Std :: function comparison for member functions
I tried to search here for similar questions:
question 1
question 2
But I cannot compare member functions anyway. Here's an example:
class ClassA
{
public:
int add(int a, int b)
{
return a + b;
}
};
int main()
{
ClassA a1{};
function<int(int, int)> f1 = bind(&ClassA::add, a1, placeholders::_1, placeholders::_2);
function<int(int, int)> f2 = bind(&ClassA::add, a1, placeholders::_1, placeholders::_2);
cout << boolalpha << "f1 == f2 " << (f1.target_type() == f2.target_type()) << endl; // true
cout << (f1.target<int(ClassA::*)(int, int)>() == nullptr) << endl; // true
return 0;
}
It is obvious from the code that f1 and f2 are different. The first cout shows true because the types are the same, that's fine. But why is the second cout true ? Why does :: target () function return nullptr ?
PS: I want to create a simple delegate system so that I can pass any function (global, static, member). With std :: function, I can add a callback, but I don't know how to remove it.
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That f1
target type is not int(ClassA::*)(int, int)
. Its target type will be the result of this expression bind
, which happens in gcc:
std::_Bind<std::_Mem_fn<int (ClassA::*)(int, int)> (
ClassA,
std::_Placeholder<1>,
std::_Placeholder<2>)>
What you can see with the ABI demanger:
#include <cxxabi.h>
// note, the following line technically leaks, but
// for illustrative purposes only it fine
cout << abi::__cxa_demangle(f1.target_type().name(), 0, 0, 0) << endl;
Note that if the target type was actually a class method, you could not name it with two int
- you will need it as well ClassA*
. For example, this target function type int(ClassA::*)(int, int)
:
function<int(ClassA*, int, int)> f3 = &ClassA::add;
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