May 7, 2020 – MAlabdali

gamma function

by malabdaliMay 7, 2020math

in cmath is called tgamma

std::move is used to indicate that an object t may be “moved from”, i.e. allowing the efficient transfer of resources from t to another object.
In particular, std::move produces an xvalue expression that identifies its argument t. It is exactly equivalent to a static_cast to an rvalue reference type.

#include <iostream>
#include <utility>
#include <vector>
#include <string>
 
int main()
{
    std::string str = "Hello";
    std::vector<std::string> v;
 
    // uses the push_back(const T&) overload, which means 
    // we'll incur the cost of copying str
    v.push_back(str);
    std::cout << "After copy, str is \"" << str << "\"\n";
 
    // uses the rvalue reference push_back(T&&) overload, 
    // which means no strings will be copied; instead, the contents
    // of str will be moved into the vector.  This is less
    // expensive, but also means str might now be empty.
    v.push_back(std::move(str));
    std::cout << "After move, str is \"" << str << "\"\n";
 
    std::cout << "The contents of the vector are \"" << v[0]
                                         << "\", \"" << v[1] << "\"\n";
}

After copy, str is "Hello"
After move, str is ""
The contents of the vector are "Hello", "Hello"

forward

template<class T>
void wrapper(T&& arg) 
{
    // arg is always lvalue
    foo(std::forward<T>(arg)); // Forward as lvalue or as rvalue, depending on T
}

struct Arg
{
    int i = 1;
    int  get() && { return i; } // call to this overload is rvalue
    int& get() &  { return i; } // call to this overload is lvalue
};

#include <iostream>
#include <memory>
#include <utility>
 
struct A {
    A(int&& n) { std::cout << "rvalue overload, n=" << n << "\n"; }
    A(int& n)  { std::cout << "lvalue overload, n=" << n << "\n"; }
};
 
class B {
public:
    template<class T1, class T2, class T3>
    B(T1&& t1, T2&& t2, T3&& t3) :
        a1_{std::forward<T1>(t1)},
        a2_{std::forward<T2>(t2)},
        a3_{std::forward<T3>(t3)}
    {
    }
 
private:
    A a1_, a2_, a3_;
};
 
template<class T, class U>
std::unique_ptr<T> make_unique1(U&& u)
{
    return std::unique_ptr<T>(new T(std::forward<U>(u)));
}
 
template<class T, class... U>
std::unique_ptr<T> make_unique2(U&&... u)
{
    return std::unique_ptr<T>(new T(std::forward<U>(u)...));
}
 
int main()
{   
    auto p1 = make_unique1<A>(2); // rvalue
    int i = 1;
    auto p2 = make_unique1<A>(i); // lvalue
 
    std::cout << "B\n";
    auto t = make_unique2<B>(2, i, 3);
}

rvalue overload, n=2
lvalue overload, n=1
B
rvalue overload, n=2
lvalue overload, n=1
rvalue overload, n=3

exchange

template<class T, class U = T>
constexpr // since C++20
T exchange(T& obj, U&& new_value)
{
    T old_value = std::move(obj);
    obj = std::forward<U>(new_value);
    return old_value;
}

swap

int main()
{
   int a = 5, b = 3;
 
   // before
   std::cout << a << ' ' << b << '\n';
 
   std::swap(a,b);
 
   // after
   std::cout << a << ' ' << b << '\n';
}

bitset
set,reset,flip,size,all,any,none ~^|&
https://en.cppreference.com/w/cpp/utility/bitset

#include <bitset>
 
int main()
{
    std::bitset<8> b1(42);
    for (std::size_t i = 0; i < b1.size(); ++i) {
        std::cout << "b1[" << i << "]: " << b1[i] << '\n';
    }
    b1[0] = true; // modifies the first bit through bitset::reference
    std::cout << "After setting bit 0, the bitset holds " << b1 << '\n';
}

type_traits
https://en.cppreference.com/w/cpp/header/type_traits

Composite type categories	Primary type category
`is_arithmetic`	`is_floating_point` or `is_integral`
`is_fundamental`	`is_arithmetic` or `is_void`
`is_object`	`is_arithmetic` or `is_enum` or `is_pointer` or `is_member_pointer`
`is_reference`	`is_lvalue_reference` or `is_rvalue_reference`
`is_compound`	complement of `is_fundamental`
`is_member_pointer`	`is_member_object_pointer` or `is_member_function_pointer`

`template class Base, class Derived>`	Checks if `Derived` is derived from `Base`.
`struct is_base_of`
`template <class From, class To>`	Checks if `From` can be converted to `To`.
`struct is_convertible`
`template <class T, class U>`	Checks if the types `T` and `U` are the same.
`struct is_same`

#include <type_traits>
using std::cout;

cout << std::is_void<void>::value;             // true
cout << std::is_integral<short>::value;        // true
cout << std::is_floating_point<double>::value; // true
cout << std::is_array<int [] >::value;         // true
cout << std::is_pointer<int*>::value;          // true
cout << std::is_reference<int&>::value;        // true

struct A{
  int a;
  int f(int){ return 2011; }
};
cout << std::is_member_object_pointer<int A::*>::value;          // true
cout << std::is_member_function_pointer<int (A::*)(int)>::value; // true

enum E{
  e= 1,
};
cout << std::is_enum<E>::value;                  // true

union U{
  int u;
};
cout << std::is_union<U>::value;                 // true

cout << std::is_class<std::string>::value;       // true
cout << std::is_function<int * (double)>::value; // true
cout << std::is_lvalue_reference<int&>::value;   // true
cout << std::is_rvalue_reference<int&&>::value;  // true
cout << is_const<int>::value;                     // false
cout << is_const<const int>::value;               // true
cout << is_const<add_const<int>::type>::value;    // true

typedef add_const<int>::type myConstInt;
cout << is_const<myConstInt>::value;              //true

typedef const int myConstInt2;
cout << is_same<myConstInt, myConstInt2>::value;  // true

cout << is_same<int, remove_const<add_const<int>::type>::type>::value;    // true
cout << is_same<const int, add_const<add_const<int>::type>::type>::value; // true

functional
https://en.cppreference.com/w/cpp/header/functional

...
#include <functional>
...
// for placehoder _1 and _2
using namespace std::placeholders; f

using std::bind;
using std::function
...
double divMe(double a, double b){ return a/b; };
function < double(double, double) > myDiv1= bind(divMe, _1, _2);
function < double(double) > myDiv2= bind(divMe, 2000, _1);

divMe(2000, 10) == myDiv1(2000, 10) == myDiv2(10);

#include <functional>
...
using std::make_pair;
using std::map;

map<const char, std::function<double(double, double)>> tab;
tab.insert(make_pair('+', [](double a, double b){ return a + b; }));
tab.insert(make_pair('-', [](double a, double b){ return a - b; }));
tab.insert(make_pair('*', [](double a, double b){ return a * b; }));
tab.insert(make_pair('/', [](double a, double b){ return a / b; }));

std::cout << tab['+'](3.5, 4.5);  // 8
std::cout << tab['-'](3.5, 4.5);  // -1
std::cout << tab['*'](3.5, 4.5);  // 15.75
std::cout << tab['/'](3.5, 4.5);  // 0.777778

Function type	Return type	Type of the arguments
`double(double, double)`	`double`	`double`
`int()`	`int`
`double(int, double)`	`double`	`int, double`
`void()`

std::variant

std::variant is a type-safe union. An instance of std::variant has a value from one of its types. The type must not be a reference, array or void.

#include <variant>

...

std::variant<int, float> v, w;
v = 12;                             // v contains int
int i = std::get<int>(v);
w = std::get<int>(v);
w = std::get<0>(v);                 // same effect as the previous line
w = v;                              // same effect as the previous line
 
//  std::get<double>(v);            // error: no double in [int, float]
//  std::get<3>(v);                 // error: valid index values are 0 and 1
 
try{
  std::get<float>(w);               // w contains int, not float: will throw
}
catch (std::bad_variant_access&) {}
 
std::variant<std::string> v("abc"); // converting constructor must be unambiguous
v = "def";                          // converting assignment must be unambiguous

optional
can have value or empty

#include <optional>

std::optional<int> getFirst(const std::vector<int>& vec){
  if (!vec.empty()) return std::optional<int>(vec[0]);
  else return std::optional<int>();
}

...
    
std::vector<int> myVec{1, 2, 3};
std::vector<int> myEmptyVec;
    
auto myInt= getFirst(myVec);
    
if (myInt){
  std::cout << *myInt << std::endl;                       // 1
  std::cout << myInt.value() << std::endl;                // 1
  std::cout << myInt.value_or(2017) << std::endl;         // 1
}
    
auto myEmptyInt= getFirst(myEmptyVec);
    
if (!myEmptyInt){
  std::cout << myEmptyInt.value_or(2017) << std::endl;   // 2017
}

std::any

std::any is a type-safe container for single values of any type which is copy-constructible.

#include <algorithm>
#include <any>
#include <iostream>
#include <string>
#include <vector>
 
class Star
{
    std::string name;
    int id;
 
public:
    Star(std::string name, int id) : name { name }, id { id }
    {
        std::cout << "Star::Star(string, int)\n";
    }
 
    void print() const
    {
        std::cout << "Star{ \"" << name << "\" : " << id << " };\n";
    }
};
 
auto main() -> int
{
    std::any celestial;
    // (1) emplace( Args&&... args );
    celestial.emplace<Star>("Procyon", 2943);
    const auto* star = std::any_cast<Star>(&celestial);
    star->print();
 
    std::any av;
    // (2) emplace( std::initializer_list<U> il, Args&&... args );
    av.emplace<std::vector<char>>({ 'C', '+', '+', '1', '7' } /* no args */ );
    std::cout << av.type().name() << '\n';
    const auto* va = std::any_cast<std::vector<char>>(&av);
    std::for_each(va->cbegin(), va->cend(), [](char const& c) { std::cout << c; });
    std::cout << '\n';
}

#include <any>

struct MyClass{};

...

std::vector<std::any> anyVec{true, 2017, std::string("test"), 3.14,  MyClass()};
std::cout << std::any_cast<bool>(anyVec[0]);                          // true
int myInt= std::any_cast<int>(anyVec[1]);                                        
std::cout << myInt << std::endl;                                      // 2017
    
std::cout << anyVec[0].type().name();                                 // b 
std::cout << anyVec[1].type().name();                                 // i

any a=any(12);
a.emplace<B>(B());
cout<<a.type().name();//1B

Conversion in c++

by malabdaliMay 7, 2020c++

static_cast
Converts between types using a combination of implicit and user-defined conversions

#include <vector>
#include <iostream>
 
struct B {
    int m = 0;
    void hello() const {
        std::cout << "Hello world, this is B!\n";
    }
};
struct D : B {
    void hello() const {
        std::cout << "Hello world, this is D!\n";
    }
};
 
enum class E { ONE = 1, TWO, THREE };
enum EU { ONE = 1, TWO, THREE };
 
int main()
{
    // 1: initializing conversion
    int n = static_cast<int>(3.14); 
    std::cout << "n = " << n << '\n';
    std::vector<int> v = static_cast<std::vector<int>>(10);
    std::cout << "v.size() = " << v.size() << '\n';
 
    // 2: static downcast
    D d;
    B& br = d; // upcast via implicit conversion
    br.hello();
    D& another_d = static_cast<D&>(br); // downcast
    another_d.hello();
 
    // 3: lvalue to xvalue
    std::vector<int> v2 = static_cast<std::vector<int>&&>(v);
    std::cout << "after move, v.size() = " << v.size() << '\n';
 
    // 4: discarded-value expression
    static_cast<void>(v2.size());
 
    // 5. inverse of implicit conversion
    void* nv = &n;
    int* ni = static_cast<int*>(nv);
    std::cout << "*ni = " << *ni << '\n';
 
    // 6. array-to-pointer followed by upcast
    D a[10];
    B* dp = static_cast<B*>(a);
 
    // 7. scoped enum to int or float
    E e = E::ONE;
    int one = static_cast<int>(e);
    std::cout << one << '\n';
 
    // 8. int to enum, enum to another enum
    E e2 = static_cast<E>(one);
    EU eu = static_cast<EU>(e2);
 
    // 9. pointer to member upcast
    int D::*pm = &D::m;
    std::cout << br.*static_cast<int B::*>(pm) << '\n';
 
    // 10. void* to any type
    void* voidp = &e;
    std::vector<int>* p = static_cast<std::vector<int>*>(voidp);
}

dynamic_cast
Safely converts pointers and references to classes up, down, and sideways along the inheritance hierarchy

struct V {
    virtual void f() {};  // must be polymorphic to use runtime-checked dynamic_cast
};
struct A : virtual V {};
struct B : virtual V {
  B(V* v, A* a) {
    // casts during construction (see the call in the constructor of D below)
    dynamic_cast<B*>(v); // well-defined: v of type V*, V base of B, results in B*
    dynamic_cast<B*>(a); // undefined behavior: a has type A*, A not a base of B
  }
};
struct D : A, B {
    D() : B(static_cast<A*>(this), this) { }
};
 
struct Base {
    virtual ~Base() {}
};
 
struct Derived: Base {
    virtual void name() {}
};
 
int main()
{
    D d; // the most derived object
    A& a = d; // upcast, dynamic_cast may be used, but unnecessary
    D& new_d = dynamic_cast<D&>(a); // downcast
    B& new_b = dynamic_cast<B&>(a); // sidecast
 
 
    Base* b1 = new Base;
    if(Derived* d = dynamic_cast<Derived*>(b1))
    {
        std::cout << "downcast from b1 to d successful\n";
        d->name(); // safe to call
    }
 
    Base* b2 = new Derived;
    if(Derived* d = dynamic_cast<Derived*>(b2))
    {
        std::cout << "downcast from b2 to d successful\n";
        d->name(); // safe to call
    }
 
    delete b1;
    delete b2;
}

const_cast
Converts between types with different cv-qualification.

struct type {
    int i;
 
    type(): i(3) {}
 
    void f(int v) const {
        // this->i = v;                 // compile error: this is a pointer to const
        const_cast<type*>(this)->i = v; // OK as long as the type object isn't const
    }
};
 
int main() 
{
    int i = 3;                 // i is not declared const
    const int& rci = i; 
    const_cast<int&>(rci) = 4; // OK: modifies i
    std::cout << "i = " << i << '\n';
 
    type t; // if this was const type t, then t.f(4) would be undefined behavior
    t.f(4);
    std::cout << "type::i = " << t.i << '\n';
 
    const int j = 3; // j is declared const
    int* pj = const_cast<int*>(&j);
    // *pj = 4;      // undefined behavior
 
    void (type::* pmf)(int) const = &type::f; // pointer to member function
    // const_cast<void(type::*)(int)>(pmf);   // compile error: const_cast does
                                              // not work on function pointers
}

user-defined conversion
Enables implicit conversion or explicit conversion from a class type to another type.

struct X {
    //implicit conversion
    operator int() const { return 7; }
 
    // explicit conversion
    explicit operator int*() const { return nullptr; }
 
//   Error: array operator not allowed in conversion-type-id
//   operator int(*)[3]() const { return nullptr; }
    using arr_t = int[3];
    operator arr_t*() const { return nullptr; } // OK if done through typedef
//  operator arr_t () const; // Error: conversion to array not allowed in any case
};
class A{
public:
   explicit A(int i){
            
   }
};
 
int main()
{
    X x;
 
    int n = static_cast<int>(x);   // OK: sets n to 7
    int m = x;                     // OK: sets m to 7
 
    int* p = static_cast<int*>(x);  // OK: sets p to null
//  int* q = x; // Error: no implicit conversion
 
    int (*pa)[3] = x;  // OK
    A a=(A)12;
}

Template in c++

by malabdaliMay 7, 2020January 27, 2022c++

Function templates

template <class myType>
myType GetMax (myType a, myType b) {
 return (a>b?a:b);
}

int x,y;
GetMax <int> (x,y);

#include <iostream>
using namespace std;

template <class T>
T GetMax (T a, T b) {
  T result;
  result = (a>b)? a : b;
  return (result);
}

int main () {
  int i=5, j=6, k;
  long l=10, m=5, n;
  k=GetMax<int>(i,j);
  n=GetMax<long>(l,m);
  cout << k << endl;
  cout << n << endl;
  return 0;
}

Class templates

#include <iostream>
using namespace std;

template <class T>
class mypair {
    T a, b;
  public:
    mypair (T first, T second)
      {a=first; b=second;}
    T getmax ();
};

template <class T>
T mypair<T>::getmax ()
{
  T retval;
  retval = a>b? a : b;
  return retval;
}

int main () {
  mypair <int> myobject (100, 75);
  cout << myobject.getmax();
  return 0;
}

#include <iostream>
using namespace std;

// class template:
template <class T>
class mycontainer {
    T element;
  public:
    mycontainer (T arg) {element=arg;}
    T increase () {return ++element;}
};

// class template specialization:
template <>
class mycontainer <char> {
    char element;
  public:
    mycontainer (char arg) {element=arg;}
    char uppercase ()
    {
      if ((element>='a')&&(element<='z'))
      element+='A'-'a';
      return element;
    }
};

int main () {
  mycontainer<int> myint (7);
  mycontainer<char> mychar ('j');
  cout << myint.increase() << endl;
  cout << mychar.uppercase() << endl;
  return 0;
}

template <class T>
struct A{
    template<class AA>
    void fun();
};

template <class T>
template<class AA>
void A<T>::fun(){

}
template <>
template<>
void A<QString>::fun<QString>(){

}

template<>
struct A<int>{
    void fun();
};
void A<int>::fun(){

}

Non-type parameters for templates

#include <iostream>
using namespace std;

template <class T, int N>
class mysequence {
    T memblock [N];
  public:
    void setmember (int x, T value);
    T getmember (int x);
};

template <class T, int N>
void mysequence<T,N>::setmember (int x, T value) {
  memblock[x]=value;
}

template <class T, int N>
T mysequence<T,N>::getmember (int x) {
  return memblock[x];
}

int main () {
  mysequence <int,5> myints;
  mysequence <double,5> myfloats;
  myints.setmember (0,100);
  myfloats.setmember (3,3.1416);
  cout << myints.getmember(0) << '\n';
  cout << myfloats.getmember(3) << '\n';
  return 0;
}

Parameter pack or Variadics template

template<class ... Types> void f(Types ... args);
f();       // OK: args contains no arguments
f(1);      // OK: args contains one argument: int
f(2, 1.0); // OK: args contains two arguments: int and double

template<typename... Ts, typename U> struct Invalid; // Error: Ts.. not at the end
 
template<typename ...Ts, typename U, typename=void>
void valid(U, Ts...);     // OK: can deduce U
// void valid(Ts..., U);  // Can't be used: Ts... is a non-deduced context in this position
 
valid(1.0, 1, 2, 3);      // OK: deduces U as double, Ts as {int,int,int}

Pack expansion

template<class ...Us> void f(Us... pargs) {}
template<class ...Ts> void g(Ts... args) {
    f(&args...); // “&args...” is a pack expansion
                 // “&args” is its pattern
}
g(1, 0.2, "a"); // Ts... args expand to int E1, double E2, const char* E3
                // &args... expands to &E1, &E2, &E3
                // Us... pargs expand to int* E1, double* E2, const char** E3

// The base case: we just have a single number.
template <typename T>
double sum(T t) {
    cout<<"call base sum\n";// call once at the last of recursion
    return t;
}

// The recursive case: we take a number, alongside
// some other numbers, and produce their sum.
template <typename T, typename... Rest>
double sum(T t, Rest... rest) {
    cout<<"call main sum\n";//call first 3 times
    return t + sum(rest...);
}

int main(int argc,char* argv[]){
    cout<<sum(1,2,3,4);//10
    return 0;
}

Template Specialize style


template <class T,class ...TT>
class AA;
template <class T,class ...TT>
class AA<T(TT...)>{
};

MAlabdali

Day: May 7, 2020