Comparing std::functions for equality?

Question

How can I compare two C++11 std::functions with operator==, and return true if both of said functions refer to the same f

Answer 1

If the std::function<T(U...)> f is a member function,the fnPointer will be null.

Answer 2

You can actually get it to work with .target:

template<typename T, typename... U>
size_t getAddress(std::function<T(U...)> f) {
    typedef T(fnType)(U...);
    fnType ** fnPointer = f.template target<fnType*>();
    return (size_t) *fnPointer;
}

if (getAddress(f) == getAddress(g)) {...}

(Ref: C++ trying to get function address from a std::function)

Answer 3

operator== for std::function compares a std::function with a null pointer, as far as I can tell the standard does not provide any details as to why.

Although, this boost FAQ entry, Why can't I compare boost::function objects with operator== or operator!=? provides a rationale and as far as I can tell should be applicable to std::function as well. Quoting the FAQ:

Comparison between boost::function objects cannot be implemented "well", and therefore will not be implemented. [...]

it then outlines requested solutions similar to Preet's and goes on to say:

The problem occurs when the type of the function objects stored by both f and g doesn't have an operator==[...]

and explains why this has to has to be dealt with in either the assignment operator or constructor and then goes on to say:

All of these problems translate into failures in the boost::function constructors or assignment operator, even if the user never invokes operator==. We can't do that to users.

Update

Found a standards rationale in Accessing the target of a tr1::function object, which is pretty old but is consistent with the boost FAQ and says:

operator== is unimplementable for tr1::function within the C++ language, because we do not have a reliable way to detect if a given type T is Equality Comparable without user assistance.

Answer 4

Well, if you are not afraid of hacks, you can do something like this:

// Simple function means no std::bind was used
bool IsSimpleFunction(std::function<void(Args...)> function)
{
    typedef void(functionType)(Args...);
    functionType** functionPointer = function.template target<functionType*>();
    return functionPointer != NULL;
}

bool AreEqual(std::function<void(Args...)> left, std::function<void(Args...)> right)
{
    const int size = sizeof(std::function<void(Args...)>);
    std::byte leftArray[size] = { {(std::byte)0} };
    std::byte rightArray[size] = { {(std::byte)0} };
    std::byte* leftByte = (std::byte*) new (&leftArray) std::function<void(Args...)>(left);
    std::byte* rightByte = (std::byte*) new (&rightArray) std::function<void(Args...)>(right);

    // PrintFunctionsBytes(leftByte, rightByte, size);

    // Here the HACK starts
    // By resetting certain values we are able to compare functions correctly
    // When values are reset it has the same effect as when these values are ignored
    bool isSimpleFunction = IsSimpleFunction(left);
    if (!isSimpleFunction)
    {
        ResetAt(leftArray, rightArray, 16);
    }
    ResetAt(leftArray, rightArray, 56);
    ResetAt(leftArray, rightArray, 57);
    // Here the HACK ends

    for (int i = 0; i < size; i++, leftByte++, rightByte++)
    {
        if (*leftByte != *rightByte)
        {
            return false;
        }
    }
    return true;
}

void ResetAt(std::byte* leftArray, std::byte* rightArray, int i)
{
    leftArray[i] = (std::byte)0;
    rightArray[i] = (std::byte)0;
}

// Only for debug
void PrintFunctionsBytes(std::byte* leftFirstByte, std::byte* rightFirstByte, unsigned long long size)
{
    std::vector<std::byte> leftVector(leftFirstByte, leftFirstByte + size);
    std::vector<std::byte> rightVector(rightFirstByte, rightFirstByte + size);
    std::cout << "Left: ";
    for (int i = 0; i < size; i++)
    {
        std::cout << i << ':' << (int)leftVector[i] << std::endl;
    }
    std::cout << "Right: ";
    for (int i = 0; i < size; i++)
    {
        std::cout << i << ':' << (int)rightVector[i] << std::endl;
    }
}

This was tested in MSVC and on 64 bit release configuration. And it was working for simple functions and for std::bind converted to std::function.

If you have different compiler or build configuration, you'll have to adjust ignored bytes for your environment.

Complete example here: https://github.com/linksplatform/Delegates

Answer 5

What about comparing two shared_ptr?

using MessageFilter = std::function<void(const int msgID)>;

static void onMessageReceived(const int msgID)
{
    std::cout << "msg id => " << msgID << std::endl;
}

static void someFunc()
{
    auto filter = std::make_shared<MessageFilter>(&onMessageReceived);

    if (filter && *filter)
    {
        (*filter)(1234);
    }
}

As you can see, 'filter' is a shared_ptr, so it is easy to compare with another.

Answer 6

You could try comparing a and b first by comparing their .target_type() and if these target type ids are the same, then you can compare their .target() pointers. You can use a mismatching target type as an early out false.