Dear Stackoverflow community,
I'm still bit fresh in c++ and I've been scratching my head and haven't found a solution to my problem yet. I've been searching and trying things for a while now and I've gotten to the point where asking a question would be more beneficial and educational.
Problem:
I'd like to make a class or function that wraps/decorates a given function with or without parameters. Like a good old fashioned @wrapthis in python or c# and the like.
The closest thing I found so far (that is elegant, short and easy to use) is from this stackoverflow answer: C++ Equivalent Decorator
The scratching-my-head part is trying to pass a pointer-function. The error I'm receiving:
Error (active) E0300 a pointer to a bound function may only be used to call the function
Which obviously means that somehow passing a pointer in this fashion is not allowed, so what are my options here?
A working example as answer would be great!
Example of what I'm trying to achieve can be found below:
Something.h
class Something
{
private:
public:
Something() {}
void v_func_with_nothing() { std::cout << "v_func_with_nothing" << "\n"; }
void v_func_with_void(void) { std::cout << "v_func_with_void" << "\n"; }
void v_func_with_one_arg(int x) { std::cout << "v_func_with_one_arg" << x << " " << "\n"; }
void v_func_with_args(int x, int y) { std::cout << "v_func_with_args" << x << " " << y << "\n"; }
int return_func_with_nothing() { return 1; }
int return_func_with_void(void) { return 3; }
int return_func_with_one_arg(int x) { return x; }
int return_func_with_args(int x, int y) { return x+y; }
};
Decorator.h [Again source: C++ Equivalent Decorator]
template<typename T>
auto decorator(T&& func)
{
auto new_function = [func = std::forward<T>(func)](auto&&... args)
{
std::cout << "BEGIN decorating...\n";
auto result = func(std::forward<decltype(args)>(args)...);
std::cout << "END decorating\n";
return result;
};
return new_function;
}
main.cpp
#include <iostream>
#include "Something.h"
#include "Decorator.h"
int main()
{
Something* something = new Something();
auto somedeco = decorator(&something->return_func_with_one_arg);//<-- error here in argument
//int value = somedeco(**enter an argument**);
//std::cout << value << "\n";
return 0;
}
Thank you!
EDIT: SOLUTION
Based on the kind answers given down below I thought of editing this post with an example. The solution to the problem was using lambda.
Decorator.h: I created 2 decorators (one for return-functions, one for void-functions):
template<typename T>
auto DECO_R(T&& func)
{
try
{
auto new_function = [func = std::forward<T>(func)](auto&&... args)
{
std::cout << "BEGIN RETURN decorating...\n";
auto result = func(std::forward<decltype(args)>(args)...);
std::cout << "END RETURN decorating\n";
return result;
};
return new_function;
}
catch (const std::exception& ex)
{
std::cout << ex.what() << "\n";
}
}
template<typename T>
auto DECO_V(T&& func)
{
try
{
auto new_function = [func = std::forward<T>(func)](auto&&... args)
{
std::cout << "BEGIN VOID decorating...\n";
func(std::forward<decltype(args)>(args)...);
std::cout << "END VOID decorating\n";
};
return new_function;
}
catch (const std::exception& ex)
{
std::cout << ex.what() << "\n";
}
}
Main.cpp: 2 examples
int main()
{
Something* something = new Something();
auto somedeco = DECO_R(
[&](int x) {
return something->return_func_with_one_arg(x);
});
int value = somedeco(255);
std::cout << value << "\n";
auto some_v_deco = DECO_V(
[&](int x) {
return something->v_func_with_one_arg(x);
});
some_v_deco(2);
return 0;
}
Output
BEGIN RETURN decorating...
END RETURN decorating
255
BEGIN VOID decorating...
v_func_with_one_arg2
END VOID decorating
I hope this helps others out there.
The call decorator(&something->return_func_with_one_arg) is invalid. There's no such thing as a pointer to a bound function in C++.
If you want somedeco to be a function-like object that wraps a call to something->return_func_with_one_arg(42), for example, you will need to wrap the call either in a lambda:
auto somedeco = decorator(
[&]() {
return something->return_func_with_one_arg(42);
}
);
somedeco();
Or you could pass the parameter through the decorator:
auto somedeco = decorator(
[&](int x) {
return something->return_func_with_one_arg(x);
}
);
somedeco(42);
Keep in mind that this will require that the object pointed to by something outlives the object returned by decorator.