When Windows cannot detect a second monitor, first check whether the physical connection is normal, including power supply, cable plug-in and interface compatibility, and try to replace the cable or adapter; secondly, update or reinstall the graphics card driver through the Device Manager, and roll back the driver version if necessary; then manually click "Detection" in the display settings to identify the monitor to confirm whether it is correctly identified by the system; finally check whether the monitor input source is switched to the corresponding interface, and confirm whether the graphics card output port connected to the cable is correct. Following the above steps to check in turn, most dual-screen recognition problems can usually be solved.

In C, the POD (PlainOldData) type refers to a type with a simple structure and compatible with C language data processing. It needs to meet two conditions: it has ordinary copy semantics, which can be copied by memcpy; it has a standard layout and the memory structure is predictable. Specific requirements include: all non-static members are public, no user-defined constructors or destructors, no virtual functions or base classes, and all non-static members themselves are PODs. For example structPoint{intx;inty;} is POD. Its uses include binary I/O, C interoperability, performance optimization, etc. You can check whether the type is POD through std::is_pod, but it is recommended to use std::is_trivia after C 11.

In C, there are three main ways to pass functions as parameters: using function pointers, std::function and Lambda expressions, and template generics. 1. Function pointers are the most basic method, suitable for simple scenarios or C interface compatible, but poor readability; 2. Std::function combined with Lambda expressions is a recommended method in modern C, supporting a variety of callable objects and being type-safe; 3. Template generic methods are the most flexible, suitable for library code or general logic, but may increase the compilation time and code volume. Lambdas that capture the context must be passed through std::function or template and cannot be converted directly into function pointers.

When encountering the problem of printing task stuck, clearing the print queue and restarting the PrintSpooler service is an effective solution. First, open the "Device and Printer" interface to find the corresponding printer, right-click the task and select "Cancel" to clear a single task, or click "Cancel all documents" to clear the queue at one time; if the queue is inaccessible, press Win R to enter services.msc to open the service list, find "PrintSpooler" and stop it before starting the service. If necessary, you can manually delete the residual files under the C:\Windows\System32\spool\PRINTERS path to completely solve the problem.

The key steps to install PHP on Windows include: 1. Download the appropriate PHP version and decompress it. It is recommended to use ThreadSafe version with Apache or NonThreadSafe version with Nginx; 2. Configure the php.ini file and rename php.ini-development or php.ini-production to php.ini; 3. Add the PHP path to the system environment variable Path for command line use; 4. Test whether PHP is installed successfully, execute php-v through the command line and run the built-in server to test the parsing capabilities; 5. If you use Apache, you need to configure P in httpd.conf

In C, the mutable keyword is used to allow the object to be modified, even if the object is declared as const. Its core purpose is to maintain the logical constants of the object while allowing internal state changes, which are commonly found in cache, debug counters and thread synchronization primitives. When using it, mutable must be placed before the data member in the class definition, and it only applies to data members rather than global or local variables. In best practice, abuse should be avoided, concurrent synchronization should be paid attention to, and external behavior should be ensured. For example, std::shared_ptr uses mutable to manage reference counting to achieve thread safety and const correctness.

The methods to enable Windows dark mode are as follows: 1. The system-level dark mode can be implemented by selecting "Dark" in "Settings>Personalization>Color", which affects the system interfaces such as the Start Menu and Taskbar; 2. Change the application theme color, you can turn on the accent color in the same menu and enable the dark background, which is suitable for some native applications; 3. Browsers and third-party software need to be set separately, such as enabling dark mode on the flags page of Chrome or Edge, or switching in WeChat, QQ and other software; 4. Automatic switching can be achieved through tools such as AutoDarkMode to improve the user experience.

The key to an abstract class is that it contains at least one pure virtual function. When a pure virtual function is declared in the class (such as virtualvoiddoSomething()=0;), the class becomes an abstract class and cannot directly instantiate the object, but polymorphism can be realized through pointers or references; if the derived class does not implement all pure virtual functions, it will also remain an abstract class. Abstract classes are often used to define interfaces or shared behaviors, such as designing Shape classes in drawing applications and implementing the draw() method by derived classes such as Circle and Rectangle. Scenarios using abstract classes include: designing base classes that should not be instantiated directly, forcing multiple related classes to follow a unified interface, providing default behavior, and requiring subclasses to supplement details. In addition, C