C Member Functions: What Do `&` and `&&` Mean in Parameter Lists?

Member Function Parameter Lists: Unveiling the & Symbol

When declaring member functions in C , one may encounter the use of & and && symbols after the parameter list. These symbols, known as ref-qualifiers, indicate the reference type of the implicit object parameter.

Single Ampersand (&)

The single ampersand (&) signifies that the member function will be invoked when the object is an lvalue reference. An lvalue reference refers to an existing object stored at a particular memory location. The following code snippet demonstrates its usage:

class wrap {
public:
   operator obj() const & { ... }
   operator obj() && { ... }
private:
   obj data_;
};

The operator obj() function with the & ref-qualifier will be invoked when the object is an lvalue reference. This means it can be called using both lvalue and rvalue expressions.

No Ampersand

In the absence of a ref-qualifier, the implicit object parameter is treated as an lvalue reference. This allows the member function to be invoked with both lvalue and rvalue expressions. The code below showcases this behavior:

struct foo
{
    void bar() {}
};

int main()
{
    foo().bar(); // (always fine)
    foo f;
    f.bar();      // (always fine)
}

Rvalue Reference (&&)

The && ref-qualifier indicates that the member function will be invoked when the object is an rvalue reference. An rvalue reference refers to a temporary or expression-evaluated object that cannot be bound to a specific memory location.

Key Differences

The following table summarizes the differences between the single ampersand (&) and no ampersand:

Ref-Qualifier	Reference Type	Can be Invoked With
&	lvalue reference	lvalue and rvalue expressions
None	lvalue reference	lvalue and rvalue expressions

Understanding these ref-qualifiers is crucial for correctly implementing member functions and tailoring their behavior based on the context.

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