How to Read Binary Files into a Vector of Unsigned Chars Efficiently?

Efficient Binary File Reading into a Vector of Unsigned Chars

Methods for Reading Binary Files

To efficiently read a binary file into a vector of unsigned chars (std::vector), there are several methods available:

Method 1:

<code class="cpp">std::vector<BYTE> readFile(const char* filename) {
  std::ifstream file(filename, std::ios::binary);
  std::streampos fileSize = file.tellg();
  std::vector<BYTE> fileData(fileSize);
  file.read((char*) &fileData[0], fileSize);
  return fileData;
}</code>

This method opens the file and uses file.seekg/file.tellg to determine its size. It then allocates a vector of the appropriate size and reads the data. However, it requires an explicit cast to char*, which can be inconvenient.

Method 2:

<code class="cpp">std::vector<BYTE> readFile(const char* filename) {
  std::ifstream file(filename, std::ios::binary);
  return std::vector<BYTE>((std::istreambuf_iterator<char>(file)),
                            std::istreambuf_iterator<char>());
}</code>

This method utilizes std::istreambuf_iterator to iterate over the file's characters and construct the vector, but it requires casting to char even though the vector stores unsigned chars.

Method 3:

<code class="cpp">std::vector<BYTE> readFile(const char* filename) {
  std::basic_ifstream<BYTE> file(filename, std::ios::binary);
  return std::vector<BYTE>((std::istreambuf_iterator<BYTE>(file)),
                            std::istreambuf_iterator<BYTE>());
}</code>

This method directly uses a std::basic_ifstream with the desired type, BYTE, avoiding the need for casting.

Behind-the-Scenes Operation:

When using std::istreambuf_iterator, the underlying operations involve reading from the file's internal buffer and converting the char data into the desired type (BYTE in this case). However, due to this conversion process, some performance concerns may arise.

Performance Optimization:

To optimize performance, consider:

• Using std::basic_ifstream for direct type handling.
• Reserving capacity for the vector before reading to avoid multiple resizes.
• Employing std::copy with specialized iterators to efficiently transfer data from the file to the vector.
• Ensuring that the file's binary mode is enabled (std::ios::binary) and newline skipping is disabled (std::noskipws), as these settings can affect binary data reading.

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