Java-like null for string and ofstream arguments in c++

Question

I have some Java code (constructor)

RecursiveDescentParser
(
  std::string inputStream, 
  bool fileService, 
  std::string filePathName, 
  std::ofstream writer
)
{
  input_ = inputStream;
  tokenizer_ = new Tokenizer(inputStream);
  if (fileService == true){
      error = new ErrorHandling(fileService, std::move(writer));
  }
  else{
      error = new ErrorHandling(fileService, std::ofstream());
  }     
  compiled_ = "";
}

Tokenizer *tokenizer_;

std::string input_, compiled_;

I would like to emulate a call within c++

RecursiveDescentParser *parser = new RecursiveDescentParser
(
  stream, 
  false, 
  null, 
  null
);

If I use pointer arguments

std::string *str; std::ofstream *out

I can pass in nullptr but if I choose not to use pointer arguments, I can't pass null. What can I do to simulate passing null to a std::string and std::ofstream?

you can use boost::optional or make another mechanism to tell your function that the parameter is not passed. — andre
– andre, Commented Apr 5, 2013 at 13:37
@andre please read the question more carefully. OP says if I use pointers I can pass NULL but if I don't what can I do? — Ivaylo Strandjev
– Ivaylo Strandjev, Commented Apr 5, 2013 at 13:40

Mike Seymour · Accepted Answer · 2013-04-05 14:04:18Z

2

Pointers are one option as you say; but create complications as you need to either manage the objects separately, or use smart pointers. Shared pointers are a possibility, and give semantics quite similar to Java's object references (only with reference counting rather than garbage collection).

For a nullable object type, you could use Boost.Optional, or implement your own nullable wrapper class. There is talk of optional being included in a future standard library; but for now, you need Boost for that.

Alternatively, it might make more sense to provide a second constructor which does not take those arguments at all.

edited Apr 5, 2013 at 14:04

answered Apr 5, 2013 at 13:40

Mike Seymour

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6 Comments

Mushy Over a year ago

I read Boost.Optional and would like a clear concrete example of how it would be implemented for parameters.

Mike Seymour Over a year ago

@Mushy: The parameter would be boost::optional<std::string>, and you could pass either a string or boost::none.

Mushy Over a year ago

What is the #include<> path for Boost.Optional?

Mike Seymour Over a year ago

@Mushy: boost/optional.hpp

Mushy Over a year ago

I having a problem with boost::optional<std::ofstream> related to no copy/no assign. If I specify, for example, const boost::optional<std::ofstream> &writer as an argument, how am I to do this?

|

metalhead · Accepted Answer · 2013-04-05 13:44:56Z

1

This is how I would change your usage:

RecursiveDescentParser
(
  std::string inputStream, 
  bool fileService = false, 
  std::string filePathName = "", 
  std::ofstream* writer = NULL
)

RecursiveDescentParser *parser = new RecursiveDescentParser(stream);

answered Apr 5, 2013 at 13:44

metalhead

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Comments

Ivaylo Strandjev · Accepted Answer · 2013-04-05 13:37:50Z

0

You have two options - either create a wrapper class that represents an optional value or use pointers. I strongly recommend the first option.

Also in some cases you may considered an empty string to be equivalent to not set value.

answered Apr 5, 2013 at 13:37

Ivaylo Strandjev

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Comments

Dmytro · Accepted Answer · 2013-04-05 14:00:14Z

0

sadly, strings in C++ are objects, and unlike java, C++ objects cannot be null.

From what I understand, you are creating a constructor to a RecursiveDescentParser. The constructor's job is to initialize the object.

If the constructor was allowed to accept a pointer, it could accept a null path string, then it will have been created with a null path string, because it is constructor's job is to initialize all fields, and null fields are not initialized, this would not be a good solution.

My suggestion is to make the constructor private, but create a public method that can accept the parameters the constructor would, but also a pointer to the string rather than the object. If the path string pointer is null, throw an exception, or return an error object. If the path string is valid, create a new parser from reference to the string pointed to by path string pointer, and return this reference.

Although, I could be totally off, but this is just a suggestion, and I welcome all constructive criticisms on why this would not work.

edited Apr 5, 2013 at 14:00

answered Apr 5, 2013 at 13:44

Dmytro

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5 Comments

Yakk - Adam Nevraumont Over a year ago

C++ strings are no more references than ints are references. C++ std::string are full on value types.

Mike Seymour Over a year ago

Strings are not references, they're objects. But you're correct that objects can't be null.

Dmytro Over a year ago

Modified it. No need to downvote, I am willing to correct all inaccuracies, I am merely suggesting a solution.

Mike Seymour Over a year ago

You might want to change or remove the paragraph about "null fields" too, since that makes no sense at all.

Dmytro Over a year ago

I modified it to make more sense.

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