Some syntax issues - C++

**HelpMeC** · 12-24-2019

Hello everyone!

1. If I'm declaring some static array as a member of a class, for example:

Code:

Matrix matrices[4];

Now, suppose I have a constructor which receives an array 4 matrices to assign to that member.

Now isn't it right to define the elements of our matrices member this way, for example:

Code:

matrices[0](matricesFromCont[0]); // Assuming we have a copy constructor in Matrix class

What's the problem with that?

2. Another, maybe related question:
Why isn't it correct to pass as an argument the following one:

Code:

const Matrix& arr[]

Thank you!

**laserlight** · 12-24-2019

Originally Posted by HelpMeC

What's the problem with that?

That looks like an attempt to call an overloaded operator() on matrices[0], passing matricesFromCont[0] as an argument. It has nothing to do with a copy constructor unless the overloaded operator() has a parameter that results in copy construction.

Originally Posted by HelpMeC

Why isn't it correct to pass as an argument the following one:

If that's what you have in mind as an actual argument... the syntax is simply invalid.

**HelpMeC** · 12-25-2019

@laserlight
Tha first issue - got it.

The second one - So, how can one to pass such an argument?

Code:

const Matrix arr[]

But by reference, as we don't want to create another copy...

Thank you!!

**laserlight** · 12-25-2019

What are you trying to pass as an argument?

**HelpMeC** · 12-25-2019

@laserlight -
In this particular case, we know that we have to receive an array of 4 matrices (Matrix object) actually, so maybe the right form of writing it, is such that:

Code:

const Matrix arr[4]

But, this way the array is passing as a copy (to my understanding) where I want it to be passed by reference; I want to avoid copying...

**laserlight** · 12-25-2019

Ohhh... you want to pass an array as an argument. An array is converted to a pointer to its first element when passed as an argument. This rule remains true even when the array elements are objects of class type.

So, what you want is a pointer parameter, more specifically a pointer to const parameter, along with a second parameter to specify the number of elements. Or, you might use a std::initializer_list.

Or, it might make more sense to have four parameters if you really need exactly four matrices.

**HelpMeC** · 12-25-2019

@laserlight -
Oh yea, C -> C++ - this is actually a pointer so there is no any copy except for the address of the first object...

BTW, the copy constructor is invoked only when the name of the class is followed by the instance?

**laserlight** · 12-25-2019

Originally Posted by HelpMeC

BTW, the copy constructor is invoked only when the name of the class is followed by the instance?

No. Think about things like parameters.

**HelpMeC** · 12-25-2019

@laserlight
You mean some cases like:

Code:

void func(const Matrix m) {}

But in this case when we are passing some matrix into this function, it performs the assignment operator on it, doesn't it?

Thanks.

**laserlight** · 12-25-2019

Write a program to find out.

**HelpMeC** · 12-25-2019

@laserlight -
Yea, according to the PPT of my class - it does invoke the the copy constructor (the default one or the implemented one).
Strange language

Thank you!

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