A n（）; 
 

声明一个名为 n 的函数没有参数，并返回 A 。

因为它是一个声明，没有代码被调用/执行没有构造函数）。

在声明之后，你可能会写

  A myA = n（）; 
 

这将编译。但它不会链接！因为没有函数 n 的定义。

I am trying to understand how default constructor (provided by the compiler if you do not write one) versus your own default constructor works.

So for example I wrote this simple class:

class A
{
    private:
        int x;
    public:
        A() { std::cout << "Default constructor called for A\n"; }
        A(int x)
        {
            std::cout << "Argument constructor called for A\n";
            this->x = x;
        }
};

int main (int argc, char const *argv[])
{
    A m;
    A p(0);
    A n();

    return 0;
}

The output is :

Default constructor called for A

Argument constructor called for A

So for the last one there is another constructor called and my question is which one and which type does n have in this case?

 A n();

declares a function, named n, that takes no arguments and returns an A.

Since it is a declaration, no code is invoked/executed (especially no constructor).

After that declaration, you might write something like

A myA = n();

This would compile. But it would not link! Because there is no definition of the function n.