c++ error: call to non-static member function without an object argument

Question

I inherited some code that uses the class adapter pattern and I want to convert it to use the object adapter pattern.

The custom class string is adapting std::string , and it's in the MyString namespace.
Here's a snippet of how the code looks before I alter it.

// mystring.h
namespace MyString
{

// StringInterface is the new (abstract) interface that the client will use.
// Inheriting the implementation of std::string to build on top of it.
class string : public StringInterface, private std::string
{
  ...
};

}

---

// mystring.cpp
namespace MyString
{

...

string& MyString::string::operator=(const string& s) // copy assignment operator
{
  if (this != &s) std::string::operator=(s);
  return *this;
}

...

}

Once I remove the private inheritance of std::string (which I do because--correct me if I'm wrong--the object adapter pattern uses composition and not inheritance of the implementation), the statement std::string::operator=(s); causes the error " call to non-static member function without an object argument ".

So I'm not really sure how to accomplish this. It's my first time dealing with the adapter pattern (and C++ isn't my strongest language); maybe I'm overlooking something simple.

Answer 1

So assuming you have made the std::string a member of your class

class string : public StringInterface
{
   ...
   std::string m_str;
   ...
};

you should then modify all your operations on the once "inherited" (but it's privately... well) std::string to your now member std::string , which in my example, is m_str . For example, instead of doing std::string::size() , you should do m_str.size() .

For your operator=() , you should then do it this way:

string& MyString::string::operator=(const string& s) // copy assignment operator
{
  if (this == &s) return *this;  // So you'll only have to do this test once

  m_str = s.m_str; // instead of std::string::operator=(s);
  // ...

  return *this;
}