Class constructors in C++

Question

I'm a beginner to c++ so there are a lot of things quite not clear in my mind.

I have this code I need to write and in a class I make a constructor. However, I don't need any parameters because I read from a file-stream inside the constructor. So my questions are:

1.Can I make a constructor like this:

class myClass {
private:
  string title;
  string organizer;
public:
  myClass() {
    title = stringRead();
    organizer = stringRead();
  }
}

where stringRead() is a function I have written to read from my file??

2.How do I call it afterwards when I need it? I know that the default constructror is being called like that:

myClass A;
A = myClass();

Is it the same?

3.If I have a pointer, how do I call the constructor again? This doesn't seem like it should be right...

myClass *B;
B = myClass();

Thanks in advance! =D

Useful read: see Constructors section at cplusplus.com/doc/tutorial/classes — jarmod
– jarmod, Commented Jun 6, 2015 at 16:54

Ryan Haining · Accepted Answer · 2015-06-06 17:00:54Z

3

1) This constructor will work but you should favor using an initialization list (assuming stringRead() isn't a member function of myClass

class myClass {
private:
  string title;
  string organizer;
public:
  myClass() 
    : title(stringRead()),
      organizer(stringRead())
  { }
};

2) myClass A; is what you should be doing. You could alternatively have auto A = myClass(); which, after optimizations, will be the same thing. Without optimizations a temporary will be constructed, and then A will be move constructed from it, so this won't work with unmovable objects (your object is movable)

3) If you want to use a raw pointer then you would use

myClass *ptr = new myClass;
// bunch of code
delete ptr;

However, you'd be better using a smart pointer to control its lifetime. This way you won't need to manually delete

std::unique_ptr<myClass> ptr(new myClass);

or make_unique in c++14

auto ptr = std::make_unique<myClass>();

And of course use a shared_ptr if you have shared ownership

answered Jun 6, 2015 at 17:00

Ryan Haining

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4 Comments

Anina7 Over a year ago

about 2: I have a class in a class so there I have to write class myClass1: private: myClass2 element; public: myClass1() { /*call constructor of myClass2*/ }

Anina7 Over a year ago

check it again. I hit 'enter' by mistake before finishing writing. =]

Ryan Haining Over a year ago

@Anina7 that doesn't parse. Are you trying to inherit or you have a myClass2 data member? In either case you'd still use the initialization list syntax for your constructor but the name will vary.

Ryan Haining Over a year ago

probably: myClass1() : myClass2(/* ctor args */) { }

ForceBru · Accepted Answer · 2015-06-06 16:54:36Z

2

I think it's OK to assign the value returned by a function to a member of a class.

You can initialize it as you suggested (with myClass A;)

When you use pointers, you need myClass *k=new myClass();. You should remember to delete the object you created with delete k;.

edited Jun 6, 2015 at 16:54

answered Jun 6, 2015 at 16:52

ForceBru

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Comments

Bathsheba · Accepted Answer · 2015-06-06 16:58:27Z

2

Your constructor is fine, so long as the functions used within it are globals or static functions of this or another class.
myClass A; will invoke the constructor you have written.
To use a pointer, you need B = new myClass(). That will also call the same constructor. Don't forget to delete B at some point else you'll leak memory.

Do bear in mind that if an exception is thrown in a constructor then the destructor is not called.

edited Jun 6, 2015 at 16:58

answered Jun 6, 2015 at 16:52

Bathsheba

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Comments

Stefano Sanfilippo · Accepted Answer · 2015-06-06 17:06:51Z

2

Yes, you can, but it might not be the best approach. Reading from input can fail, failure in a constructor is often a non-recoverable event you'll want to handle. A good approach is reading the values outside the costructor, handling errors and calling the constructor only when you have "everything ready". Like this:

class myClass {
private:
string _title;
string _organizer;
public:
  myClass(const string &title, const string &organizer) {
    _title = title;
    _organizer = organizer;
}

or, by using a more idiomatic C++ initializer list:

class myClass {
private:
string _title;
string _organizer;
public:
  myClass(const string &title, const string &organizer):
    _title(title), _organizer(organizer) {}
}

and then, somewhere else:

string title = stringRead();
string organizer = stringRead();
myClass A(title, organizer);

No, in this snippet:
```
myClass A;
A = myClass();
```
two different things happen: at line 1 the default constructor is called; at line 2, a temporary object is constructed (again, by calling the default constructor) and then assigned to A using the (rval for C++11) copy operator. This expression:
```
myClass A;
```
calls the default constructor. If you have parameters:
```
myClass A(title, organizer);
```
Nope, this does not even work. A pointer is not an object, you have to allocate the object. At that point, you can get a pointer to it:
```
myClass A;
myClass *B = &A;
```
you could also resort to dynamic allocation:
```
myClass *B = new myClass;
```
in this case, either remember to call delete B somewhere else or wrap B in a smart pointer:
```
std::unique_ptr<myClass> B(new myClass());
```

edited Jun 6, 2015 at 17:06

answered Jun 6, 2015 at 16:56

Stefano Sanfilippo

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2 Comments

Ryan Haining Over a year ago

your constructor has added leading underscores on the data members, and it is a java style constructor. You should use an initialization list

Stefano Sanfilippo Over a year ago

@RyanHaining the initializer list is certainly more idiomatic, but it might not be clear to a true beginner like OP is. Anyway, I added both.

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