Creating an array of a class with arguments in constructor

Question

I'm trying to instantiate an array of a class where the constructor takes two arguments, and initialize it in the same line.

Conceptually, I want to do something like this:

foo::foo (int A, int B = 10)
{
    V1 = A;
    V2 = B;
}

foo Myfoo[3] = { (1, 100), (2, 300), (5, 100) };
// what I _don't_ want to do is create individual objects like this:
// foo Myfoo1(1, 100); 
// foo Myfoo2(2, 300);
// foo Myfoo3(5, 100);

What I found is that when the constructor is called the arguments are not as expected. The B argument always shows up as the default value of 10. Just in tinkering I threw in additional arguments in the array initialization.

foo Myfoo[3] = { (0, 1, 100), (2, 300, 0), (0, 5, 100, 0) };

To my surprise it compiled without error, but I didn't pursue this too far, because it didn't make sense to me - but I was able to affect the problem

Does anyone have an idea on how I should code this? I've already worked around the problem but I'm curious how it should be done properly.

songyuanyao · Accepted Answer · 2019-07-03 04:00:44Z

3

With the usage of (1, 100), you're just passing one int with value 100 to the constructor of foo. The comma-operator just discards the 1st operand, and returns the 2nd operand here. (It doesn't work in the way as you expected like foo Myfoo1(1, 100); or foo(1, 100);.)

You should use {} ( list initialization (since C++11) ) instead of (), i.e.

foo Myfoo[3] = { {1, 100}, {2, 300}, {5, 100} };

edited Jul 3, 2019 at 4:00

answered Jul 3, 2019 at 3:28

songyuanyao

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2 Comments

El Ronaldo Over a year ago

I tried this using MS Visual Studio 2010 and it will not compile, giving the error message: 1>d:\dev\arraytest\arraytest\arraytest.cpp(24): error C2552: 'Myfoo' : non-aggregates cannot be initialized with initializer list 1> 'foo' : Types with user defined constructors are not aggregate

songyuanyao Over a year ago

@ElRonaldo Does your compiler support C++11? Or it might have some trouble with list initialization. I trid a live demo here.

R Sahu · Accepted Answer · 2019-07-03 03:40:30Z

2

The cause of the problem has already been explaind by @songhuanyao.

I can think of the following ways to resolve the problem.

Option 1

Use {} instead of () to construct objects.

foo Myfoo[3] = { {1, 100}, {2, 300}, {5, 100}};

Option 2

Use foo explitly with () to construct objects.

foo Myfoo[3] = { foo(1, 100), foo(2, 300), foo(5, 100) };

Option 3

Use foo explitly with {} to construct objects.

foo Myfoo[3] = { foo{1, 100}, foo{2, 300}, foo{5, 100}};

answered Jul 3, 2019 at 3:40

R Sahu

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2 Comments

El Ronaldo Over a year ago

your option 2 worked for me! but doesn't that create an additional foo each time? I'm wondering if this is one of those cases where you really need to have a copy constructor defined (but I'm not sure I used the right terminology)

R Sahu Over a year ago

@ElRonaldo, In theory, there is a copy no matter which option you choose. I am not sure whether a compiler is able to elide any of those copy operations.

Community · Accepted Answer · 2020-06-20 09:12:55Z

1

We need to use {} instead of () and code is working as expected

#include<iostream>

using namespace std;

class Test
{
    public:
            Test(int a, int b = 10)
            {
                    x = a;
                    y = b;
                    cout << "C-tor" << endl;
            }
            void show()
            {
                    cout << " x = " << x << " y = " << y << endl;
            }
    private:
            int x;
            int y;
};


int main()
{

    Test obj[3] = {{10,20},{30,40}, {50,60}};
    obj[0].show();
    obj[1].show();
    obj[2].show();
    return 0;
 }

Above code produces expected output:

C-tor

C-tor

C-tor

x = 10 y = 20

x = 30 y = 40

x = 50 y = 60

I hope it helps!

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jul 3, 2019 at 3:39

Abhishek Sinha

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3 Comments

El Ronaldo Over a year ago

Thanks for your answer Sinha! I copied your example into Visual Studio 2010 and it would not compile - getting the same error message as songyuanyao's. What compiler did you use?

Abhishek Sinha Over a year ago

Thanks @ElRonaldo I'm using g++ on Ubuntu 18.04LTS

Abhishek Sinha Over a year ago

gcc version 7.3.0 (Ubuntu 7.3.0-27ubuntu1~18.04) On this version I compile the code.

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