Why does this class object declaration work?

Question

Suppose I have a class

class Test{
  public:
    int x;
    Test(const Test& obj){x=obj.x;}
};

Why does

Test object_name(Test random_name);

run and does not need another object as a parameter?. .Something like Test random_name(Test another_random(...)), making it a never ending way of declaring the object?

Answer 1

This line:

Test object_name(Test random_name);

declares a function called object_name that takes a Test as a paramter and returns a Test. It does not declare an object. It is perfectly legal to declare a function like this inside another function, it's just implicitly extern.

Answer 2

Substitute Test with a PoD like int and you will see what is happening

Test object_name(Test random_name); //1

int object_name(int random_name); //2

You can see that the second statement is a function declaration which takes an int as argument and returns an int.

This is due to a well-known rule in CPP related to ambiguity resolution.
From the CPP working draft (N4713):

9.8 Ambiguity resolution [stmt.ambig]

1 There is an ambiguity in the grammar involving expression-statements and declarations: An expression-statement with a function-style explicit type conversion as its leftmost subexpression can be indistinguishable from a declaration where the first declarator starts with a (. In those cases the statement is a declaration.

2 [ Note: If the statement cannot syntactically be a declaration, there is no ambiguity, so this rule does not apply. The whole statement might need to be examined to determine whether this is the case.