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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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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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class Base
{
public:
~Base() {}
private:
int val;
};
Base base; // a global variable
Look, the destructor does nothing, just same as the default destructor provided by c++ complier. But the destructor is still also a non-trivial destructor accroding to the post What is a non-trivial destructor in C++?
I know, each RULE from the standard must be strict.
But, by the upper codes I pasted, the user-defined destructor does nothing really! Why it is also non-trivial? I don't understand....
Is there any magic I don't konw?
Having the rule absolutely prohibit a definition gives a meaning to providing an empty one: it specifies that its instances must not be forgotten in an array that provides their storage. What the destructor does, if anything, can be considered an implementation detail that might change in future versions. It also avoids a change in meaning based on whether the “empty” destructor is defined in the class definition; if it isn’t, it can even be binary-compatible to change it to do something.
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Basically I want to convert abstract class reference or object to pointer of that object, I couldn't find anything on the internet. I know i can do just &object but i want to make a function that takes object and returns pointer. For example:
GameObject * convert(GameObject & object)
I need a function for that becouse i have a lua scripts that handle objects so i can mod my game easier.
Thanks in advance.
.This function is in the standard library. It is called std::addressof.
It was introduced in C++11, so in case your standard library is too old, then:
I know i can do just &object
Simply write a function and use the address of operator there, as you just know you can do. (The trivial implementation has a caveat that the addressof operator may be overloaded to not return the address. This can be avoided, but I feel is out of scope of my answer).
The problem is GameObject is an abstract class so it will give an error: cannot instantiate abstract class.
That is not a problem with getting the address of a GameObject. That is a problem with creating an instance of GameObject. You cannot instantiate objects of an abstract class except as base class sub object.
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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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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 = ..;