Function Overloading

Function overloading allows multiple functions with the same name but different parameters. The compiler selects the best match based on arguments.

Compile-Time Selection

Overload resolution happens at compile-time based on argument types, not runtime values.

Basic Overloading

void print(int x) {
    std::cout << "int: " << x << "\n";
}

void print(double x) {
    std::cout << "double: " << x << "\n";
}

void print(const char* x) {
    std::cout << "string: " << x << "\n";
}

print(42);       // Calls print(int)
print(3.14);     // Calls print(double)
print("hello");  // Calls print(const char*)

Overloading Rules

Functions can differ by:

✅ Number of parameters

void func(int x);
void func(int x, int y);

✅ Type of parameters

void func(int x);
void func(double x);

✅ const/volatile qualifiers

void func(int x);
void func(const int x);  // ❌ Same (top-level const ignored)

void func(int* p);
void func(const int* p);  // ✅ Different (low-level const)

void process(std::string& s);      // Lvalue
void process(const std::string& s); // Lvalue (different)
void process(std::string&& s);     // Rvalue

✅ Reference qualifiers

void func(int& x);   // Lvalue reference
void func(int&& x);  // Rvalue reference

❌ Return type only (not allowed)

int func(int x);
double func(int x);  // ❌ Error: differs only by return type

Overload Resolution Process

Find candidates: Functions with matching name
Filter viable: Can be called with given arguments
Select best match: Most specific match wins

void func(int x);          // #1
void func(double x);       // #2
void func(int x, int y);   // #3

func(42);        // Calls #1: exact match
func(3.14);      // Calls #2: exact match
func(42, 10);    // Calls #3: exact match
func('c');       // Calls #1: char promotes to int

Exact Match vs Conversion

void print(int x);
void print(double x);

print(42);       // Exact match → print(int)
print(3.14);     // Exact match → print(double)
print(3.14f);    // float → double conversion → print(double)
print('A');      // char → int promotion → print(int)

Ranking:

Exact match
Promotions (char→int, float→double)
Standard conversions (int→double)
User-defined conversions

Ambiguous Overloads

void func(int x, double y);
void func(double x, int y);

func(42, 3.14);    // ✅ OK: exact match on both
// func(42, 42);   // ❌ Error: ambiguous!
                   // Both require one conversion

Resolving Ambiguity

// Explicit cast
func(42, static_cast<double>(42));  // Calls first overload
func(static_cast<double>(42), 42);  // Calls second overload

// Or add more overloads
void func(int x, int y);  // Exact match for func(42, 42)

const Overloading

class Widget {
public:
    int getValue() const {        // const version
        std::cout << "const\n";
        return value;
    }
    
    int& getValue() {             // non-const version
        std::cout << "non-const\n";
        return value;
    }
    
private:
    int value = 42;
};

Widget w;
w.getValue() = 100;  // Calls non-const, returns int&

const Widget cw;
int x = cw.getValue();  // Calls const version

Reference Overloading

void process(const std::string& s) {
    std::cout << "lvalue: " << s << "\n";
}

void process(std::string&& s) {
    std::cout << "rvalue: " << s << "\n";
    // Can move from s
}

std::string str = "hello";
process(str);              // Calls lvalue version
process("temporary");      // Calls rvalue version
process(std::move(str));   // Calls rvalue version

Pointer vs Array

void func(int* arr);
void func(int arr[]);     // Same as int*
void func(int arr[10]);   // Still same as int*

// All three are identical - no overloading!

void func(int (&arr)[10]); // ✅ Different: reference to array

Default Arguments

void func(int x, int y = 0);
void func(int x);  // ❌ Error: ambiguous with func(x, 0)

// Call
func(42);  // Which one? Ambiguous!

Template Overloading

template<typename T>
void func(T x) {
    std::cout << "template\n";
}

void func(int x) {
    std::cout << "non-template\n";
}

func(42);    // Calls non-template (exact match preferred)
func(3.14);  // Calls template (only option)

Rule: Non-template functions preferred over templates for exact matches.

Variadic Overloading

void print(int x) {
    std::cout << x << "\n";
}

template<typename T, typename... Args>
void print(T first, Args... rest) {
    std::cout << first << ", ";
    print(rest...);  // Recursive
}

print(1, 2, 3, 4);  // 1, 2, 3, 4

Name Hiding in Inheritance

class Base {
public:
    void func(int x) {
        std::cout << "Base::func\n";
    }
};

class Derived : public Base {
public:
    void func(double x) {  // Hides Base::func!
        std::cout << "Derived::func\n";
    }
};

Derived d;
d.func(42);    // Calls Derived::func(double), not Base::func(int)!
               // Even though int→int is better than int→double

Solution: Use using declaration:

class Derived : public Base {
public:
    using Base::func;  // Bring Base::func into scope
    
    void func(double x) {
        std::cout << "Derived::func\n";
    }
};

Derived d;
d.func(42);  // Now calls Base::func(int) - better match

Operator Overloading

class Complex {
    double real, imag;
public:
    Complex(double r, double i) : real(r), imag(i) {}
    
    // Member function
    Complex operator+(const Complex& other) const {
        return Complex(real + other.real, imag + other.imag);
    }
    
    // Can also overload as friend/global function
};

Complex a(1, 2), b(3, 4);
Complex c = a + b;  // Calls operator+

Best Practices

Overload for different types/counts
Use const overloading for getter/setter pairs
Use reference qualifiers (lvalue/rvalue) for optimization
Keep overload sets intuitive

DON'T

Overload with same parameter types (differs only by names)
Create ambiguous overload sets
Differ only by return type
Use overloading when different names are clearer

Common Pitfalls

Surprising Conversions

void func(long x);
void func(double x);

func(42);  // ❌ Ambiguous! int→long and int→double both conversions

Pointer/Bool Conversion

void func(int x);
void func(bool x);

func(nullptr);  // ❌ Ambiguous in some cases

Forwarding References

template<typename T>
void func(T&& x);  // Universal reference

void func(int& x);

int x = 42;
func(x);  // Which one? Template is exact match!

Summary

Function overloading:

Same name, different parameters
Resolved at compile-time
Ranked: Exact > Promotion > Conversion

Can overload on:

Parameter count
Parameter types
const qualifiers (low-level)
Reference type (lvalue/rvalue)

Cannot overload on:

Return type only
Parameter names
Top-level const

// Good overload set
void process(int x);              // Integer
void process(double x);           // Floating-point
void process(const std::string& s); // String lvalue
void process(std::string&& s);    // String rvalue

Overloading enables type-safe, expressive APIs while maintaining performance through compile-time resolution.

Basic Overloading​

Overloading Rules​

Overload Resolution Process​

Exact Match vs Conversion​

Ambiguous Overloads​

Resolving Ambiguity​

const Overloading​

Reference Overloading​

Pointer vs Array​

Default Arguments​

Template Overloading​

Variadic Overloading​

Name Hiding in Inheritance​

Operator Overloading​

Best Practices​

Common Pitfalls​

Surprising Conversions​

Pointer/Bool Conversion​

Forwarding References​

Summary​