What Does the Single Ampersand (&) Mean in a C Member Function Declaration?

Understanding the Single Ampersand in Member Function Declaration

In the given code snippet:

class wrap {
   operator obj() const & { ... }   //Copy from me.
   operator obj() && { ... }  //Move from me.
};

we encounter two member function declarations with different ref-qualifiers (& and &&). However, what does the single ampersand (&) signify?

The single ampersand in a non-static member function declaration indicates that the member function is invoked when the object is an lvalue reference. By default, non-static member functions can be invoked on both lvalue and rvalue objects. However, by specifying &, the function can only accept lvalues.

To illustrate the difference:

struct foo
{
   void bar() {}
   void bar1() & {}
   void bar2() && {}
};

• bar() accepts both lvalues and rvalues.
• bar1() requires an lvalue reference, so it can only be invoked on lvalue objects.
• bar2() requires an rvalue reference, so it can only be invoked on rvalue objects.

Example:

int main()
{
   foo().bar();  // (always fine)
   foo().bar1(); // doesn't compile because `bar1()` requires an lvalue
   foo().bar2(); // doesn't compile because `bar2()` requires an rvalue

   foo f;
   f.bar();      // (always fine)
   f.bar1();
   f.bar2();     // doesn't compile because `bar2()` requires an rvalue
}

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