C++ read the whole file in buffer [duplicate]

Question

What is a good approach to read the whole file content in a buffer for C++?

While in plain C I could use fopen(), fseek(), fread() function combination and read the whole file to a buffer, is it still a good idea to use the same for C++? If yes, then how could I use RAII approach while opening, allocating memory for buffer, reading and reading file content to buffer.

Should I create some wrapper class for the buffer, which deallocates memory (allocated for buffer) in it's destructor, and the same wrapper for file handling?

Hyperplasia

Answer 1

There's no need for wrapper classes for very basic functionality:

std::ifstream file("myfile", std::ios::binary | std::ios::ate);
std::streamsize size = file.tellg();
file.seekg(0, std::ios::beg);

std::vector<char> buffer(size);
if (file.read(buffer.data(), size))
{
    /* worked! */
}

Suspense

Wouldn't uint8_t be better than char? – Bois

read() doesn't return bool. You need to check file.bad() (or in your case file.good()). – Lory

Jamie - read() returns a reference to the ifstream itself. By using it in a boolean context, you implicitly call operator bool(), which returns the same as !fail() => this code is correct – Waldo

You can't rely on tellg() for the file size, AFAICR. – Bookcraft

if the file is binary, that works. If it's text, then yes you cannot. – Subaqueous

The constructor of std::Vector initializes all elements, which is unnecessary. I would expect this solution to be slightly less efficient than the old C equivalent with malloc. – Bridgettebridgewater

He said you do not use tellg() to get the file size. – Janik

@КонстантинВан good point, that's right by the language specification, but in practice this answer would work fine in Unix. Also works in Windows, since the answer uses the binary option. – Larner

Answer 2

You can access the contents of a file with a input file stream std::ifstream, then you can use std::istreambuf_iterator to iterate over the contents of the ifstream,

std::string
getFileContent(const std::string& path)
{
  std::ifstream file(path);
  std::string content((std::istreambuf_iterator<char>(file)), std::istreambuf_iterator<char>());

  return content;
}

In this case im using the iterator to build a new string using the contents of the ifstream, the std::istreambuf_iterator<char>(file) creates an iterator to the begining of the ifstream, and std::istreambuf_iterator<char>() is a default-constructed iterator that indicate the special state "end-of-stream" which you will get when the first iterator reach the end of the contents.

Mignonmignonette

Answer 3

Something I have in most of my programs:

/** Read file into string. */
inline std::string slurp (const std::string& path) {
  std::ostringstream buf; 
  std::ifstream input (path.c_str()); 
  buf << input.rdbuf(); 
  return buf.str();
}

Can be placed in a header.
I think I have found it here: https://mcmap.net/q/55254/-how-do-i-read-an-entire-file-into-a-std-string-in-c

Ghat

Great function name! – Tb

Thanks! The name comes up in Perl. – Ghat