Does there exist REAL pass by reference in any language?

Question

As far as I know there's no pass by reference in c and java essentially passes everything by value, there're dozens of stack overflow posts discussing about this.

Now I wonder that is there any example of REAL call by reference? Because during function call the value of all parameters (including pointers or mutable object identifiers) are always copied to local variables in callee's frame, in that sense everything surely passes by value.

Answer 1

Sure, there is. For example, C? has pass-by-reference. In order for pass-by-reference to occur, both the method parameter in the parameter list at the declaration site as well as the method call argument in the argument list at the call site must be annotated with the ref modifier. The same applies to Visual Basic.NET (here, the modifier is ByRef, I believe.)

C++ also has pass-by-reference, the modifier is &. PHP also has pass-by-reference and uses the same modifier. The same applies to E.

Rust also offers call-by-reference.

In contrast to all the languages listed above, where pass-by-value is the default and pass-by-reference has to be explicitly requested, Fortran II is a pass-by-reference language.

Now I wonder that is there any example of REAL call by reference? Because during function call the value of all parameters (including pointers or mutable object identifiers) are always copied to local variables in callee's frame, in that sense everything surely passes by value.

What you describe is pass-by-value. That's not pass-by-reference. With pass-by-reference, the reference itself is passed, not the value that is referenced.

Here is an example in C? that demonstrates pass-by-value of a value type, pass-by-value of a reference type, pass-by-reference of a value type, and pass-by-reference of a reference type:

struct MutableCell
{
    public string value;
}

class Program
{
    static void IsCSharpPassByValue(string[] foo, MutableCell bar, ref string baz, ref MutableCell qux)
    {
        foo[0] = "More precisely, for reference types it is call-by-object-sharing, which is a special case of pass-by-value.";
        foo = new string[] { "C# is not pass-by-reference." };

        bar.value = "For value types, it is *not* call-by-sharing.";
        bar = new MutableCell { value = "And also not pass-by-reference." };

        baz = "It also supports pass-by-reference if explicitly requested.";

        qux = new MutableCell { value = "Pass-by-reference is supported for value types as well." };
    }

    static void Main(string[] args)
    {
        var quux = new string[] { "Yes, of course, C# *is* pass-by-value!" };

        var corge = new MutableCell { value = "For value types it is pure pass-by-value." };

        var grault = "This string will vanish because of pass-by-reference.";

        var garply = new MutableCell { value = "This string will vanish because of pass-by-reference." };

        IsCSharpPassByValue(quux, corge, ref grault, ref garply);

        Console.WriteLine(quux[0]);
        // More precisely, for reference types it is call-by-object-sharing, which is a special case of pass-by-value.

        Console.WriteLine(corge.value);
        // For value types it is pure pass-by-value.

        Console.WriteLine(grault);
        // It also supports pass-by-reference if explicitly requested.

        Console.WriteLine(garply.value);
        // Pass-by-reference is supported for value types as well.
    }
}