default

If a class is defined with any constructors, the compiler will not generate a default constructor. This is useful in many cases, but it is some times vexing. For example, we defined the class as below:

class A
{
public:
    A(int a){};
};

Then, if we do:

A a;

The compiler complains that we have no default constructor. That's because compiler did not make the one for us because we've already had one that we defined.

class A
{
public:
    A(int a){}
    A() = default;
};

Then, compiler won't complain for this any more:

A a;

Suppose we have a class with a constructor taking an integer:

class A
{
public:
    A(int a){};
};

Then, the following three operations will be successful:

A a(10);     // OK
A b(3.14);   // OK  3.14 will be converted to 3
a = b;       // OK  We have a compiler generated assignment operator

However, what if that was not we wanted. We do not want the constructor allow double type parameter nor the assignment to work.

C++11 allows us to disable certain features by using delete:

class A
{
public:
    A(int a){};
    A(double) = delete;         // conversion disabled
    A& operator=(const A&) = delete;  // assignment operator disabled
};

Then, if we write the code as below:

A a(10);     // OK
A b(3.14);   // Error: conversion from double to int disabled
a = b;       // Error: assignment operator disabled

In that way, we could achieve what we intended.