在定义期间将std :: string设置为0，而将std :: string设置为0 [英] Setting std::string to 0 during definition vs setting std::string to 0

问题描述

我正在处理一个我的项目，当我正在为一个类的构造函数，我将一些我的变量设置为默认值。我去设置 std :: string 到 NULL ，它给了一个错误。但是当我在同一行上定义一个 std :: string 并将其设置为 NULL 。我想知道为什么

第一个例子

  std :: string text = NULL; 
  

工作，

第二个示例

  std :: string text; 
 text = NULL; 
  

无法使用。现在我知道你不应该将字符串设置为NULL或0，但我发现这是意外。

第一个例子调用一个构造函数， c $ c> char * ，认为0是指向char的指针？我认为 = 也调用了一个构造函数，所以我不知道为什么他们不会工作，除非 std :: string 具体重载 = 运算符。

我使用Microsoft Visual Studio Express 2013

解决方案

NULL 计算为 0 。

现在， std :: string text = NULL; 将调用构造函数 const char * 并尝试复制数据，但由于 NULL 没有指向任何东西，运行时会出现错误，我得到了（与gcc 5.2）：

 在抛出'std :: logic_error'实例后调用终止
 what（）：basic_string :: _ M_construct null无效
  std中没有定义 operator = ，因此 
 
 $ b < ：string 和 int （或 size_t ）， text = NULL; 将无法编译。
 
I was working on one my projects, and while I was making the constructor for a class, I was setting some of my variables to a default value. I went to set an std::string to NULL, and it gave me an error. But when I define an std::string and set it to NULL on the same line, it works without any errors. I was wondering why

First Example
std::string text = NULL;
worked, and

Second Example
std::string text;
text = NULL;
didn't work. Now I know you shouldn't set a string to NULL, or 0, but I found this on accident.

Does the first example call a constructor that takes a char*, and thinking that 0 is a pointer to a char? I thought = called a constructor too, so I don't get why they wouldn't both work, unless std::string specifically overloads the = operator.

I am using Microsoft Visual Studio Express 2013
解决方案
Value of NULL evaluates to 0. 

Now, std::string text = NULL; will call constructor taking const char* and try to copy the data, but since NULL doesn't point to anything, an error will occur at run-time, I am getting (with gcc 5.2) : 
terminate called after throwing an instance of 'std::logic_error'
  what():  basic_string::_M_construct null not valid
Since, there is no operator= defined for std::string and int(or size_t), text = NULL; will not compile.

                        
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