C++ - Creating a Copy function without = assignment for a Dynamic String Array

Question

Trying to write a copy function for a dynamically allocated array.

In my header file I have:

#include <memory>
#include <string>


using std::string;
using std::unique_ptr;
using std::make_unique;

class StringSet{
public:
    //create an empty set
    StringSet() = default;
    StringSet(int capacity);

    //copy a set
    StringSet(const StringSet&);

    StringSet& operator[](const int);

    //Insert a string to the set
    bool insert(string);

    //Remove a string from the set
    bool remove(string);

    //Test whether a string is in the set
    int find(string) const;

    //Get the size of the set
    int size() const;

    //get string at position i
    string get(int i) const;

    //Return the set union of the set and another StringSet
    StringSet setunion(const StringSet&) const;

    //Return the intersection of the set and another StringSet
    StringSet intersection(const StringSet&) const;

    //Return the set diffference of the set and another StringSet
    StringSet difference(const StringSet&) const;

    //prevent default copy assignment
    StringSet& operator=(const StringSet&) = delete;

    int NOT_FOUND = -1;
    static constexpr int def_capacity {4};

private:
    int arrSize {def_capacity};
    int currentSize {0};
    unique_ptr<string[]> arr {make_unique<string[]>(def_capacity)};

};

In my implementation file I have:

#include "StringSet.h"
#include <iostream>
#include <utility>



StringSet::StringSet(int capacity)
: arrSize{capacity},
    arr{make_unique<string[]>(capacity)}
{
}

StringSet::StringSet(const StringSet& a)
{
    auto a2 = StringSet(currentSize);

    for (auto i=0; i < currentSize ; i++ )
        {
        a2[i] = a[i];
        }
}

Compiler error:

error: constructors may not be cv-qualified
error: no match for 'operator=' (operand types are 'StringSet' and 'std::string {aka std::basic_string<char>}')
error: passing 'const StringSet' as 'this' argument discards qualifiers [-fpermissive]
error: use of deleted function 'StringSet& StringSet::operator=(const StringSet&)'

My assignment has overloaded the assignment operator= and thus I'm not able to use that here. Is there another way of implementing a copy function without using the assignment operator - is there anything in std::string that allows us to copy contents easier in this manner?

If there's anything else I need to add here for details please let me know.

Thank you.

shouldn't it be "string& operator[](const int);" instead of "StringSet& operator[](const int);"? Also, you should define a "const string& operator[](const int) const;" — Mr_Hic-up
– Mr_Hic-up, Commented Sep 26, 2016 at 20:00

Barry · Accepted Answer · 2016-09-26 20:15:33Z

1

The problem with this code:

StringSet::StringSet(const StringSet& a)
{
    auto a2 = StringSet(currentSize);

    for (auto i=0; i < currentSize ; i++ )
    {
        a2[i] = a[i];
    }
}

is that, even if it compiled, you're never actually initializing the members of this... you're initializing some temporary a2 that goes out of scope at the end of the constructor. You actually want:

StringSet::StringSet(const StringSet& a)
    : StringSet(a.arrSize)
{
    currentSize = a.currentSize;

    for (auto i=0; i < currentSize; i++ )
    {
        arr[i] = a.arr[i];
    }
}

Also, your operator[] returns a StringSet& where it should probably return a std::string&.

Also, you should avoid bringing names into the global namespace like you're doing. Keep it local. Writing std:: is not a burden.

answered Sep 26, 2016 at 20:15

Barry

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6 Comments

Jarod42 Over a year ago

auto a2 = StringSet(currentSize); would even be infinite loop.

TigerCode Over a year ago

Yea I was mistaken about doing the auto a2 = StringSet(currentSize) when I actually wanted arrSize. I didn't know arr was the way of referring to the local instance of the array initialized in the copy function though. That definitely clears some of the problems up. error: constructors may not be cv-qualified What does this mean though?

Barry Over a year ago

@TigerCode I'm guessing some constructor you have that you didn't include in your question is cv-qualified. That's not allowed.

TigerCode Over a year ago

In the header file - for the copy function i forgot to include a place holder for the variable.

Barry Over a year ago

@TigerCode Comments aren't for followup questions. I have no idea what a2 or a1 are, or why you have that extra &

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