Creating an instance of a class with ()

I have a question : what constructor is used when you create an instance of a class with ClassName instance() in C++ ?

Example:

#include <iostream>

using namespace std;

class Test
{
private:
    Test()
    {
        cout << "AAA" << endl;
    }

public:
    Test(string str)
    {
        cout << "String = " << str << endl;
    }
};

int main()
{
    Test instance_1(); // instance_1 is created... using which constructor ?
    Test instance_2("hello !"); // Ok

    return 0;
}

Thanks !