混合ifstream getline和>> [英] Mixing ifstream getline and >>

问题描述

使用>> 输入 float 值后，getline停止工作并读取垃圾桶。如何避免此问题？

  string settingsFile =TerrainSettings.txt; 
 
 ifstream settingsStream; 
 
 settingsStream.open（settingsFile.data（））; 
 assert（settingsStream.is_open（））; 
 
 string line; 
 
 getline（settingsStream，line）; // fine here 
 getline（settingsStream，line）; // fine here 
 getline（settingsStream，line）; // fine here 
 
 float frequency; 
 float octaves; 
 float amplitude; 
 
 settingsStream>> frequency; // fine here 
 
 getline（settingsStream，line）; //这里是垃圾桶，为什么？ 
 
 settingsStream>>八度; // trash 
 
 getline（settingsStream，line）; 
 
 settingsStream>> amplitude; 
  

编辑：包含ignore（），会产生以下错误：

  settingsStream>> frequency; 
 
 settingsStream.ignore（256，'\\\
'）; 
 
 getline（settingsStream，line）; // fine here 
 
 settingsStream>> octaves; // trash here 
 
 getline（settingsStream，line）; 
 
 settingsStream>> amplitude; 
  

示例输入：

  / *地形渲染设置请输入各自的行* / 
 / ** / 
频率：
 24 
八度：
 120 
幅度：
 1.25 
  

解决方案

首先，使用 std :: string s和 std :: getline 开始。其次，垃圾的原因是缓冲区中还有一个换行符。如果你希望用户一次输入一行，你应该使用 istream :: ignore 跳过所有直到并包括下一个换行符。例如：

  std :: string s; 
 float f; 
 std :: cin>> F; 
 std :: cin.ignore（big_num，'\\\
'）; 
 std :: getlines（std :: cin，s）; 
  

什么 big_num 应该取决于您的期望关于输入。如果你不介意编写很多，并且想要安全，使用 std :: numeric_limits< std :: streamsize> :: max（）。

例如，上面的代码将解析下面的代码，使得 f = 5.0f ， s = Hello！。

  5 
您好！但是，它将按照完全相同的方式解析：
 
 
 
 
 
 
 
  5嗨！ 
你好！ 
  
如果您要保留 Hi！ ，你不应该忽略的东西，而是定义一个正确的语法，你将用来解析文档。
 
After using a >> to input a float value, getline ceases to work correctly and reads trash. How can I avoid this?
   string settingsFile = "TerrainSettings.txt";

   ifstream settingsStream;

   settingsStream.open(settingsFile.data());
   assert (settingsStream.is_open());

   string line;

   getline(settingsStream,line); // fine here
   getline(settingsStream,line); // fine here
   getline(settingsStream,line); // fine here

   float frequency;
   float octaves;
   float amplitude;

   settingsStream>>frequency; // fine here

   getline(settingsStream,line); // here it gets trash, why? 

   settingsStream>>octaves; // trash

   getline(settingsStream,line);

   settingsStream>>amplitude;
Edit: An inclusion of ignore(), generates the following error:
   settingsStream>>frequency;

   settingsStream.ignore(256,'\n');

   getline(settingsStream,line); // fine here 

   settingsStream>>octaves; // trash here

   getline(settingsStream,line);

   settingsStream>>amplitude;
sample input:
/*Settings for Terrain Rendering, please input values on their respective lines*/
/**/
Frequency:                
24
Octaves:                   
120
Amplitude:                 
1.25

解决方案
First of all, start using std::strings and std::getline.  Secondly, the reason for the trashing is that there's still a newline in the buffer.  If you expect the user to be entering values one line at a time, you should use istream::ignore to skip everything up to and including the next newline character.  For example:
std::string s;
float f;
std::cin >> f;
std::cin.ignore(big_num, '\n');
std::getline(std::cin, s);
What big_num should be depends on your expectations about the input.  If you don't mind writing a lot and want to be on the safe side, use std::numeric_limits<std::streamsize>::max().  If you do mind writing a lot, make a constant of the suitable type that you use everywhere.

For example, the above code will parse the following so that f = 5.0f, s = Hello!.
5
Hello!
However, it will parse the following exactly the same way:
5 Hi!
Hello!
If you want to preserve the Hi!, you shouldn't ignore things, and instead define a proper grammar that you'll use to parse the document.

                        
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