在构造函数中创建带有参数的类的数组

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.

Answer 1

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 () , ie

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

Answer 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}};

Answer 3

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!