Why Can’t the Default Constructor Be Called with Empty Brackets?

You may assume that calling a default constructor in C++ is simple with empty brackets, but did you know that it does not work as it should? With the most troublesome parse rule, your object declaration may not even be an object! In this article, we will explore why default constructors have such a method, the issue with empty parentheses, and the most common approach for object initialization in C++ without surprising compiler interpretation.

Table of Contents:

• Default Constructor in C++
• Why Can’t the Default Constructor Be Called with Empty Brackets?
• Problem of Calling the Default Constructor with Empty Brackets
• Approaches for Correct Constructor Declaration
  • Use Direct Initialization (No Parentheses)
  • Use Uniform Initialization ({})
  • Use new for Dynamic Allocation (If Needed)
• Effects of Incorrect Object Declaration in Different Scenarios
  • Case 1: Constructor with Parameters
  • Case 2: Function Returning an Object
• Conclusion

Default Constructor in C++

In C++, the default constructor is a constructor that does not take any arguments or default values for its parameters. The default constructor arises when an object of a class is created without passing any arguments. In some cases, if no constructor is explicitly declared in a class, then automatically C++ compiler provides the default constructor.

Syntax:

class Example {
public:
    Example() {  // Default constructor
        std::cout << "Default constructor called!" << std::endl;
    }
};

Here, the class Example contains a default constructor that initializes the objects of the class. It has the same name as a class, but doesn’t take any parameters. The constructor automatically executes when an object of Example is created. After execution, it prints the message as “Default constructor called!”.

Why Can’t the Default Constructor Be Called with Empty Brackets?

In C++, due to the most vexing parse rule, we cannot call the default constructor with empty parentheses. This rule states that if a statement is initialized as a function declaration, then the compiler will treat it as one.

1. Function declaration interpretation
2. Ambiguity in syntax
3. Avoiding confusions

Problem of Calling the Default Constructor with Empty Brackets

If you create an object using empty parentheses in C++, due to the most vexing parse rule, it may lead to unexpected issues. The rule says that the declaration can be interpreted as either a variable declaration or a function declaration.

Example of incorrect declaration:

class Test {
public:
    Test() { std::cout << "Default Constructor Called!" << std::endl; }
};

int main() {
    Test obj(); // This is interpreted as a function declaration, NOT an object!
}

Explanation: The above snippet calls the default constructor, but in the main() function, we initialize the test object with the empty parentheses Test obj(). So that it doesn’t create an object, but is treated as a function declaration returning a Test object. This is known as the Most Vexing Parse problem.

Approaches for Correct Constructor Declaration

The approaches below ensure the correct declaration in C++

1. Use Direct Initialization (No Parentheses)

Using Direct Initialization means object declaration by simply stating its type and name, such as Test obj. It makes the compiler treat it as an object declaration and not as a function declaration. In contrast to Test obj(), which is mistakenly considered a function prototype, Test obj properly calls the constructor to create an object.

Example:

Output:

In the above code, using the direct initialization, the Test obj is correctly declared as an object of class Test. The above declaration avoids the most vexing parse issue and also ensures that the default constructor is called.

2. Use Uniform Initialization ({})

In C++, using the uniform initialization with { } makes sure, without ambiguity, that the object is initialized properly. To avoid the most vexing parse issue, the Test obj{ } directly calls the constructor. This makes the code more readable.

Example:

Output:

The above code uses uniform initialization with { } and correctly initializes an object of class Test. This method avoids the most vexing parse issues, and rather than a function prototype, it ensures the obj is treated as an object declaration.

3. Use new for Dynamic Allocation (If Needed)

The new keyword in C++ dynamically allocates the memory for an object on the heap. This guarantees avoiding the ambiguity in the declaration, and the object is created explicitly. Also, remember to use the delete function to prevent memory leaks.

Example:

Output:

The above codes use a new keyword for dynamic memory allocation. By ensuring the explicit initialization, the Test object is created on the heap. However, to prevent memory leaks, use the delete obj.

Effects of Incorrect Object Declaration in Different Scenarios

Case 1: Constructor with Parameters

In this case, the constructor with parameters using parentheses does not cause any issues like the most vexing parse. Because the compiler can easily separate it as an object creation with arguments. 

Example:

Output:

In the above code, the constructor takes an argument. Here, the Test obj(5) is not mistaken as a function declaration, and the object is also created correctly.

Case 2: Function Returning an Object

In this scenario, if a function is declared to return an object, this may lead to some confusion because this declaration sometimes resembles an object declaration.

Example:

Output:

The above code uses the function called createObject() to return a Test object. However, if the Test obj() is assigned inside the main(), then it would be assumed to be a function declaration instead of object instantiation.

Conclusion

Dealing with the default constructors with empty parentheses may cause the most vexing parse rule in C++. Because the compiler assumes the default constructor as a function declaration instead of object instantiation. To avoid these types of mistakes, we can use approaches like direct initialization, uniform initialization, and dynamic allocation using new. Understanding these issues ensures the correct object creations and prevents unexpected compilation errors.

You can also explore C++ Interview Questions prepared by our industry experts.

Why Can’t the Default Constructor Be Called with Empty Brackets? – FAQs