C++中关于=default和=delete问题

// use of defaulted functions
#include <iostream>
using namespace std;
class A {
public:
    // A user-defined
    A(int x){
        cout << "This is a parameterized constructor";
    }
    // Using the default specifier to instruct
    // the compiler to create the default implementation of the constructor.
    A() = default;
};
int main(){
    A a;          //call A()
    A x(1);       //call A(int x)
    cout<<endl;
    return 0;
} 

// non-special member functions can't be defaulted(非特殊成员函数不能使用default) 
class B { 
public: 
    // Error, func is not a special member function. 
    int func() = default;  
    // Error, constructor B(int, int) is not a special member function. 
    B(int, int) = default;  
    // Error, constructor B(int=0) has a default argument. 
    B(int = 0) = default;  
}; 
int main() { 
    return 0; 
} 

// copy-constructor using delete operator 
#include <iostream> 
using namespace std; 
class A { 
public: 
    A(int x): m(x) { } 
    // Delete the copy constructor 
    A(const A&) = delete;      
    // Delete the copy assignment operator 
    A& operator=(const A&) = delete;  
    int m; 
}; 
int main() { 
    A a1(1), a2(2), a3(3); 
    // Error, the usage of the copy assignment operator is disabled 
    a1 = a2;   
    // Error, the usage of the copy constructor is disabled 
    a3 = A(a2);  
    return 0; 
} 

// type conversion using delete operator 
#include <iostream> 
using namespace std; 
class A { 
public: 
    A(int) {} 
    // Declare the conversion constructor as a  deleted function. Without this step,  
    // even though A(double) isn't defined,  the A(int) would accept any double value
    //  for it's argumentand convert it to an int 
    A(double) = delete;  
}; 
int main() { 
    A A1(1); 
    // Error, conversion from  double to class A is disabled. 
    A A2(100.1);  
    return 0; 
} 

class C {
public:
         C(C& a) = delete;
};

// incorrect syntax of declaring a member function as deleted 
class C  { 
public: 
    C(); 
}; 
// Error, the deleted definition of function C must be the first declaration of the function. 
C::C() = delete;