Forem: yeonseong

division operator in javascript

yeonseong — Sat, 17 Dec 2022 16:23:13 +0000

42 / 2

is 21 of course

42 / 5

is 8.4, not 8

in javascript / operator no discard the decimal point.

you should use Math.round(), Math.ceil() or Math.floor()

Math.round(42 / 5) // 8

singleton in javascript

yeonseong — Fri, 16 Dec 2022 01:29:37 +0000

singleton in javascript

class Singleton
{
    constructor()
    {
        if (!Singleton.instance)
        {
            Singleton.instance = this;
        }
        return Singleton.instance;
    }

    getInstance()
    {
        return this.instance;
    }
}

test

const testA = new Singleton();
const testB = new Singleton();

console.log(testA === testB); // true

singleton example (db connection)

const URL = "db_url";
const createConnection = url => ({"url" : url});
class DB
{
    constructor(url)
    {
        if (!DB.instance)
        {
            DB.instance = createConnection(url);
        }
        return DB.instance;
    }

    connect()
    {
        return this.instance;
    }
}

profits of singleton

save cost of creating instance.

disadvantage of singleton

high coupling
(can be resolved DI(dependency injection))

virtual in C++

yeonseong — Thu, 15 Dec 2022 09:50:14 +0000

implement virtual in C++

C++에서 클래스를 컴파일하면 그 클래스의 모든 메서드를 담은 바이너리 객체가 생성됨.

컴파일러는 virtual로 선언되지 않는 메서드를 호출하는 부분을 정적 바인딩(static binding)함.

virtual로 선언하면 vtable이라는 특수한 메모리 영역을 활용하여 메서드를 호출함. vtable을 보고 적합한 버전의 메서드를 실행함. (동적 바인딩, dynamic binding)

virtual 메서드가 하나 이상 정의도니 클래스마다 vtable이 하나씩 있고, vtable에 대한 포인터를 가짐.

정적바인딩(static binding)

이른 바인딩(early binding)이라고도 함. 컴파일 시간에 바인딩이 결정되는 것

C style cast vs C++ style cast in C++

yeonseong — Wed, 07 Dec 2022 10:20:00 +0000

conclusion

USE C++ STYLE CAST

    ⭕️ static_cast<float>(_value) 
    ❌ (float)_value

reasons

C++ style cast can be check in compile time.
C++ style cast is more spotted easily!

reference

Oct 22 '09 Comments: 1 Answers: 7

270

Is there any reason to prefer static_cast<> over C style casting? Are they equivalent? Is there any sort of speed difference?

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Memory layout in C++

yeonseong — Thu, 01 Dec 2022 12:58:31 +0000

automatic storage

exists till the function ends.** -> only useable in the scope
automatic variable is variable used in function.
allocated in stack -> last-in, first-out ## example in automatic storage

int main()
{
    char temp[42]; // this is automatic variable
    return (0);
}

static storage

exists till program ends.
allocated in data

example in static storage

int main()
{
    static char temp[42]; // this is static variable
    return (0);
}

dynamic storage

exits till delete is called.
new and delete operators manage memory poll (a.k.a free store)
allocated in heap
depend to programmer.
- WATCH MEMORY LEAKS
  - recommend : locate new and delete close.

example in dymaic storage

int main()
{
    int *ptr = new int(42);
    delete ptr;
    return (0);
}

reference

https://hackthedeveloper.com/memory-layout-c-program/
C++ Primer Puls sixth edition(Stephen Prata)

When Constructors is called in C++

yeonseong — Thu, 01 Dec 2022 08:49:30 +0000

Constructor

Constructor in C++ is a special non-static member function of a class that is used to initialize objects of its class type as cpp reference say.

it means Constructors dose not construct anywhere, Constructor is just invoked at the time of object creation.

Constructor is just function that is invoked automatically at the time of object creation. so you can invoke other function in constructor.

example

ClassName::init(const int& id_in) // other function
{
    m_id = id_in;
}

ClassName::ClassName() // constructor
{
    init(42);
}

reference

https://www.geeksforgeeks.org/constructors-c/

https://en.cppreference.com/w/cpp/language/constructor

why seperate hpp and cpp

yeonseong — Tue, 29 Nov 2022 09:53:39 +0000

separating the interface from the implementation.

The header declares "what" a class (or whatever is being implemented) will do

The cpp file defines "how" it will perform those features.

class reusability

other cpp file that wanna use our class, only need the interface(hpp file).

if hpp file have implementation code. it will be included in other cpp file.

refereces

Aug 20 '09 Comments: 2 Answers: 13

195

I've never really understood why C++ needs a separate header file with the same functions as in the .cpp file. It makes creating classes and refactoring them very difficult, and it adds unnecessary files to the project. And then there is the problem with having to include header files, but…

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Dec 2 '08 Comments: 5 Answers: 9

543

Why does C++ have header files and .cpp files?

Open Full Question

Header only vs hpp/cpp styles

Feb 21 '21 Author: DmitryiKh

I would like to ask you about your opinion about header only style of c++ programming.

I’m stick with header only approach for the next reasons:

Not repeating of functions signatures.
Define class method outside of class is a boilerplate (thinking of template methods of template class)
There are lot of templated c++ code that anyway needs to be placed…

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'\n' vs endl in Cpp

yeonseong — Tue, 29 Nov 2022 09:31:57 +0000

std::cout << '\n';

just inserts a new line in cout buffer

std::cout << endl;

inserts a new line in cout buffer, and flushes the cout buffer(a.k.a stream)

?? in Javascript -Nullish coalescing operator-

yeonseong — Fri, 25 Nov 2022 18:22:18 +0000

?? in javascipt

Nullish coalescing operatior(??) is return right operand when left operand is null or undefined. if left operand is not nullish it return left operand.

example basic

const foo = null ?? 'default string';
console.log(foo);
// expected output: "default string"

const bar = 0 ?? 42;
console.log(bar);
// expected output: 0

example in react

const [who, setWho] = useState(initWho ?? "all");

reference

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Nullish_coalescing

Process meaning, Process State

yeonseong — Thu, 24 Nov 2022 17:55:26 +0000

process meaning

Process is the program in execution

process state

running : process has the CPU and executing the mechain language.
ready : can execute immediately. but no allocated CPU
wating(blocked) : can not execute immediately

storage devices hierarchy

yeonseong — Thu, 24 Nov 2022 15:43:55 +0000

source

em vs rem in css

yeonseong — Wed, 23 Nov 2022 18:11:12 +0000

em

1em = 10px * 1
the standard of em is the current element's font-size in using em

rem

1rem = 10px * 1
the standard of em is the root element's (<html>) font-size in using rem