E-COM-NET

Question

PayTableModelLayer::PayTableModelLayer()

:m_sName(NULL)

{

// for (int i = 0; i < MAX_SYMBOL_COUNT; i++)

// {

// m_nodePayTable[i] = NULL;

// }

}

string m_sName

问题出在了m_sname是个字符串变量不应该初始化成NULL，指针初始化成NULL，查阅stackFlow给出如下解释

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.

try to use pointer, you can assign NULL to it, but dont forget to use properly new and delete — Dec 16 '12 at 0:18

Timo Geusch 19.7k 4 35 60 · Accepted Answer · Dec 16 '12 at 0:05

up vote 9 down vote accepted

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.

answered Dec 16 '12 at 0:05

Timo Geusch

19.7k 4 35 60

nullptr will be UB, as it will try to construct a std::string from a null pointer, which is not allowed by the Standard. – Puppy Dec 16 '12 at 0:14

@DeadMG I think "as an aside" means it doesn't apply to this situation. – Mark Ransom Dec 16 '12 at 0:20

1

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 Dec 16 '12 at 0:20

1

@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++. – user142019 Dec 16 '12 at 0:21

2

@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. – Matteo Italia Dec 16 '12 at 0:27

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score 4 · Answer 2 · Dec 26 '12 at 11:49

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)

score -2 · Answer 3 · Dec 16 '12 at 0:18

up vote -2 down vote

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.

edited Dec 16 '12 at 0:18

answered Dec 16 '12 at 0:09

Jakub Zaverka

7,272 1 16 41

2

This is not quite correct - a void* is not an integer. – Timo Geusch Dec 16 '12 at 0:11

In C, NULL is defined as ((void*)0) but in C++ its just 0. – Zaffy Dec 16 '12 at 0:13

1

std::string a = NULL; is an initialization, not an assignment. operator= isn't called (or at least not directly, depending on the implementation). – user142019 Dec 16 '12 at 0:23

2

@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. – user142019 Dec 16 '12 at 0:25

2

@JakubZaverka, a pointer is convertible to an numerical value like int or long, but it is a distinct type. That's a big difference. – Timo Geusch Dec 16 '12 at 0:28

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c++ 中string变量的初始化

3 Answers

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