std::vector of std::functions find - c++

Question

I have a vector populated with callback functions and I would like to check whether callback to the function already exists prior to adding it. I don't know whether it will even work bu so far it doesn't even compile.
vector<std::function<void(void*)>> _callbacks;
void Event::RegisterCallback(std::function<void(void*)> callback)
{
if (callback == NULL)
return;
vector<std::function<void(void*)>>::iterator it = std::find(_callbacks.begin(), _callbacks.end(), callback);
if (it == _callbacks.end())
{
_callbacks.push_back(callback);
}
else
{
//print error
throw;
}
}
This gives a compile error:
"Overload resolution selected deleted operator '=='" in alorithm(805). This is related to the find function call.
How do I get this to work and is it even going to compare function calls to the same method properly?
Thanks

Answer 1

As noted in the comments the simplest solution is to use default C-style function pointers as they support == operator in opposite to C++11 function which does not.
using func_type = void(*)();
vector<func_type> _callbacks;
void Event::RegisterCallback(func_type callback)
{
if (callback == nullptr)
return;
auto it = std::find(_callbacks.begin(), _callbacks.end(), callback);
if (it == _callbacks.end()) {
_callbacks.push_back(callback);
}
else {
throw;
}
}
void f() {};
void g() {};
/*
evt.RegisterCallback(f); // works fine
evt.RegisterCallback(g); // works fine
evt.RegisterCallback(f); // throws exception
*/
If you don't like this approach you can write your own function-pointer class with support of equality operator.

Answer 2

Another solution is to have a class with a std::function member and another comperable member, and then overloading the () to get the std::function parameter and call it with the parameter, and the == operator to compeare the class using the comperable member.
CompareableFunction.h:
class CompareableFunction
{
public:
CompareableFunction(int nId, std::function<void(parameter)> handler);
~CompareableFunction();
void operator()(parameter param);
bool operator== (CompareableFunction compareableFunc);
private:
std::function<void(parameter)> m_handler;
int m_nId;
};
CompareableFunction.cpp:
CompareableFunction::CompareableFunction(int nId, std::function<void(parameter)> handler)
{
m_nId = nId;
m_handler = handler;
}
CompareableFunction::~CompareableFunction()
{
}
void CompareableFunction::operator()(parameter param)
{
return m_handler(param);
}
bool CompareableFunction::operator==(CompareableFunction compareableFunc)
{
return (m_nId == compareableFunc.m_nId);
}
EDIT: you can convert the std::function to a C-style function pointer and use it to compare. example to a conversion is here: http://www.cplusplus.com/forum/general/63552/