CODEWITHSUNDEEP
functionc++11referencearguments
Marco Agnese
Marco Agnese

Reputation: 369

Reference to class method

I would like to pass to a function a reference to a class method.

For example:

#include <functional>

struct Foo
{
  int f(const int& val) const
  {
    return val+2;
  }
};

int caller(const std::function<int(const int&)>& f)
{
  return f(1);
}

int main()
{
  caller([](const int& val){return val+2;}); // OK
  Foo foo;
  caller(foo.f); // WRONG
  return 0;
}

How can I fix the second call of caller() (NB: Foo:f() is not static)?

Upvotes: 2

Views: 3182

Answers (1)

Jakiša Tomić
Jakiša Tomić

Reputation: 290

In your case, function f doesn't use any member of Foo so it can be declared static

    static int f(const int& val)

and passed as:

    caller(&Foo::f);

But suppose that f cannot be declared static and you want to pass "reference" to member function of particular object.

You can use lambda in that case:

    Foo foo;
    caller(
       [&foo](const int& val){
          return foo.f(val);
       }
    );

foo object is captured in square brackets (in this case by reference) so that you can call f member function on that particular object.

Although it is not part of your question, I should add that it is not really useful to pass int by const reference, as you will not gain any performance improvement that way. Actually, your code will run slower than if you pass int by value.

Upvotes: 2

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