No match for call to std::function object which is pointer to member function

Question

I want a class to hold a function pointer to a member function of a different class.

But what I get when trying to call that member function using the function pointer, is the error:

No match for call to '(const std::function<bool(PropertyCache&)>) ()'

I don't use raw function pointers but std::function objects as this is the way to go if you want to point to member functions (they need the reference to the instance of the class which I call the member function of).

So my first class is the following:

class Condition : public ICondition
{
public:

    Condition(std::function<bool(Cache&)> cacheGetMethod, bool value)
    {
        m_getter = cacheGetMethod;
        m_value = value;
    }

    virtual bool check() const
    {
        // this is where I get the error, so the way I call the std::function object is wrong?
        return m_getter() == m_value;
    }

    virtual ~Condition() {}

private:
    std::function<bool(Cache&)> m_getter;
    bool m_value;
};

It is also a subclass of an abstract base class but I guess that this is not important for now. Basically a Condition holds the function pointer to the getter of the Cache class to then get the latest value and compare it to a given value.

The Cache class looks like this:

class Cache
{
public:
    void setIsLampOn(bool isOn);
    bool getIsLampOn() const;

private:
    bool m_isLampOn;
};

And then here is how I use it in my main function:

std::shared_ptr<Cache> cache = std::make_shared<Cache>();

std::vector<ICondition*> conditions;
conditions.push_back(new Condition(std::bind(&Cache::getLamp, cache), true));

So the one Condition that I want to use basically checks wether the value of the lamp is true or not.

Here `std::function` you say the function takes a `Cache&`. Why aren't you passing a `Cache` to it? — NathanOliver, Nov 11 '19 at 14:23
That is also, I'm pretty sure, not how to bind an instance to a member function anyway, since the implicit instance argument is a pointer (i.e. it is `this`). *Edit*: I take it back, as per my dupe suggestion: _"`std::bind()` is smart enough to use anything which looks like a pointer, anything convertible to a reference of the appropriate type (like `std::reference_wrapper`), or a [copy] of an object as the object when the first argument is a pointer to member."_ — underscore_d, Nov 11 '19 at 14:23
Possible duplicate of [How std::bind works with member functions](https://stackoverflow.com/questions/37636373/how-stdbind-works-with-member-functions) — underscore_d, Nov 11 '19 at 14:26

Some programmer dude · Accepted Answer · 2019-11-11T14:33:30.593

With

std::function<bool(Cache&)> m_getter;

you say that the "function" object m_getter needs a reference to a Cache object as argument.

While it's correct that you need to pass a Cache object to the function you call (setIsLampOn or getIsLampOn) you set this object in your call to std::bind.

With your current m_getter you need to call it as m_getter(SomeCacheObject).

You should not have m_getter need an argument:

std::function<bool()> m_getter;

Now you can call it as m_getter() and the Cache object you provided with std::bind will be used.

No match for call to std::function object which is pointer to member function

1 Answers1