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
{
privat开发者_如何转开发e:
    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 !

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Tricky! You would expect compilation to fail as default constructor is private. However, it compiles and nothing is created. The reason?

Test instance_1();

... is just a function declaration! (Which returns Test and takes nothing.)

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The statement Test instance_1(); doesn't call a constructor at all, because it's not defining a variable - instead, it's declaring a function called instance_1 that returns an object of type Test. To create an instance using the 0-argument constructor, you'd use Test instance_1;.