Problem of Calling Default Constructor with Empty Brackets
In C++, the reason we can’t call the default constructor with empty brackets is due to the language’s interpretation of function declarations. According to the C++ standard, anything that can be interpreted as a function declaration will indeed be interpreted as such. Therefore, when we use empty brackets, it signifies a function call without any arguments.
When we define a constructor with parameters, it becomes the only constructor for that class, and the compiler no longer generates the default constructor (without parameters). So, if we try to call the default constructor with empty brackets, the compiler interprets it as a declaration of a function that takes no arguments. It will then look for a function that matches that signature (no parameters), and if it’s not there, it will throw an error.
For example, if we attempt to invoke the default constructor MyClass obj() with empty brackets in the below example, the code won’t compile and the program will throw an error because the default constructor hasn’t been explicitly defined due to the presence of the parameterized constructor.
// C++ program to attempt calling default constructor with
// empty bracket
#include <iostream>
using namespace std;
class MyClass {
public:
// Constructor with parameters
MyClass(int value)
{
cout << "Constructor with parameter called. Value: "
<< value << endl;
}
};
int main()
{
// Attempt to call the default constructor with empty
// brackets
MyClass
obj(); // This line will result in a compilation error
return 0;
}
Output
warning: empty parentheses were disambiguated as a function declaration [-Wvexing-parse]
Why Can’t the Default Constructor Be Called with Empty Brackets?
In C++, the default constructor is a constructor that has no arguments or default value for all the arguments. If we don’t explicitly define a default constructor, the compiler generates a default constructor during compilation. In this article, we will learn why can’t the default constructor be called with empty brackets in C++.