Constructor chaining in C++

问题:

My understanding of constructor chaining is that , when there are more than one constructors in a class (overloaded constructors) , if one of them tries to call another constructor,then this process is called CONSTRUCTOR CHAINING , which is not supported in C++ . Recently I came across this paragraph while reading online material.... It goes like this ...

You may find yourself in the situation where you want to write a member function to re-initialize a class back to default values. Because you probably already have a constructor that does this, you may be tempted to try to call the constructor from your member function. As mentioned, chaining constructor calls are illegal in C++. You could copy the code from the constructor in your function, which would work, but lead to duplicate code. The best solution in this case is to move the code from the constructor to your new function, and have the constructor call your function to do the work of initializing the data.

Does a member function calling the constructor also come under constructor chaining ?? Please throw some light on this topic in C++ .

回答1:

The paragraph basically says this:

class X
{
   void Init(params) {/*common initing code here*/ }
   X(params1) { Init(someParams); /*custom code*/ } 
   X(params2) { Init(someOtherParams); /*custom code*/ } 
};

You cannot call a constructor from a member function either. It may seem to you that you've done it, but that's an illusion:

class X
{
public:
    X(int i):i(i){}
    void f()
    {
       X(3); //this just creates a temprorary - doesn't call the ctor on this instance
    }
    int i;
};

int main()
{
    using std::cout;
    X x(4);
    cout << x.i << "\n"; //prints 4
    x.f();
    cout << x.i << "\n"; //prints 4 again
}

回答2:

C++11 allows constructor chaining (partially). This feature is called "delegating constructors". So in C++11 you can do the following

class Foo
{
public:
    Foo(int a) : Foo() { _a = a; }
    Foo(char* b) : Foo() { _b = b; }
    Foo() { _c = 1.5; }
private:
    int _a = 0;
    char* _b = nullptr;
    double _c;
};

However, there is a severe limitation that a constructor that calls another constructor is not allowed to initialize any other members. So you cannot do the following with a delegating constructor:

class Foo
{
public:
    Foo(int a) : Foo(), _a(a) { }
    Foo(char* b) : Foo(), _b(b) { }
    Foo() { _c = 1.5; }
private:
    int _a = 0;
    char* _b = nullptr;
    double _c;
};

MSVC++2013 gives compile error "C3511: a call to a delegating constructor shall be the only member-initializer" for the latter code example.

回答3:

That's not what the text says. It's suggesting your constructor call a member function which is normal and legal. This is to avoid explicitly calling the ctor again and to avoid duplicating code between your ctor and reset function.

Foo::Foo() {
  Init();
}

void Foo::Reset() {
  Init();
}

void Foo::Init() {
  // ... do stuff ...
}

回答4:

I'm not sure if it (calling a constructor from a member function) will work or not, but it's a bad practice. moving the initialize code to a new function is the logic way.

Basically saying, Don't call the constructor unless you constructing...

回答5:

when we call constructor from a member function, then it will temporary create a object of its type. in case if we are calling in derived class function then all the parent constructors are also gets executed and destroyed using destructor once function goes out of scope.

its not a Good Practice to call the constructors in member functions since it creates objects of every class derived.