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int val1 = 0;
int val2 = 0;
int val3 = 0;
I don't understand why I have to put these = 0, is there any meaning behind it?
Initialisation of values to 0 or anything else is optional. If you don't though, the variable could contain anything...
This actually depends on where you declare the variables.
If you declare them as local variables (e.g. inside a function) then the compiler and runtime system will not initialize them, their values will be indeterminate. Using such variables except to initialize them will lead to undefined behavior.
If you declare them as global variables, then the compiler and runtime system will make sure that they are zero-initialized.
If you declare the variables as member variables inside a class or structure, then their initialization depends on if you have a constructor or not. If you don't have a constructor, or or a defaulted constructor, then the compiler will automatically generate a constructor which will default-construct the (non-static) members, which for int variables is the same as zero-initialization. If you have a constructor, the (non-static) member variables will be uninitialized just like local variables.
when declaring a new variable, it is stored on your current stack. it MIGHT have garbage in it from previous uses, so the only way to make sure you're new variable initial value is indeed 0 is setting it to zero when declaring
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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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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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Are static variables in C++ internal variables or external variables? Or can be both?
PS: It seems to me that (not sure it's correct):
Internal variables are assigned their values at compile time.
External variables are assigned their values at link time.
Initialization depends on the type of the static variable.
With fundamental types and initial values that can be computed at compile time, the variables initial value should be layed down in a section of the executable that is mapped into memory with copy on write semantics.
However, the compiler can also decide to initialize static variables at runtime, typically before main() gets executed. But afaik, the only constraint is that initialization is finished when the variable is first used, and that static variables within a compilation unit are initialized in the order they are written (in case their initializers depend on one another).
The point is, that static variables are initialized before they are used by code called from main(), but it is not specified when this initialization happens. The compiler can do what it deems most efficient.
In any case, static variables live at least until main() exits or exit() is called. I am pretty sure that C++ will also call the destructors before terminating the process, but I don't know about that.
According to the C++ Standard
3 A name having namespace scope (3.3.6) has internal linkage if it is
the name of — a variable, function or function template that is
explicitly declared static; or,
As for your statement that
PS: I learned that • Internal variables are assigned their values at
compile time.
• External variables are assigned their values at link time.
then it is wrong.
In my opinion you are trying to mix up two notions: static storage duration and program linkage.
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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 = ..;