Constructor with array parameters

Question

I want to make a constructor with array and array size so I can call the object using this: Multime m1 = Multime({1, 2, 3}, 3); Or should I use std::vector instead?

class Multime
{
private:
    int elemente[100];
    int size;
public:
    Multime(){}
    Multime(int el[50], int s){
        this -> size = s;
        for(int i = 0; i < this -> size; i++)
            this -> elemente[i] = el[i];
    }
};

int main()
{
    Multime m1 = Multime({1, 2, 3}, 3);
    return 0;
}

And i'm getting No matching constructor for initialization of 'Multime'

How about using std::vector or std::initializer_list as argument? — Some programmer dude
– Some programmer dude, Commented Jul 5, 2019 at 13:16
You cannot pass arrays as parameters. For more information, see the chapter in your C++ book that explains how function parameters types get decayed. In post-C++11 word you can declare a std::initializer_list parameter and avoid the need to pass an explicit array size parameter, and this will work; but this is a fairly advanced topic, see your C++ book for the details. — Sam Varshavchik
– Sam Varshavchik, Commented Jul 5, 2019 at 13:17
By the way, as mentioned you can't pass C-style arrays as arguments as simply as that. Like in C an argument declaration like int el[] (size is irrelevant) will be treated as int* el, in other words the function expects a pointer as argument. Your compiler should have mentioned it in the complete error message (though it might have been a little indirect). — Some programmer dude
– Some programmer dude, Commented Jul 5, 2019 at 13:19

Blaze · Accepted Answer · 2019-07-05 13:41:48Z

2

Or should I use std::vector instead?

That's a great idea.

class Multime
{
private:
    std::vector<int> elemente;
public:
    Multime() {}
    Multime(std::vector<int> el) : elemente(std::move(el)) {}
};

int main()
{
    Multime m1 = Multime({ 1, 2, 3 });
    return 0;
}

edited Jul 5, 2019 at 13:41

answered Jul 5, 2019 at 13:19

Blaze

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6 Comments

Some programmer dude Over a year ago

It would have been even better if you used a constructor initializer list. ;)

Blaze Over a year ago

@Someprogrammerdude good idea. I keep forgetting about this feature way too often.

Evg Over a year ago

Better: elemente(std::move(el)).

Andrei Over a year ago

Thanks you, I decided it is better to use std::vector but instead of move, I used push_back

Multime(vector<int> el, int s){         this -> size = s;         for(int i = 0; i < this -> size; i++)             this -> elemente.push_back(el[i]);

Evg Over a year ago

@Andrei, don't do it. 1) You take el by copy, but you don't store that copy inside your object using a move-constructor. Copying is redundant: take el by const-reference. 2) push_backing in a loop causes redundant reallocations: preallocate space with elemente.reserve(s); before the loop. 3) Don't use int for a vector's size, use std::size_t and be consistent with the standard library.

|

Adrien Givry · Accepted Answer · 2019-07-05 13:25:59Z

0

If you want your class to be able to contain a varying number of elements (Defined at compile time), you can try:

#include <array>

template <size_t _Size>
class Multime
{
private:
    int elemente[_Size];
public:
    Multime() {}

    Multime(const std::array<int, _Size>& p_elements)
    {
        for (int i = 0; i < _Size; ++i)
            this->elemente[i] = p_elements[i];
    }
};

int main()
{
    Multime<3> m1({1, 2, 3});
    return 0;
}

You can also directly store an std::array into your class, so the construction is cleaner

#include <array>

template <size_t _Size>
class Multime
{
private:
    std::array<int, _Size> elemente;
public:
    Multime() {}
    Multime(const std::array<int, _Size>& p_elements) : elemente(p_elements) {}
};

int main()
{
    Multime<3> m1({1, 2, 3});
    return 0;
}

edited Jul 5, 2019 at 13:25

answered Jul 5, 2019 at 13:20

Adrien Givry

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3 Comments

Jarod42 Over a year ago

Using std::array member also would simplify constructor too.

Adrien Givry Over a year ago

It's up to him, I didn't want to break his logic

Guillaume Racicot Over a year ago

The name _Size is reserved. Change it to _size or Size, any name that starts with an underscore followed by a capital is reseved to the standard

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