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.
3 Answers
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.
5 Comments
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.
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.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.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
Let's break down what you are in fact doing:
string a=NULL; First you execute string a. This creates a new object on the stack, with default value (an empty string). Then you execute a=NULL, which calls the assignment function of the string class. But what is NULL? NULL in C++ is macro expanded into just 0. So you are attepting to assign an integer to a string variable, which of course is not possible.
string a="abc" works, because you want to assign a char array, and the string class has the assignment operator method overloaded for char arrays, but not for integers. That's why NULL doesn't work and "abc" works.
9 Comments
user823738
In C,
NULL is defined as ((void*)0) but in C++ its just 0.
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.
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.
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NULLto it, but dont forget to use properlynewanddeletestring a;?