Code with const keyword won't compile

Question

I don't understand why this piece of code won't compile.

I get following error in line return source->GetA();

cannot convert 'this' pointer from 'const class FooStruct' to 'class FooStruct &'

If I remove the const keyword it compiles fine.

class FooStruct
{
  int a;
public:
  int GetA() {return a;};

  int Bar(const FooStruct *source);
};


int FooStruct::Bar(const FooStruct *source)
{
  return source->GetA();
}

The code itself doesn't make sense. It has been stripped down from some real code and its only purpose is to illustrate the problem.

Answer 1

It is because of this line:

return source->GetA();

Here you are trying to execute GetA function on the pointer that you got. If the pointed object is const, the function must also be const, because:

• Both const and non-const functions can be executed on non-const objects
• Only const functions can be executed on const objects.

It is a good idea to mark all the functions that do not modify the state of the object as const, so they can be used on const objects (e.g. in functions that accept const T & as a parameter).

So in your case, the class should look like:

class FooStruct
{
public:
  int GetA() const {return a;}
};

Answer 2

The function GetA itself needs to be marked const:

int GetA() const {
    return a;
}

This then allows a const source* pointer to call that function.