C++ external files

C++_external_files.md

External Files

• a way to persist data on a hard drive
  • where files are stored
• usually programs utilize RAM (main memory) to execute
  • when data is taken in from the user
  • where variables are stored
• output is when we write to a file
  • ofstream
• input is when we read from a file
  • ifstream
• #include <fstream>
  • where input and output stream come from
• both can be used to attach to a file
  • read char by char when outputting

#include <fstream>
using namespace std;

ifstream in;
ofstream out;

in.open("filename.txt"); // connects to a file
out.open("filename.txt"); // can pass in a relative or absolute path

if(in) // check that the connection was successful
{
	// MUST attempt to read in prior to the while loop
	// in order to trigger the .eof() flag
	in.get(struct.array, ARRAY_SIZE, '\n'); // stop at the delimiter
	in.ignore(100, '\n') // skips over the delimiter that we stopped at
	while(in && !in.eof()) // while we exist and have no reached the end of the file
	{
		in.get(struct.name, NAME_SIZE, '\n');
		...
	}
	in.close(); // close ifstream access to the file, releases holds
} else cout << "Error" << endl;
if(out)
{
	out << struct.array << '\n';
	out << struct.name << '\n';
	out.close(); // close hold on file
	out.clear(); // clear out any stuff in buffer?
}

• will typically only reference files that are within root
  • can call by name directly without path
• can pass in a string or array of characters for opening
• if attempt to open a file for output (ofstream)
  • does not have to exist, will create the file
  • if it does exist, will overwrite it
  • unless the file CANNOT be created, will error by returning 0
    • but do not need to clear error in the stream
• if attempt to open a file for input (ifstream)
  • will fail if file does not exist
• out.open(filename, ios::app)
  • when you want to open a file to add to it instead of overwriting it (default behavior)
  • will append to the end automatically
• when writing to a file, make sure to add a known delimiter to differentiate between different data additions
  • entry1, entry2, entry3, entry4 the deliminator is ,
• when reading from a file, ifstream will position the cursor at the start of the file
  • can choose where the cursor is placed
• when reading in from ifstream
  • >> will skip whitespace for character arrays and integers
  • .get() includes whitespace
  • .get(variable, size, deliminator) for more fine-tuning
• in.eof() a flag that gets set when the end of file is reached
  • must first attempt a read to set the flag (must fail first)
• ALWAYS make a read prior to a loop and checking .eof()

for(char ch = in.get(); in && !in.eof(); ch = in.get()) { ... };