Can C Functions be Overloaded based solely on the Constness of Non-Pointer, Non-Reference Value Types?

Function Overloading and Const Arguments: A Closer Look

In C , function overloading allows multiple functions with the same name to exist in a class, as long as they differ in their parameter types. However, a situation arises when attempting to overload functions based solely on the constness of non-pointer, non-reference value types.

Consider the following code snippet:

#include <iostream>
using namespace std;

class Test {
public:
    int foo(const int) const;
    int foo(int);
};

int main ()
{
    Test obj;
    Test const obj1;
    int variable=0;

    obj.foo(3);        // Call the const function
    obj.foo(variable); // Want to make it call the non const function
}

In the above code, an attempt is made to overload the foo function based on the constness of the argument. However, the compiler throws an error, indicating that function overloading cannot be done in this manner.

Understanding the Limitation

The reason for this limitation lies in the way value types are handled. When a value is passed by value to a function, a copy of the value is created, and any changes made to this copy within the function do not affect the original value. Therefore, the constness of the argument is only relevant within the scope of the function.

For example, in the foo function:

int Test::foo(int a)
{
    cout << "NON CONST" << endl;
    a++;
    return a;
}

Even though the function does not have the const keyword, the value of a cannot be modified because it is a copy of the original value passed to the function.

Solution

To achieve the desired functionality, one can overload the foo function based on different parameter types. For instance, one could overload foo to accept a const reference to int and a non-const reference to int.

#include <iostream>
using namespace std;

class Test {
public:
    int foo(const int &a) const;
    int foo(int &a);
};

int main() {
    Test obj;
    Test const obj1;
    int variable = 0;

    obj.foo(3);        // Call the const function
    obj.foo(variable); // Call the non-const function
}

This approach allows for overloading based on parameter types while still maintaining the desired behavior with respect to const-correctness.

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