Make a file pointer read/write to an in-memory location

Question

I can make a file pointer write to a file with fopen(). But can I make a file pointer that will make it so calling functions such as fputc or fprintf will write to a pointer

Answer 1

In C++ (and you added the C++ tag) you can write a function accepting an arbitrary input/output stream. Since std::stringstream or std::ofstream are derived classes you can pass both of them equally into this function. An example:

#include  // for std::cout
#include  // for std::ofstream
#include  // for std::stringstream

void write_something(std::ostream& stream) {
  stream << "Hello World!" << std::endl;
}

int main() {
  write_something(std::cout); // write it to the screen
  {
    std::ofstream file("myfile.txt");
    write_something(file); // write it into myfile.txt
  }
  {
    std::stringstream stream;
    write_something(stream); // write it into a string
    std::cout << stream.str() << std::endl; // and how to get its content
  }
}

And analogously with std::istream instead of std::ostream if you want to read the data:

void read_into_buffer(std::istream& stream, char* buffer, int length) {
  stream.read(buffer, length);
}

int main() {
  char* buffer = new char[255];

  {
    std::ifstream file("myfile.txt");
    read_into_buffer(file, buffer, 10); // reads 10 bytes from the file
  }
  {
    std::string s("Some very long and useless message and ...");
    std::stringstream stream(s);
    read_into_buffer(stream, buffer, 10); // reads 10 bytes from the string
  }
}