C++ string that can be NULL

Question

I'm used to passing around string like this in my C++ applications:

void foo(const std::string& input)
{
  std::cout << input.size() << std::endl;
}

void bar()
{
  foo("stackoverflow");
}

Now I have a case where I want the string to be NULL:

void baz()
{
  foo("stackoverflow");
  foo(NULL); // very bad with foo implementation above
}

I could change foo to:

void foo(const std::string* input)
{
  // TODO: support NULL input
  std::cout << input->size() << std::endl;
}

But to pass a string literal or copy a char* to that implementation of foo I need to write something like this:

void bar()
{
  string input("hi"); // annoying temporary
  foo(&input);
  foo(NULL);  // will work as long as foo handles NULL properly
}

I started thinking about inheriting from std::string and adding a null property, but I'm not so sure it's a good idea. Maybe it is better to simply use a const char* string for parameters that can be NULL, but what if I want to save a copy of the string (or NULL) without having to manage its memory myself? (See What are some of the drawbacks to using C-style strings? etc.)

Any clever solution around?

Answer 1

If you want the type to be null, then make it a pointer. Pass string pointers around instead of references, since this is precisely what pointers can do, and references cant. References always point to the same valid object. Pointers can be set to null, or be reseated to point to another object. Thus, if you need the things pointers can do, use pointers.

Alternatively, use boost::optional, which allows a more type-safe way to specify "this variable may or may not contain a value".

Or, of course, change the semantics so you either use empty strings instead of null, pass a separate bool parameter specifying whether the string is available or not, or refactor so you don't need this in the first place.

Answer 2

Function overloading to the rescue...

void foo( const std::string& input )
{
    std::cout << input << std::endl;

    // do more things ...
}

void foo( const char* input )
{
    if ( input != NULL ) foo( std::string(input) );
}

This will accept both c-style char arrays and std::strings, and will incur extra overhead on the stack if you pass in a string literal or a char array, but allows you to keep your implementation in one place and keep your nice syntax.