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constexpr 与编译期求值顺序

Mon, 31 Aug 2026 00:00:00 +0800

constexpr 函数不保证在编译期求值。只有在需要常量表达式的场合——数组长度、模板实参、static_assert——编译器才强制走编译期；其它情况下它可以退化成普通的函数调用。

Class

Thu, 21 May 2026 00:00:00 +0800

virtual

This word is supplied for c++98 standard, decorate VIRTUAL with OVERRIDE and FINAL is C++11

Declare the function is a virtual function——allow derived class overwrite the virtual function

derived class own all members which is not private from base class

Runtime Polymorphism

#include <iostream>
#include <string>

// base class
class Animal {
public:
 Animal(){
 std::cout<<"Animal Constructor" <<std::endl;
 }
 // 虚函数：允许派生类重写
 virtual std::string speak() const {

 return "Animal Constructor";
 }

 // 虚析构函数（通常都应写）
 virtual ~Animal(){
 std::cout<<"Animal Destructor"<<std::endl;
 };
};

// 派生类
class Dog : public Animal {
public:
 Dog(){
 std::cout<<"Dog Constructor" <<std::endl;
 }
 std::string speak() const override {

 return "Woof!!!";
 }
 ~Dog(){
 std::cout<< "Dog Destructor"<<std::endl;
 }
};

class Cat : public Dog {
public:
 Cat(){
 std::cout<<"Cat Constructor" <<std::endl;
 }
 std::string speak() const override {

 return "Meow!!!";
 }
 ~Cat(){
 std::cout<<"Cat Destructor" <<std::endl;
 }
};


/// @brief global function accept Animal class or pointer
/// @param a derived class from Animal class
void make_sound(const Animal& a) {
 std::cout << a.speak() << std::endl;
}

int main() {

 Animal* cat = new Cat();
 delete cat;
 std::cout << "Demo con/destructor" << std::endl;
 Dog dog;
 make_sound(dog); // 输出 "Woof!"（实际调用 Dog::speak）
 return 0;

// make_sound(cat); // 输出 "Meow!"（实际调用 Cat::speak）
}

const and override

const decorate the function which declare can’t modify self-object members
override declare the function inherit the same function signature from base class, whether thrrow except

construction and destruction

if base class add virtual keyword for destructor function, therefore all derived class for destructor implicitly own virtual attribute

pure virtual function

it indicate the derived class must overwrite the pure virtual function from base class

#include<iostream>
using namespace std;
class Shape {
public:
 virtual void draw() = 0; // 纯虚函数
};

class Circle : public Shape {
public:
 void draw() override {
 cout << "Draw Circle" << endl;
 }
};

int main() {
 // Shape s; // 错误，抽象类不能实例化
 Shape* s = new Circle();
 s->draw(); // 输出 "Draw Circle"
 delete s;
}

Draw Circle

C++ Templates

Mon, 01 Jan 0001 00:00:00 +0000

C++ Templates

Templates are C++'s mechanism for generic programming — writing code that works with any type without repeating yourself. Unlike runtime polymorphism (virtual functions), templates are resolved entirely at compile time: the compiler generates a separate copy of the function or class for each set of type arguments actually used. This is called template instantiation.

Templates are the foundation of the C++ Standard Library: std::vector<int>, std::map<std::string, double>, std::sort(begin, end) — all templates. They power everything from containers to algorithms to smart pointers. Understanding templates deeply means understanding how modern C++ achieves its combination of abstraction and zero-overhead performance.

operating-system operating-system

Mon, 01 Jan 0001 00:00:00 +0000

Process

A basic resources unit which is assigned with memory in OS - Has isolated memory address

arly System

Thread

A basic unit is called by CPU
rely on and within [Process]
use the virtual address from the Process, but the Threads must be in same process

coroutine

dimension	Process	thread	coroutine
caller	os's kernel	os's kernel	user program
memory space	Individual(no shared)	share with Memory of Process	share with Memory of Proces
switching performance overhead	very high (us-ms)	middle(us)	low(ns)
parallel	hundreds of concurrent cons	Several thousand up to tens of thousands	hundreds of thousands to millions
concurrent ability	multicore	multicore	concurrent within singal thread
usage	sandbox, dense scenario of CPU-process	Web service, dense scenario I/O	high-concurrent gateway,async web crawler

file:memory_alignment.excalidraw