CODEWITHSUNDEEP
c++functionclassparametersparameter-passing
Nickersoft
Nickersoft

Reputation: 684

Why can't I pass functions as arguments across classes in C++?

so I'm trying to pass a function as an argument across two classes of mine in C++:

void class1::func_1(QString str)
{
    class2* r = new class2();
    r->func_2(str, &process);
}

void class1::process(QString str)
{
     //Do something with str
}

Where 'r->func_2' looks like the following:

QString class2::func_2(QString str, void (*callback)(QString))
{
   //Do something else
}

Yet when I try to compile, I get the following errors:

must explicitly qualify name of member function when taking its address
    r->func_2(str, &process);
                    ^~~~~~~~
                     class1::

cannot initialize a parameter of type 'void (*)(QString)' with an rvalue of type 'void (class1::*)(QString)'
    r->func_2(str, &process);
                    ^~~~~~~~

And I can't figure out why. Any ideas? I'm clearly doing something wrong... just not sure what. Any help would be greatly appreciated. Thanks!

Upvotes: 2

Views: 7546

Answers (2)

tsragravorogh
tsragravorogh

Reputation: 3153

The functions are member functions and they are not static. I suggest you to make use of std::function and std::bind. Through these you will be able to take functions as arguments. Just keep in mind that when you will be using std::bind to pass in the argument and it is not a static/global functions but a member of a some object, you need to also make use of this thing called placement. Look it up. You don't need to use lambdas by the way.

class A
{
    std::string m_name;
    void DisplayName() { std::cout << m_name };
}

void TakeFunctionAsArgument(std::function<void(void)> func)
{
    func();
}

int main()
{
    A objectA;
    TakeFunctionAsArgument(std::bind(&A::DisplayName, objectA));
    return 0;
}

Upvotes: 2

Angus H
Angus H

Reputation: 111

You need to fully qualify the member function name when you pass its address:

void class1::func_1(QString str)
{
    class2* r = new class2();
    r->func_2(str, &class1::process);
}

Upvotes: 5

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