CS计算机代考程序代写 compiler // from: https://en.cppreference.com/w/cpp/language/default_constructor

// from: https://en.cppreference.com/w/cpp/language/default_constructor
#include
using namespace std;

struct A{
int x;
A(int x = 1) : x(x) {} // User-defined default constructor.
// If no parameter value is provided, the default value x=1 is used,
// making this a disguised default constructor.
};

struct B : A{
// B::B() is implicitly-defined, calls A::A()
};

struct C{
A a;
// C::C() is implicitly-defined, calls A::A()
};

struct D : A{
D(int y) : A(y) {}
// D::D() is not declared because another constructor exists
};

struct E : A{
E(int y) : A(y) {}
E() = default; // explicitly defaulted, calls A::A()
};

struct F {
int& ref; // reference member
const int c; // const member
// F::F() is implicitly defined as deleted because:
// c and ref cannot be assigned a value using the automatically generated
// default constructor.

// if we explicitly declare a default constructor that does nothing,
// the compiler complains that ref and c are not initialized.
// F() {};

// The only way to properly create an object of that kind is to have a constructor such as:
// F(int newref, int newc) : ref(newref), c(newc) {
// }
};

int main(){
A a;
B b;
C c;
// D d; // compilation error: default constructor does not exist
E e;
// F f; // compilation error: default constructor has been deleted

int x = 1;
int y = 2;
cout << x << y << endl; int& ref = x; ref = y; // This does not make ref refer to y. It changes the value of ref (i.e. x) to the value of y (i.e. 2). cout << x << y << endl; // if we uncomment the constructor above, it has two effects: // 1. The default constructor is not not defined (as opposed to defined but deleted. // 2. we can now properly create an object of type F. // F f = F(x, y); }