Understanding Implicit Deletion- When Default Definitions Lead to Ill-Formed Code
is implicitly deleted because the default definition would be ill-formed
In programming, it is often necessary to define default values for class members to ensure that objects can be created without providing all the necessary information. However, there are cases where the default definition may lead to ill-formed code, resulting in implicit deletion of the member. This article explores the reasons behind this issue and provides solutions to avoid it.
The concept of implicit deletion arises when a class member is not explicitly initialized, and the default definition would cause the code to be ill-formed. This can happen due to several reasons, such as conflicting types, missing members, or undefined behavior. In this article, we will focus on a common scenario where a default constructor is implicitly deleted due to an ill-formed default definition.
Consider a class called `Person` with two members: `name` and `age`. The `name` member is of type `std::string`, and the `age` member is of type `int`. If we try to define a default constructor for this class with a default value for `name`, but fail to provide a default value for `age`, the default constructor will be implicitly deleted because the default definition would be ill-formed.
Here’s an example of the code that leads to this issue:
“`cpp
include
include
class Person {
public:
std::string name;
int age;
Person() : name(“John Doe”) { // Default value for name
// No default value for age
}
};
int main() {
Person person;
std::cout << "Name: " << person.name << ", Age: " << person.age << std::endl;
return 0;
}
```
In the above code, the default constructor is implicitly deleted because the `age` member is not initialized. As a result, attempting to create an object of the `Person` class without providing an `age` value will result in a compilation error.
To resolve this issue, we can define a default constructor that initializes both members with default values:
```cpp
include
include
class Person {
public:
std::string name;
int age;
Person() : name(“John Doe”), age(0) { // Default values for both members
}
};
int main() {
Person person;
std::cout << "Name: " << person.name << ", Age: " << person.age << std::endl;
return 0;
}
```
By providing default values for both `name` and `age`, we ensure that the default constructor is well-formed, and the class can be instantiated without any issues.
In conclusion, it is crucial to define default values for all class members when using default constructors. Failing to do so may result in the implicit deletion of the constructor, leading to ill-formed code. By understanding the reasons behind this issue and implementing proper default values, we can avoid such complications in our code.
