I have some questions about constructors in C++. For each question (from (1) to (4)) I would like to know if the behaviour is perfectly defined regarding to the standard.
A) The first one is about initialization of members :
class Class
{
public:
Class()
: _x(0)
, _y(_x)
, _z(_y) // <- (1) Can I initialize a member with other members ?
{
;
}
protected:
int _x;
int _y;
int _z;
};
B) What are the functions added to each class by the compiler ?
template<typename T> class Class
{
public:
template<typename T0>
Class(const Class<T0>& other)
{
std::cout<<"My copy constructor"<<std::endl;
_x = other._x;
}
template<typename T0 = T>
Class (const T0 var = 0)
{
std::cout<<"My normal constructor"<<std::endl;
_x = var;
}
public:
T _x;
};
// (2) Are
// Class(const Class<T>& other)
// AND
// inline Class<T>& operator=(const Class<T>& other)
// the only functions automatically added by the compiler ?
As an example, if I call :
Class<int> a;
Class<int> b(a); // <- Will not write "My copy constructor"
Class<double> c(a); // <- Will write "My copy constructor"
(3) Is this behaviour perfectly normal according to the standard ?
(4) Do I have the guarantee that an empty constructor is not automatically added and that Class<int> x; will write "My normal constructor" ?
Can I initialize a member with other members ?
Yes, as long as those other members have already been initialised; i.e. as long as their declarations come before the member being initialised.
Are [the copy constructor] and [the copy-assignment operator] the only functions automatically added by the compiler ?
It will also implicitly declare a destructor, which will destroy _x using its destructor.
In C++11, a move constructor (Class(Class&&)) and move-assignment operator (Class& operator=(Class&&)) are also implicitly declared, unless you declare a copy or move constructor, or a copy or move assignment operator.
Note that your constructor template is not a copy constructor, and the implicit one will be used instead:
Class<T1> t1;
Class<T1>(t1); // prints nothing
Class<T2>(t1); // prints "My copy constructor" (which is a lie)
Is this behaviour perfectly normal according to the standard ?
Yes, see chapter 12.
Do I have the guarantee that an empty constructor is not automatically added and that
Class<int> x;will write"My normal constructor"?
Yes, a default constructor will only be implicitly declared if you don't declare any constructors at all.