Why Does Multiple Inheritance with Overloaded Functions Lead to Ambiguity in C ?

Function Overloading with Differently-Typed Multiple Inheritance

In object-oriented programming, multiple inheritance allows a subclass to inherit methods and attributes from multiple parent classes. However, when multiple parent classes define methods with the same name but different signatures (overloaded functions), it can lead to ambiguity during function calls. This article explains why overloaded functions with the same name and different signatures are not treated as such in the case of multiple inheritance and explores possible solutions.

Problem Description

Consider the following code snippet:

#include <iostream>

struct Base1 {
    void foo(int) {}
};

struct Base2 {
    void foo(float) {}
};

struct Derived : public Base1, public Base2 {
};

int main() {
    Derived d;
    d.foo(5); // Ambiguous call to 'foo'
}

In this example, the Derived class inherits two overloaded foo() functions from its parent classes, Base1 and Base2. However, when we try to call d.foo(5), we get an ambiguous call error because the compiler is unable to determine which foo() function to call. This is because both functions have the same name but different argument types.

Understanding Function Overloading in Inheritance

Function overloading allows a class to have multiple methods with the same name but different parameters. Each overloaded function is differentiated based on the number, type, and order of its arguments. In the context of single inheritance, function overloading works as expected: the compiler selects the appropriate function based on the argument types used in the function call.

Multiple Inheritance and Ambiguity

However, in the case of multiple inheritance, the rules for function lookup become more complex. When a function name is looked up in a class C, the compiler considers all declarations of that function in C and its base classes. If multiple declarations of the same function are found, any declarations that are hidden by another declaration in a derived class are eliminated.

If the remaining declarations are not all from subclasses of the same type or if the set includes a non-static member and members from distinct subclasses, an ambiguity occurs, and the program is considered ill-formed. This is exactly the situation in the previous example, where d.foo(5) calls both Base1::foo(int) and Base2::foo(float), resulting in ambiguity.

Resolving Ambiguity

To resolve the ambiguity and make the code valid, one solution is to use using declarations. A using declaration introduces a name into the scope of the current class that is associated with a name from another class. In the above example, we can use the following:

struct Derived : public Base1, public Base2 {
    using Base1::foo;
    using Base2::foo;
};

By explicitly using the using declarations, we specify which version of foo() to use from each base class. This resolves the ambiguity and allows the code to compile successfully.

Fallback Behavior

It's worth noting that the second code snippet in the original question compiles without error because the foo(float) function is defined within the Derived class's scope. Therefore, when d.foo(5) is called, it resolves to Derived::foo(float) without any ambiguity.

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