C ++：是“我的文本”一个std :: string，一个* char或一个c字符串？ [英] C++: Is "my text" a std::string, a *char or a c-string?

问题描述

我刚刚做了似乎是一个常见的新手错误：

首先，我们阅读很多教程之一，如下所示：

  #include< fstream> ; 
 int main（）{
 using namespace std; 
 ifstream inf（file.txt）; 
 //（...）
} 
  

其次，我们尝试在我们的代码中使用类似的东西，像这样：

  #include< fstream& 
 int main（）{
 using namespace std; 
 std :: string file =file.txt; //或从返回std :: string的函数中获取文件名
 //。 
 ifstream inf（file）; 
 //（...）
} 
  

被一些神秘的编译器错误消息困惑。

问题是 ifstream 以 const * char 作为构造函数参数。

解决方案是将std :: string转换为const * char 。 / p>

现在，真正的问题是，对于一个新手来说，file.txt或类似的例子在几乎所有的教程都很像一个std :: string。

那么，my text是一个std :: string，c-string还是* char，还是依赖于上下文？

你能提供例子说明我的文本如何根据上下文不同的解释？

谢谢。

• C和C ++都不具有内置字符串数据类型，因此任何双引号字符串你的代码实质上是 const char * （或 const char [] ）。


• 在C ++中， std :: string 是一个字符数组，一个将原始字符串包装到对象中的方便类。通过使用这个，你可以避免自己做（杂乱的）指针算术和内存重分配。


• 大多数标准库函数仍然只需要 char * （或 const char * ）参数。


• 可以隐式转换 char * / code>到 std :: string ，因为后者有一个构造函数。


• c_str（）将 std :: string 转换为 const char * 感谢Clark Gaebel指出 const 方法。 ness，jalf和GMan提到它实际上是一个数组。
  
  

  I have just done what appears to be a common newbie mistake:

  First we read one of many tutorials that goes like this:

   #include <fstream>
   int main() {
        using namespace std;
        ifstream inf("file.txt");
        // (...)
   }  
  

  Secondly, we try to use something similar in our code, which goes something like this:

  #include <fstream>
  int main() {
      using namespace std;
      std::string file = "file.txt"; // Or get the name of the file 
                                     // from a function that returns std::string.
      ifstream inf(file);
      // (...)
  }
  

  Thirdly, the newbie developer is perplexed by some cryptic compiler error message.

  The problem is that ifstream takes const * char as a constructor argument.

  The solution is to convert std::string to const * char.

  Now, the real problem is that, for a newbie, "file.txt" or similar examples given in almost all the tutorials very much looks like a std::string.

  So, is "my text" a std::string, a c-string or a *char, or does it depend on the context?

  Can you provide examples on how "my text" would be interpreted differently according to context?

  [Edit: I thought the example above would have made it obvious, but I should have been more explicit nonetheless: what I mean is the type of any string enclosed within double quotes, i.e. "myfilename.txt", not the meaning of the word 'string'.]

  Thanks.

  解决方案

  So, is "string" a std::string, a c-string or a *char, or does it depend on the context?

  • Neither C nor C++ have a built-in string data type, so any double-quoted strings in your code are essentially const char * (or const char [] to be exact). "C string" usually refers to this, specifically a character array with a null terminator.
  • In C++, std::string is a convenience class that wraps a raw string into an object. By using this, you can avoid having to do (messy) pointer arithmetic and memory reallocations by yourself.
  • Most standard library functions still take only char * (or const char *) parameters.
  • You can implicitly convert a char * into std::string because the latter has a constructor to do that.
  • You must explicitly convert a std::string into a const char * by using the c_str() method.

  Thanks to Clark Gaebel for pointing out constness, and jalf and GMan for mentioning that it is actually an array.

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