Default value for non-const reference argument

defaultref.cpp

// $ clang++ --std=c++11 defaultref.cpp
// $ ./a.out
// Calling without the default argument:
//
// foo in main pre-increment        = xyzzy
// foo in main post-incement        = yzxxz
// foo in withFoo pre-increment     = yzxxz
// foo in withFoo post-increment    = zxyyx
// foo in main post-withFoo         = zxyyx
//
// Calling with the default argument:
//
// defaultFoo in main pre-increment = xyzzy
// defaultFoo in main post-incement = yzxxz
// foo in withFoo pre-increment     = yzxxz
// foo in withFoo post-increment    = zxyyx
// defaultFoo in main post-withFoo  = zxyyx

#include <iostream>

enum class Foo
{
    xyzzy,
    yzxxz,
    zxyyx
};

std::ostream &
operator<<(std::ostream &o, Foo &foo)
{
    switch (foo)
    {
        case Foo::xyzzy:
            o << "xyzzy";
            break;
        case Foo::yzxxz:
            o << "yzxxz";
            break;
        case Foo::zxyyx:
            o << "zxyyx";
            break;
    }
    return o;
}

Foo &
operator++(Foo &foo)
{
    switch (foo)
    {
        case Foo::xyzzy:
            foo = Foo::yzxxz;
            break;
        case Foo::yzxxz:
            foo = Foo::zxyyx;
            break;
        case Foo::zxyyx:
            foo = Foo::xyzzy;
            break;
    }
    return foo;
}

static Foo defaultFoo = Foo::xyzzy;


void
withFoo(Foo &foo = defaultFoo)
{
    std::cout << "foo in withFoo pre-increment     = " << foo << std::endl;
    ++foo;
    std::cout << "foo in withFoo post-increment    = " << foo << std::endl;
}

int
main()
{
    std::cout << "Calling without the default argument:" << std::endl
              << std::endl;

    Foo foo = Foo::xyzzy;
    std::cout << "foo in main pre-increment        = " << foo << std::endl;
    ++foo;
    std::cout << "foo in main post-incement        = " << foo << std::endl;
    withFoo(foo);
    std::cout << "foo in main post-withFoo         = " << foo << std::endl;

    std::cout << std::endl
              << "Calling with the default argument:" << std::endl
              << std::endl;

    std::cout << "defaultFoo in main pre-increment = " << defaultFoo << std::endl;
    ++defaultFoo;
    std::cout << "defaultFoo in main post-incement = " << defaultFoo << std::endl;
    withFoo();
    std::cout << "defaultFoo in main post-withFoo  = " << defaultFoo << std::endl;
    return 0;
}