CODEWITHSUNDEEP
c++arraysobject
Asssasssiino
Asssasssiino

Reputation: 13

c++ create object with an array as member

i'm new to c++ (coming from java) and i'm actually struggeling with the following: Let foo be a class

int bar[10] = {};

Foo::Foo()
{
    bar[1] = 42;

}

and doSmth() a method in the main class:

Foo doSmth(){
Foo f;
f.bar[0] = 10;
return f;
}

the main will be something like:

int main(int argc, char *argv[])
{
Foo f = doSmth();

cout << f.bar[1] << endl;
cout << f.bar[0] << endl;
return 0;
}

Is this the right way to return the foo-object in doSmth()? I want to create the object on stack, but I'm worried about the array (bar) from the foo object, when will it be deleted from stack?

Upvotes: 0

Views: 63

Answers (3)

Jonathan Mee
Jonathan Mee

Reputation: 38919

Is this the right way to return the foo object in doSmth()?

Yes, objects created locally must be returned by value. Typically the compiler will create the object in place and the copy will be optimized out. You can read more about returning local objects here: https://isocpp.org/wiki/faq/ctors#return-local-var-by-value-optimization

I want to create the object on stack, but I'm worried about bar from the foo object, when will it be deleted from stack?

As mentioned above, because of return value optimization the compiler will typically not even create this object within doSmith's stack frame. Furthermore, even if it was created in doSmith's stack frame, the behavior of the default copy constructor and the default assignment operator is such that:

If the subobject is an array, each element is assigned, in the manner appropriate to the element type

See 15.8.1[class.copy.ctor]14.1 and 15.8.2[class.copy.assign]12.2

Upvotes: 0

eerorika
eerorika

Reputation: 238311

Is this the right way to return the foo object in doSmth()?

It sure is.

I want to create the object on stack

You have.

but i'm worried about the array (bar) from the foo object

bar is not "from foo". bar is a global static object.

f.bar[1]

This is ill formed, since bar is not a member of Foo. To declare a member, it has to be inside the definition of the class:

struct Foo {
    int bar[10] = {};
};

when will it be deleted from stack?

If bar has static storage, such as in your code, then it is destroyed at the end of the program. If it is a (non static) member, then it is destroyed when its complete object is destroyed i.e. the instance of Foo that contains it.

Upvotes: 2

Stephan Lechner
Stephan Lechner

Reputation: 35154

If bar is a non-static data member of your class Foo, then the bar-content of a foo-object will be part of the foo-object; it will reside where foo resides (on the stack, on the heap, ...) and it will be created and destroyed once foo gets created or destroyed.

If bar is a static data member, then it will exist once in your complete program and will "live" until the program finishes.

Upvotes: 0

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