Function Pointers

Function Pointers

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Prerequisites

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1. What is a Function Pointer?

A function pointer is a variable that stores the address of a function. Just like pointers to variables, but pointing to functions

return_type (*pointer_name)(parameter_types);

Use when:

• You need C-style callbacks
• You want low-overhead dynamic dispatch
• You are working with legacy or embedded systems

✔️ Example

#include <iostream>

int add(int a, int b)
{
    return a + b;
}

int main()
{
    int (*funcPtr)(int, int) = add;

    std::cout << funcPtr(2, 3);  // 5
}

funcPtr now points to add

2. Why Use Function Pointers?

2-1. Callback Functions

void execute(int (*func)(int, int))
{
    std::cout << func(2, 3);
}

2-2. Dynamic Behavior

int add(int a, int b) { return a + b; }
int sub(int a, int b) { return a - b; }

int (*op)(int, int);

op = add;
std::cout << op(5, 3);  // 8

op = sub;
std::cout << op(5, 3);  // 2

2-3. Table of Functions

int add(int a, int b) { return a + b; }
int sub(int a, int b) { return a - b; }

int (*ops[2])(int, int) = {add, sub};

std::cout << ops[0](3, 2);  // add
std::cout << ops[1](3, 2);  // sub

3. Performance Characteristics

✔️ Direct Call

add(2, 3);

✔️ Function Pointer Call

funcPtr(2, 3);

Aspect	Direct Call	Function Pointer
Inline Optimization	✅ Possible	❌ Not possible
Branch Prediction	Better	Slightly worse
Overhead	Minimal	Small overhead
Flexibility	Low	High

3-1. No Inlining

int add(int a, int b) { return a + b; }

Compiler may inline:

int result = 2 + 3;  // optimized

But with function pointer:

funcPtr(2, 3);

Compiler cannot inline (target unknown at compile time)

3-2. Indirect Call

Function pointer introduces an indirect jump

Direct call: call add
Pointer call: call [address]

This affects CPU pipeline & branch prediction

• Usually very small overhead
• Becomes noticeable in:
  • tight loops
  • high-frequency calls
  • performance-critical systems

4. Function Pointer vs Modern Alternatives

✔️ Function Pointer

int (*func)(int, int);

✔️ std::function

#include <functional>

std::function<int(int, int)> func = add;

More flexible but slower

✔️ Lambda

auto func = [](int a, int b) { return a + b; };

Often inlined → fastest

Method	Speed	Flexibility
Direct Call	🔥 Fastest	❌
Lambda	🔥 Fast	✅
Function Pointer	⚡ Medium	✅
std::function	🐢 Slowest	🔥 Most flexible

5. Common Mistakes

❌ Forgetting pointer syntax

int *func(int, int);  // ❌ wrong

✔️ Correct:

int (*func)(int, int);

❌ Null pointer call

int (*func)(int, int) = nullptr;

func(2, 3);  // ❌ crash

✔️ Always check:

if (func)
{
    func(2, 3);
}

6. Function Pointer Typedef

typedef int (*FuncPtr)(int, int);

FuncPtr f = add;

Or (modern C++):

using FuncPtr = int (*)(int, int);

FuncPtr f = add;

Example

#include <iostream>

using Operation = int (*)(int, int);

int add(int a, int b) { return a + b; }
int mul(int a, int b) { return a * b; }

int compute(Operation op, int a, int b)
{
    return op(a, b);
}

int main()
{
    std::cout << compute(add, 2, 3);  // 5
    std::cout << compute(mul, 2, 3);  // 6
}