How do you open a file in C++?

Question

I want to open a file for reading, the C++ way. I need to be able to do it for:

• text files, which would involve some sort of read line function.

• binary files, which would provide a way to read raw data into a char* buffer.

Answer 1

You need to use an ifstream if you just want to read (use an ofstream to write, or an fstream for both).

To open a file in text mode, do the following:

ifstream in("filename.ext", ios_base::in); // the in flag is optional 

To open a file in binary mode, you just need to add the "binary" flag.

ifstream in2("filename2.ext", ios_base::in | ios_base::binary );  

Use the ifstream.read() function to read a block of characters (in binary or text mode). Use the getline() function (it's global) to read an entire line.

Answer 2

There are three ways to do this, depending on your needs. You could use the old-school C way and call fopen/fread/fclose, or you could use the C++ fstream facilities (ifstream/ofstream), or if you're using MFC, use the CFile class, which provides functions to accomplish actual file operations.

All of these are suitable for both text and binary, though none have a specific readline functionality. What you'd most likely do instead in that case is use the fstream classes (fstream.h) and use the stream operators (<< and >>) or the read function to read/write blocks of text:

int nsize = 10; std::vector<char> somedata(nsize); ifstream myfile; myfile.open("<path to file>"); myfile.read(somedata.data(), nsize); myfile.close(); 

Note that, if you're using Visual Studio 2005 or higher, traditional fstream may not be available (there's a new Microsoft implementation, which is slightly different, but accomplishes the same thing).