Problem with const

[Joseph Hesse]

This is a simplification of my problem.
The array Values in class Algorithm will be populated by the constructor
reading from a data file.
Once the array Values is populated it never changes, hence the const in
"const int d;"
The problem is how can I give values to Values[i].d?
I probably have a bad design, nevertheless in the non-simplified version
it reflects the real problem.

Thank you, Joe

==========================================================
#include <iostream>
#include <string>
using namespace std;

class Data
{
private:
const int d;
// ...
public:
Data () : d(0) {} // so we don't have junk in d
// ...

friend class Algorithm;
};

class Algorithm
{
private:
Data Values[100]; // no need to use vector since 100 will never change
// ...
public:
// ctor opens data file and reads ints into Values[i].d
Algorithm(const string & DataFile); // code not shown
// ...
};
==========================================================

[Daniel]

On Sunday, August 14, 2016 at 12:01:52 PM UTC-4, Joseph Hesse wrote:
> This is a simplification of my problem.
> The array Values in class Algorithm will be populated by the constructor
> reading from a data file.
> Once the array Values is populated it never changes, hence the const in
> "const int d;"
> The problem is how can I give values to Values[i].d?

How about

class Data
{
private:
const int d;
// ...
public:

Data(int val) : d(val) {}

friend class Algorithm;
};

class Algorithm
{
private:

std::vector<Data> Values;

// ...
public:
// ctor opens data file and reads ints into Values[i].d
Algorithm(const string & DataFile)

{
Values.reserve(100);

// For each integer i in Datafile
{
Values.push_back(Data(i));
}
assert(Values.size() == 100);
}
// ...
};

Although I don't think most people would worry about Data having a const
value, I think most people would be fine with d private and no public
modifier.

Daniel

[Marcel Mueller]

On 14.08.16 18.01, Joseph Hesse wrote:
> The problem is how can I give values to Values[i].d?
> I probably have a bad design, nevertheless in the non-simplified version
> it reflects the real problem.

Maybe your const is at the wrong place.

> class Algorithm
> {
> private:
> const Data Values[100]; // no need to use vector since 100 will never change
^^^^^

> // ...
> public:
> // ctor opens data file and reads ints into Values[i].d
> Algorithm(const string & DataFile); // code not shown
> // ...
> };
> ==========================================================

Of course, you still have to take some care about the initialization of
const arrays. std::array might fit your needs.


Marcel

[Alf P. Steinbach]

On 14.08.2016 18:01, Joseph Hesse wrote:
> This is a simplification of my problem.
> The array Values in class Algorithm will be populated by the constructor
> reading from a data file.
> Once the array Values is populated it never changes, hence the const in
> "const int d;"
> The problem is how can I give values to Values[i].d?
> I probably have a bad design, nevertheless in the non-simplified version
> it reflects the real problem.
>
> Thank you, Joe
>
> ==========================================================
> #include <iostream>
> #include <string>
> using namespace std;
>
> class Data
> {
> private:
> const int d;
> // ...
> public:
> Data () : d(0) {} // so we don't have junk in d
> // ...
>
> friend class Algorithm;
> };

The `const` only makes `Data` non-assignable.

Have you considered that that makes it difficult to e.g. sort a sequence
of `Data` values?

> class Algorithm
> {
> private:
> Data Values[100]; // no need to use vector since 100 will never change

With the present `const`, `std::vector` enables you to put Data values
in the array.

Simply use `std::vector<>::push_back` or `std::vector<>::emplace_back`.

> // ...
> public:
> // ctor opens data file and reads ints into Values[i].d
> Algorithm(const string & DataFile); // code not shown
> // ...
> };

An instance of Algorithm is an object whose initialization reads values
from a file, and that is not copy assignable. Somehow that doesn't make
much sense to me. I think you should differentiate between algorithms
and the data they operate on, much as the standard library does.


Cheers & hth.,

- Alf