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I am fairly new to the language, and I can't seem to find a good explanation on constructors.
When I don't create a constructor, according to many resources, a default constructor is created, which doesn't do anything. However, is there something that the constructor does behind the scenes that instantiates an object?
A comparison between the default constructor and my own defined constructors would help me understand this.
Thanks a ton in advance!
~novice
The constructor initializes the variables(fields) of a class. The default constructor initializes to default values. Example, string to "", integers to zero, doubles to 0.0, boolean to false an so on. When you create a constructor you're customizing the variables initialization.
A constructor is essentially the conditions called upon the object being created. If you want to input an int, for example, into the initialization of the object, you would create a constructor that takes "int x" in the parentheses, which is then referenced within the constructor statements.
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As you can see, the destructor complains "Use of undeclared identifier 'a'
Is my array out of scope? How can i delete it, when i call the destructor?
a has to be a member variable of the class. As you have it now its a local
a needs to be a data member of hashtable in order for this to work. Right now it is a variable local to the constructor, and the memory allocation is therefore leaked when the constructor terminates.
However, even if you correct this problem, beware the rule of five: if you implement any of the following you need to implement or delete all of them:
Destructor
Copy constructor
Copy-assignment operator
Move constructor
Move-assignment operator
If you do not, the compiler will generate them for you and it will get them wrong in this case. Simply copying the pointer value to the new object is not enough; a new allocation needs to be made and the contents copied (except in the case of a move). The compiler-generated versions will result in a use-after-free or double-delete.
You can avoid all of this by using std::vector<int> instead of trying to manage your own memory.
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So, just for my peace of mind, isn't it a logical error to assign a value to a static member variable within a class constructor?
Edit : I mean using the = operator
Won't the value keep changing with every object declared?
Suppose I send in a value to the constructor which I then use to set the value of a static variable. With each object declared the value of the static variable would keep changing.
It is only a logical error if you consider it to be one.
If however the change of the value of the static variable with each constructor is exactly what you WANT it to do, then it is not a logical error.
As in the comments already mentioned, one example is to count all constructors. This is often accompanied by also counting all DEstructors and in the end yields a count of existing instances.
The example could be implemented by using the = operator on the static variable,
count_of_ctors = count_of_ctors +1;;
though it probably would usually use ++,
count_of_ctors++;.
In the special case outlined in the question, setting a variable to a value given as parameter to the ctor, it could achieve the same, i.e. by giving the current value of the static variable, increased by one.
But you probably meant a value which is not derived from the current value of the static variable.
That still could be intended behaviour. I imagine that recording the last date+time of an instantiation could be such a value, which could serve debugging or logging purposes. That would be used by instantiating always with the current time.
Since the current time would probably better be read by the ctor (for convenience and for tampering protection), here is another idea. For each instantiation, the ID of the user (and maybe a matching passwort) has to be given, so that the static variable always has the last user ID which created an instance.
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Suppose we declare A to be of type Type:
Type A;
Question: is the Type constructor for A called at this point? Or is it only after we initialize A that a constructor is called?
is the Type constructor for A called at this point? Or is it only after we initialize A that a constructor is called?
You are initializing A here, whether you explicitly provided a value for that process or not.
There is no opportunity to initialise A "later"; you can only assign to it later. (Early C texts talk about "initialising" numeric values long after declaration, but that's a different language, a different century, and a different set of descriptive idioms.)
So, yes, the constructor is called as soon as the object's lifetime begins. That's either right there and then, on that line, or if a class member then of course it's when the encapsulating class is being initialised and it's this member's turn.
Of course you could have proven this with a simple std::cout pair.
Yes, it is called immediately. When you assign it a value, the assignment operator is called instead.
The only exception is this:
Type a = value;
Here some compiler with some settings may relax it and call constructor directly with the value as parameter. But it still needs to be part of declaration.
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C++ will create copy constructor and copy assignment operator for class. My questions is why it is creating two member function ? What is the significance and what is the difference by copying the object by copy constructor and copy assignment operator? Thank you in advance.
Copy assignment and copy construction do different things. Copy assignment has to take a fully constructed object and change it, while copy construction has to take a non-fully constructed object and do that initial construction.
For example copy assignment on a class that manages a resource has to ensure that its old resource is properly disposed of after it has taken ownership of the 'copied' resource, whereas the copy constructor doesn't have any previous resource to deal with.
If you have pointers as data members in your class, and if they are directly getting copied to another object, more than one object will access that pointer memory (unintentionally). To avoid that we can override copy constructor/assignment operator.
If you do not override these two functions, compiler copies bit by bit to another object.
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One of the main motto of cpp is to avoid uninitialized variables. Then what is the use of default constructor that compiler provides as it doesn't initialize variables.
The implicitly generated default constructor calls the default constructors of all members and base classes. They may or may not be implicitly generated (meaning, some member, or member's member, etc, may have a non-implicitly generated default constructor, one that actually does something).
There are some circumstances in which a default constructor is required. For example
MyClass arrayOfObjects[10];
Here the default constructor is called even if then you are going to assign new values to objects in the array. Or a derived class ctor which doesn't explicitly call a parent constructor. Or even a simple declaration of a variable:
MyClass x; // calls default constructor
...
x = ..;