C++ - String Array Parameter for SetUnion function

Question

I wrote a function to compute the union of two sets.

I'm running into several compilation errors and I believe that's in part due to how I made the StringUnion array and declared it but nothing I do is working thus far.

This is my header file.

#ifndef StringSet_header
#define StringSet_header
#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)};

};

#endif

And this is my implementation of my SetUnion function.

StringSet StringSet::setunion(const StringSet &Array2) const
{
    StringSet StringUnion = make_unique<string[]>(arrSize);

    if (currentSize > 0)
    {
        for (auto i=0; i < currentSize; i++)
        {
            auto s = arr[i];
            StringUnion.insert(s);
        }
        for (auto i=0; i < Array2.currentSize; i++)
        {
            auto s = Array2[i];
            if (StringUnion.find(s) == NOT_FOUND)
            {
                StringUnion.insert(s);
            }
        }
    }
    else    
    {
        auto result = StringSet();
        return result;          //return empty StringSet}
    }
}

Errors:

|error: conversion from 'std::_MakeUniq<std::basic_string<char> []>::__array {aka std::unique_ptr<std::basic_string<char> []>}' to non-scalar type 'StringSet' requested|

error: passing 'const StringSet' as 'this' argument discards qualifiers [-fpermissive]

error: no matching function for call to 'StringSet::find(StringSet&)'

error: no matching function for call to 'StringSet::insert(StringSet&)'

Insert and find work as intended to and I was able to use insert and find functions within my remove function and some others so why can't I use them here?

Answer 1

In your line

StringSet StringUnion = make_unique<string[]>(arrSize);

The RHS uses the c++14 construct that takes an std::size_t , and returns an std::unique_ptr<std::string> internally pointing to an array .

The LHS, however, is a StringSet object.

You did not define a constructor taking such a type, so it's a problem.

Looking at your code, StringSet does have a std::unique_ptr<std::string> member, so you could add a ctor taking such an object, and initializing the member from it. However, it's unclear what would be the benefit of such a ctor, as you already have a ctor

StringSet(int capacity);

which already essentially does the same.

As Leon writes, you should just use this one instead of the line you have

StringSet StringUnion(arrSize);

Answer 2

The errors provides by your compiler seem pretty clear. Let's check them.

• conversion from std::make_unique ... to non-scalar type StringSet requested

It's because of the definition of the function std::make_unique , which returns a std::unique_ptr<T> . But you're trying to assign it to a value of type StringSet . There is no constructor or operator for creating a StringSet from a std::unique_ptr , so the compiler complains that he can't do that.

• error: no matching function for call to 'StringSet::find(StringSet&)'

Your class StringSet has an operator[] that returns a reference on a StringSet so auto s = Array2[i]; is of type StringSet . But your functions find and insert ask for a std::string . As there is no constructor that can provide implicit conversion from StringSet to std::string , the compiler complains.