C++ Const Correctness

Example 1

class MyClass {

public:

    const int dosomething(const int const number) const; // Red const no use

};

In this example, `doSomething` is a const member function of a class named `MyClass` that takes an integer parameter `number` and returns a constant integer. The final `const` at the end of the function declaration indicates that the function does not modify the state of the object on which it is called.

Return Type:

   - `const int`: This indicates that the function returns a constant integer. 

The `const` keyword after the data type means that the value returned by the function should not be modified.

Parameter:

   - `(const int const number)`: This declares a parameter named `number` of type `const int const`. However, the use of `const` twice in this context is not valid and would result in a compilation error. The correct syntax would be either `const int` or `int const`. Using both is redundant.

Const Qualifier for Member Function:

   - `const`: The final `const` at the end of the function declaration indicates that this member function is a "const member function." In C++, when a member function is declared as `const`, it means that the function does not modify the state of the object it is called on. This is a promise to the compiler and allows the function to be called on constant objects.

Example 2

class MyClass {

public:

    int doSomething(const int* const number) const;

}

The function declaration `int doSomething(const int* const number) const;` is using pointers, and it indicates that the parameter `number` is a constant pointer to a constant integer. Let's break down each part of the declaration:

1. Return Type:

   - `int`: This indicates that the function returns an integer.

2. Function Name:

   - `doSomething`: This is the name of the function.

3. Parameter:

   - `const int* const number`: This declares a parameter named `number`. Breaking it down:

      - `const int*`: This indicates that `number` is a pointer to a constant integer. The data pointed to by `number` cannot be modified through this pointer.

      - `const`: This additional `const` means that the pointer itself (`number`) is constant, meaning the pointer cannot be reassigned to point to a different memory location.

4. Const Qualifier for Member Function:

   - `const`: The final `const` at the end of the function declaration indicates that this member function is a "const member function." In C++, when a member function is declared as `const`, it means that the function does not modify the state of the object it is called on.

Here's an example of how this function could be implemented:

class MyClass {

public:

    int doSomething(const int* const number) const {

        // Access the value pointed to by number (read-only)

        int value = *number;

        // Rest of the function...

        return value; // Return some result

    }

};

In this example, `dosomething` is a const member function of a class named `MyClass` that takes a constant pointer to a constant integer as a parameter. 

The function can read the value pointed to by `number`, but it cannot modify the pointed-to value, and it cannot reassign the pointer `number` to point to a different memory location.

Example 3

class MyClass {

public:

    int dosomething(const int& const number) const;

};

The function declaration `int dosomething(const int& const number) const` contains a similar issue as before with the redundant use of `const`. In C++, you can use either `const int&` or `int const&` to declare a constant reference, but using both is not necessary. 

Additionally, when declaring a const member function, the final `const` at the end is valid and indicates that the function does not modify the state of the object it is called on.

Here's the corrected version:

int dosomething(const int& number) const {

    // Function implementation...

}

In this corrected example:

- `const int&` specifies that `number` is a constant reference to an integer. The `const` indicates that the value referred to by `number` cannot be modified within the function.

- The final `const` after the parameter list indicates that `dosomething` is a const member function, meaning it does not modify the state of the object it is called on.

So, the corrected function declaration is:

int dosomething(const int& number) const;