string initializing as NULL in C++

Question

string a=NULL;

it gives error. Why and how can I initialize string as NULL?

but when I write

string a="foo";

this it works fine.

Solution 1

Actually to get an empty std::string, you just write

std::string a;

std::string's default constructor will give you an empty string without further prompting.

As an aside, using NULL in C++ is generally discouraged, the recommendation would be to either use 0 (which NULL tends to be defined to anyway) or if you have a modern enough compiler, nullptr.

Solution 2

There is a difference between null and empty string (an empty string is still a valid string). If you want a "nullable" object (something that can hold at most one object of a certain type), you can use boost::optional:

boost::optional<std::string> str; // str is *nothing* (i.e. there is no string)
str = "Hello, world!"; // str is "Hello, world!"
str = ""; // str is "" (i.e. empty string)

Comments

[Zaffy]

try to use pointer, you can assign NULL to it, but dont forget to use properly new and delete

[sbi]

@Borgleader: What's wrong with string a;?

[Borgleader]

@sbi: Nothing, in fact it's probably better.

[Zaffy]

In C, NULL is defined as ((void*)0) but in C++ its just 0.

[Puppy]

nullptr will be UB, as it will try to construct a std::string from a null pointer, which is not allowed by the Standard.

[Fergus In London]

Note the typecast (void*)0 - which means a pointer to 0; not am integer representation of 0. In C++ I think it largely depends on the compiler.

[Puppy]

No, it is just zero. There are plenty of other wrongs in this answer.

[Mark Ransom]

@DeadMG I think "as an aside" means it doesn't apply to this situation.

[Matteo Italia]

As an aside, I don't see how using NULL is discouraged in pre-nullptr C++... in practice it's the same as 0 (it's guaranteed to be defined as 0), and it makes more clear that we are talking about pointers.

[Admin]

@MatteoItalia foo(NULL) will it call foo(char*) or foo(int)? You'd say foo(char*) because NULL is used for pointers, but it's actually calling foo(int). Clear code is important. Don't ever use NULL in C++.

[Jakub Zaverka]

@Timo I edited the question, but I still claim that a pointer is an integer (or long). Otherwise you would not be able to perform pointer arithmetics on it.

[Admin]

std::string a = NULL; is an initialization, not an assignment. operator= isn't called (or at least not directly, depending on the implementation).

[Jakub Zaverka]

@DeadMg please enlighten me.

[Zaffy]

"abc" is also an "integer" (pointer to ROM address where "abc" is)

[Admin]

@Zaffy no. It's an array of chars. It decays to a pointer, and how that pointer is stored (if it is stored at all) is implementation-defined. It may be a floating point number on crazy architectures.

[Matteo Italia]

@Zoidberg'--: no, I'd say foo(int) because I know that NULL is plain zero. But if I make the mistake and later someone else notices that the wrong overload is called he can immediately understand what I meant and add the relevant cast (otherwise he would have to ask me if I meant "integer zero" or "pointer zero"). I agree that NULL is a botch - nullptr was introduced for a reason, but I feel it's better than using 0 also for pointers. Anyway, all this is both non relevant to the question (I'm sorry for derailing it) and non relevant in general now that nullptr solves this problem.

[Timo Geusch]

@JakubZaverka, a pointer is convertible to an numerical value like int or long, but it is a distinct type. That's a big difference.