Creating an array of a class with arguments in constructor - c++

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 (), i.e.
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!