Call Function Using Pointers in C++?

• 🔧 C++ function pointers differ between regular and member functions. This is because member functions need an object to work.
• 🧠 Templates check types well and find problems when you build the program. This happens when you call class member functions that change at runtime.
• ⚠️ Macros do not check types. Use them only when templates do not work.
• 📦 Member function pointers help you build systems in parts. You can use them for plugin systems and dispatch tables.
• 🛠️ Using classes with different return types needs template specialization or std::function.

You can call class member functions at runtime using function pointers. This helps you organize your C++ code in flexible ways. Maybe you are building a plugin system, a dispatch table, or just want to make logic work for many types. Either way, knowing how to call functions this way—with templates or C++ macros—is an important skill. Let's explain how this works and how to do it in a safe and clear way.

What Are Function Pointers in C++?

In C++, a function pointer lets you store a function's address in a variable. Then, you can call that function using the variable. This is helpful when you want to choose and run a function at runtime.

For free (non-member) functions, the syntax is clear:

void myFunction() {
    std::cout << "Called myFunction" << std::endl;
}

void (*funcPtr)() = &myFunction;
funcPtr(); // Invokes myFunction

This will output:

Called myFunction

Free functions are not part of a class or its state. So, you do not need an object to call them.

Member Function Pointers vs Regular Function Pointers

Things are different with member functions. Pointers to free functions are just memory addresses. But, pointers to member functions hold more information about the class they are part of. They also need an object to be called.

Example:

class A {
public:
    void show() {
        std::cout << "A::show()" << std::endl;
    }
};

Declaring and using a pointer to a member function:

void (A::*funcPtr)() = &A::show; // Note the special syntax
A obj;
(obj.*funcPtr)(); // Invokes A::show()

A* ptr = &obj;
(ptr->*funcPtr)(); // Also valid with a pointer to object

Key points:

• The pointer type must specify the class scope (A::*).
• To call it, you need either an object reference (obj.*ptr) or a pointer to an object (ptr->*ptr).
• The parentheses around (obj.*funcPtr) make sure the syntax is correct.

Static Member Functions: Simpler Behavior

Static member functions act like free functions. They do not use an object (this pointer). So, their pointers are treated like regular function pointers.

class B {
public:
    static void greet() {
        std::cout << "Hello from B" << std::endl;
    }
};

void (*funcPtr)() = &B::greet;
funcPtr(); // No object needed

This makes static member functions good for callbacks when you do not need an object with its own data.

Example: Two Classes Sharing Method Names

Imagine you are building a tool that works with many classes that are not related. These classes happen to have a method with the same name and the same setup.

class A {
public:
    void display() {
        std::cout << "A::display()" << std::endl;
    }
};

class B {
public:
    void display() {
        std::cout << "B::display()" << std::endl;
    }
};

// Direct invocation using pointers
void (A::*aDisplay)() = &A::display;
void (B::*bDisplay)() = &B::display;

A a;
B b;
(a.*aDisplay)(); // Output: A::display()
(b.*bDisplay)(); // Output: B::display()

This works, but it repeats itself and can easily cause errors if used with more types or functions.

Generalizing Function Calls with Templates (Preferred)

C++ templates let you create general functions. This helps you write code you can use again and that checks types. Here, templates help you call member functions for different types, all while keeping it simple to use.

Define a general function that takes an object and a pointer to a member function:

template<typename T>
void callDisplay(T& obj, void (T::*func)()) {
    (obj.*func)(); // Invoke on object
}

Usage:

callDisplay(a, &A::display); // Output: A::display()
callDisplay(b, &B::display); // Output: B::display()

Why Templates Are Better

• ✅ Check types when you build the program — if types do not match, the program will not build.
• ✅ Clear error messages — template problems are found soon.
• ✅ Clean syntax — once set up, calling is short and easy to read.
• ✅ Flexible — works with any type T that has the right setup.

In modern C++ (C++11 and newer), templates are used more than macros for general tasks.

When Should You Use Macros?

Macros have problems, but they are still useful when templates do not work, such as:

• Debug logging wrappers
• Conditional compilation
• Platform-specific optimizations

Here’s a macro version of our callDisplay logic:

#define CALL_DISPLAY(obj, func) ((obj).*(func))()

This means you do not need to set up templates, but there are downsides:

CALL_DISPLAY(a, &A::display); // A::display()
CALL_DISPLAY(b, &B::display); // B::display()

Problems with Macros

• ❌ No type checking — errors show up as strange messages from the compiler.
• ❌ Tricky syntax — you cannot see what the macro does when the program runs.
• ❌ Hard to debug — it is hard to go through step-by-step or follow in IDEs.

⚠️ Tip: If you see strange building errors with unexpected braces or tokens, macros might be why.

Templates vs. Macros: How to Choose

👉 To sum up: use templates for making general code. Keep macros for specific needs, such as including code based on conditions or for very low-level code.

Passing Function Pointers as Parameters

You can make function pointer use more general, past just the display() example. Let's define a tool to work with any member function that has a known setup.

template<typename T>
void callMemberFunction(T& obj, void (T::*func)()) {
    (obj.*func)(); // Invoke member on instance
}

Flexible usage for any class:

callMemberFunction(a, &A::display);
callMemberFunction(b, &B::display);

Even more general:

template<typename T, typename Func>
void invoke(T& obj, Func func) {
    (obj.*func)();
}

This design helps with:

• Callback registration
• Runtime dispatch tables
• Calls functions based on settings at runtime

Different Function Setups: A Problem You Might Face

What if your classes have the same function names but different setups? For example, A::display() returns void, but B::display() returns bool.

class A {
public:
    void display() { std::cout << "void display()\n"; }
};

class B {
public:
    bool display() {
        std::cout << "bool display()\n";
        return true;
    }
};

Now, your templated call function needs to change:

template<typename T, typename R>
R callFunction(T& obj, R (T::*func)()) {
    return (obj.*func)();
}

Calling the function works smoothly:

callFunction(a, &A::display); // OK
bool result = callFunction(b, &B::display); // OK

Or, you can use std::function for even more flexible polymorphism. But, this adds a small amount of extra work.

Real-Use Case: Dispatching via Map Lookup

Suppose you want to link function names with the real functions at runtime. A map used for dispatch links strings with function pointers:

class Plugin {
public:
    void start() { std::cout << "Started" << std::endl; }
    void stop()  { std::cout << "Stopped" << std::endl; }
};

using MemberFunc = void (Plugin::*)();

std::map<std::string, MemberFunc> dispatchTable = {
    {"start", &Plugin::start},
    {"stop", &Plugin::stop}
};

void dispatch(Plugin& p, const std::string& command) {
    auto it = dispatchTable.find(command);
    if (it != dispatchTable.end()) {
        (p.*(it->second))();
    } else {
        std::cerr << "Unknown command" << std::endl;
    }
}

This pattern is very useful for:

• Plugin managers
• Scripting language integrations
• Command interfaces

Common Mistakes With Function Pointers

Here are common problems and how to avoid them:

• ⚠️ Forgetting & — Always assign function pointers with &Class::function.
• ⚠️ Wrong syntax — Do not forget parentheses: (obj.*func)(); not obj.*func();.
• ⚠️ Null pointers — A nullptr object can cause issues that are hard to track.
• ⚠️ Setup not matching — Make sure pointer types match exactly. Even return types must match.

Debugging Tips:

• Use decltype to find types.
• Use static_asserts inside templates to limit how they are used.
• Break down complex call sites into smaller test units.

Best Practices: Keeping Things Easy to Handle

To make long-lasting solutions with member function pointers:

• ✅ Store templates that you can use again in a utils/ or helpers/ library file.
• ✅ Use the same way of naming pointers.
• ✅ Document functions and the setups you expect using type alias or using.
• ✅ Wrap pointer dispatch with traits or CRTP to make them easier to extend.

More advanced: Define function traits to get setup types and parameters from function pointers at runtime, using template rules.

Performance Considerations

For how fast things run:

• Function pointers are faster than virtual function calls, especially outside inheritance trees.
• Templates result in calls set up when the program builds. The compiler often makes them disappear.
• Avoid using std::function too much unless you truly need it to work with different types. It is slow because of how it removes type info and uses memory from the heap sometimes.

If your system is slowed down by runtime calls, using template-based lookup may be faster than usual polymorphism.

How This Is Used In The Real World

• 🧩 Plugin frameworks — load functions when the program runs without virtual inheritance.
• 🎮 Game engines — parts of the system call actions using callbacks.
• 💻 GUI frameworks — events are sent based on function name/ID.
• 🛠️ Logic based on settings — spreadsheets or scripts control when functions run.

These scenarios are helped by how flexible and organized function pointers and general ways of calling functions are.

C++ provides good tools to build flexible systems that change at runtime using function pointers. You can call class member functions well, for types that are not related, using type-safe templates or fallback macros. Templates should be your main tool because of their checks when you build the program, how easy they are to read, and how easy they are to extend. Macros still have a place sometimes, especially in systems that use the preprocessor a lot or for conditional compilation.

By learning these techniques well—handling objects, understanding setups, and choosing between templates or macros—you make your C++ systems more reliable and able to grow.

References

• Stroustrup, B. (2013). The C++ Programming Language. Addison-Wesley.
• Meyers, S. (2005). Effective C++: 55 Specific Ways to Improve Your Programs and Designs.
• Sutter, H., & Alexandrescu, A. (2004). C++ Coding Standards. Pearson Education.
• ISO/IEC 14882:2017 – C++17 Standard.