Question

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

Answer 1

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.