Function Pointers with Default Arguments
Can we use member function pointers with member functions that have default arguments?
Yes, you can use member function pointers with member functions that have default arguments in C++. However, there are some important considerations and limitations to be aware of.
Basic Usage
When creating a pointer to a member function with default arguments, the pointer doesn't carry information about the default arguments. You must provide all arguments when calling through the pointer:
#include <iostream>
class MyClass {
public:
void foo(int a, int b = 10) {
std::cout << "a: " << a << ", b: " << b;
}
};
int main() {
void (MyClass::*ptr)(int, int) = &MyClass::foo;
MyClass obj;
// (obj.*ptr)(5); // Error: too few arguments
(obj.*ptr)(5, 20); // OK
}a: 5, b: 20Overloading and Default Arguments
If you have overloaded functions where one version uses default arguments, you need to be explicit about which version you're pointing to:
#include <iostream>
class MyClass {
public:
void bar(int a) {
std::cout << "bar(int): " << a << '\n';
}
void bar(int a, int b = 10) {
std::cout << "bar(int, int): "
<< a << ", " << b << '\n';
}
};
int main() {
void (MyClass::*ptr1)(int) = &MyClass::bar;
void (MyClass::*ptr2)(int, int) = static_cast<
void (MyClass::*)(int, int)>(&MyClass::bar);
MyClass obj;
(obj.*ptr1)(5); // Calls bar(int)
(obj.*ptr2)(5, 20); // Calls bar(int, int)
}bar(int): 5
bar(int, int): 5, 20Using std::function
When using std::function, you need to specify the full function signature, including all parameters:
#include <functional>
#include <iostream>
class MyClass {
public:
void baz(int a, std::string b = "default") {
std::cout << "a: " << a << ", b: " << b;
}
};
int main() {
std::function<void(
MyClass&, int, std::string
)> func = &MyClass::baz;
MyClass obj;
// func(obj, 5); // Error: too few arguments
func(obj, 5, "custom"); // OK
}a: 5, b: customWrapper Functions
To leverage default arguments, you can create wrapper functions:
#include <iostream>
class MyClass {
public:
void process(int a, int b = 10, int c = 20) {
std::cout << "a: " << a << ", b: "
<< b << ", c: " << c << '\n';
}
};
void wrapper(MyClass& obj, int a) {
obj.process(a);
}
int main() {
void (*ptr)(MyClass&, int) = wrapper;
MyClass obj;
ptr(obj, 5);
}a: 5, b: 10, c: 20We can create a similar solution using a lambda:
#include <functional>
#include <iostream>
class MyClass {
public:
void process(int a, int b = 10, int c = 20) {
std::cout << "a: " << a << ", b: " << b <<
", c: " << c << '\n';
}
};
int main() {
auto bound = [](MyClass& obj, int a,
int b = 10, int c = 20){
obj.process(a, b, c);
};
MyClass obj;
// Uses default for b and c
bound(obj, 5);
// Uses default for c
bound(obj, 5, 15);
// Specifies all arguments
bound(obj, 5, 15, 25);
}a: 5, b: 10, c: 20
a: 5, b: 15, c: 20
a: 5, b: 15, c: 25Template Solutions
For more flexibility, you can use templates to create generic wrappers:
#include <iostream>
class MyClass {
public:
void process(int a, int b = 10, int c = 20) {
std::cout << "a: " << a
<< ", b: " << b
<< ", c: " << c << '\n';
}
};
template <typename T, typename... Args>
auto makeWrapper(void (T::*func)(Args...)) {
return [func](T& obj, auto... args){
// Capture default values
int b = 10, c = 20;
// Determine how many arguments are passed
if constexpr (sizeof...(args) == 1) {
// Only a is provided
(obj.*func)(args..., b, c);
} else if constexpr (sizeof...(args) == 2) {
// a and b are provided
(obj.*func)(args..., c);
} else {
// All are provided
(obj.*func)(args...);
}
};
}
int main() {
auto wrapper = makeWrapper(&MyClass::process);
MyClass obj;
// Uses default for b and c
wrapper(obj, 5);
// Uses default for c
wrapper(obj, 5, 15);
// Specifies all arguments
wrapper(obj, 5, 15, 25);
}a: 5, b: 10, c: 20
a: 5, b: 15, c: 20
a: 5, b: 15, c: 25Limitations and Considerations
- Member function pointers don't store information about default arguments. This information is part of the function declaration, not the pointer.
- When using function pointers or
std::function, you generally need to provide all arguments explicitly. - Default arguments are resolved at compile-time, while function pointers are typically used for runtime polymorphism.
Best Practices
- Be explicit about function signatures when working with member function pointers and default arguments to avoid ambiguity.
- Consider using wrapper functions or
std::bindto leverage default arguments when needed. - Document the expected usage clearly, especially when the function pointed to has default arguments.
While member function pointers can be used with functions that have default arguments, it's important to understand the limitations and use appropriate techniques to handle default values when needed.
This often involves creating wrappers or using standard library utilities to achieve the desired behavior.
Member Function Pointers and Binding
Explore advanced techniques for working with class member functions