##Overwriting means that the derived class function covers the base class function. The characteristics are: (1) Different scopes (located in derived classes and base classes respectively);
(2) The function names are the same;
(3) The parameters are the same;
(4) The base class function must have the virtual keyword .

26## If you use VC to develop a program, there are several common mistakes, C2001, c2005 ,c2011, what are the causes of these errors. In the process of learning VC, the error messages of LNK2001 errors encountered are mainly:

unresolved external symbol "symbol" (undefined external "symbol").

This error message will be generated if the linker cannot find the referenced function, variable or label in all libraries and object files.

Generally speaking, there are two reasons for errors: First, the referenced function or variable does not exist, is spelled incorrectly, or is used incorrectly;

Secondly, different versions of the connection may be used. library.

We often encounter LNK2005 errors in programming - repeated definition errors. In fact, LNK2005 errors are not difficult to solve.

27Introduce STL and explain in detail how STL implements vector. STL (Standard Template Library) is composed of container algorithm iterators.

STL has the following advantages:

It can easily implement a series of algorithms such as searching for data or sorting data;

It is safer and more convenient to debug programs;

You can easily understand even the code written by people using STL under the UNIX platform (because STL is cross-platform).

        Vector is essentially a dynamic array, which will dynamically increase the array space according to the increase of data.

28Introduce templates and containers. How to achieve? (Maybe I will give you an example to implement it on the spot) ## Templates can be said to be relatively old, but the current generic programming is essentially template programming.

It embodies a universal and generalized idea.

STL has 7 main containers: vector, list, deque, map, multimap, set, multiset.

29 : Briefly describe the principle of polymorphic implementation

​​​​​ When the compiler finds that there is a virtual function in a class, it will immediately Generate virtual function table vtable for this class. Each entry in the virtual function table is a pointer to the corresponding virtual function. The compiler also implicitly inserts a pointer vptr into this class (for the vc# compiler, it is inserted into the class ) points to the virtual function table. When calling the constructor of this class, the compiler will implicitly perform the association between vptr and vtable in the class constructor. Code, point vptr to the corresponding vtable, and connect the class with the vtable# of this class ## got in touch. In addition, when the constructor of the class is called, the pointer to the base class has now become the this pointer to the specific class, thus relying on this thisThe pointer can get the correct vtable. Only in this way can we truly connect with the function body. This is dynamic binding and the basic principle of realizing polymorphism.

30: Talk about your understanding of object-oriented

Analysis:Object-oriented can be understood as treating every problem by first determining that the problem consists of several parts, and each part is actually an object. Then design these objects separately, and finally get the entire program. Traditional programming is mostly considered and designed based on functional ideas, while object-oriented programming considers problems based on the object's perspective. Doing so can make the program more concise and clear.

Explanation: The "Object-oriented programming technology" that is most commonly encountered in programming is just It is an integral part of object-oriented technology. Taking advantage of object-oriented technology is a comprehensive technical issue that not only requires object-oriented analysis, design and programming technology, but also requires the help of necessary modeling and development tools.

#31 Why use template classes in C.

#32 What is the difference between function templates and class templates?

Answer: The instantiation of function templates is automatically completed by the compiler when processing function calls, while the instantiation of class templates

Must be explicitly specified by the programmer in the program.

33. What are the main steps to achieve a winsock connection? (Very important, must ask)

Answer: Server side: socker()Establish a socket, bind (bind) and listen (listen), use accept()

Waiting for client connection.

Client: socker()Establish a socket and connect (connect ) server, after connecting, use send() and recv (

) , write and read data on the socket until the data exchange is completed, closesocket()Close the socket.

Server side: accept() finds a client connection, establishes a new socket, and restarts waiting for the connection

catch. The newly generated socket uses send() and recv() to write and read data until the data exchange is completed, closesock

et()Close the socket.

#34 The difference between processes and threads.

Answer: A thread refers to an execution unit within a process and is also a schedulable entity within the process. The difference from a process:

(1) Scheduling : Threads are the basic unit of scheduling and allocation, and processes are the basic units of resource ownership.

(2) Concurrency: Not only processes can be executed concurrently, but multiple threads of the same process can also be executed concurrently.

(3) Own resources: A process is an independent unit that owns resources. Threads do not own system resources, but they can access resources belonging to the process.

(4) System overhead: When creating Or when canceling a process, because the system has to allocate and reclaim resources for it, the system overhead is significantly greater than the overhead when creating or canceling threads.

35. Is C type-safe?

Answer: No. Two pointers of different types can be cast (using reinterpret cast). C# is type safe.

1. If you use VC to develop a program, there are several common errors, C2001, c2005, c2011, what are the reasons for these errors.

37 The difference between macros and inline functions

Analysis: Both inline functions and macros are expanded where the program appears. Inline functions are not implemented through function calls. They are expanded at the program that calls the function (completed during compilation); macros are also ;

The difference is that inline functions can complete compilation functions such as type detection and whether statements are correct during compilation; macros do not have such functions, and the macro expansion time is different from that of inline functions. (Expand during runtime)

#38 Where is the entry point of the message loop Windows program in win32? Write the process of Windows message mechanism

# Analysis: The entry point of Windows program is the WinMain() function.

Windows application message processing mechanism:

A. The operating system receives the window message of the application and delivers the message to the application's message queue

B. Application The program calls the GetMessage function in the message loop to retrieve messages one by one from the message queue. After retrieving the messages, the application can perform some preprocessing on the messages.

C. The application calls DispatchMessage to send the message back to the operating system.

D. The system uses the pointer to the window procedure function saved in the lpfnWndProc member of the WNDCLASS structure to call the window procedure to process the message.

39: Talk about your understanding or knowledge of programming specifications

Programming specifications can be summarized as: Program Feasibility, readability, portability and testability.

Note: This is the general outline of programming specifications. Interviewers do not have to recite the examples given above. They should understand the problems illustrated by these examples and think about how they can solve the feasibility and The questions of readability, portability and testability are combined with the above examples and your usual programming habits to answer this question.

40 Why do stream operator overloads return references

 using namespace std;
   class A1{
   public:
    int a;
    static int b;
    A1();
    ~A1();
   };
   int main()   {
    cout << sizeof(A1) <<endl;
   }