C++ read binary file char by char - c++

Question

I have an assignment of saving some objects data in a specific order of its data members, I'll try to simplfy . Consider this Base class constructor from a binary file. (Please note that it was not my choice to use char *) .
Base(ifstream & in_file) {
int n;
in_file.read((char *)&n, sizeof(n));
m_var = new char[n + 1];
in_file.read(m_var, n);
m_var[n] = '\0';
in_file.read((char *)&m_intvar, sizeof(m_intvar));
}
It has to initialize m_var (char *) and another integer variable. This code works, though it requiers to save the length of the char * for me to allocate the memory.
The problem starts here. I was instructed not to save the size of the string, but to only enter a \n after each value i write to the file. So I need some how to read the file, and get the string until the \n character.
I was thinking about reading char by char, but couldn't find a way to do it, I assume there is an istream function that offers that. Some similar function to >> of a text file would also be good I assume.

Answer 1

After consulting cppreference.com I end up as follows:
#include <fstream>
#include <iostream>
#include <cstring>
class Base
{
public:
Base(std::istream & in_file) { // NOTE: changed to istream to allow reading from any stream not just files
in_file.read((char *)&n, sizeof(n));
char buffer[1024];
in_file.get(buffer, sizeof(buffer), '\n');
size_t gcount = in_file.gcount();
if (in_file.get() != '\n')
{
throw std::runtime_error("binary string to long"); // you may want to implement a loop here using peek() to check for newline
}
m_var = new char[gcount];
std::copy(buffer, buffer + gcount, m_var);
}
Base(int num, const char* strg)
: n(num)
, m_var(strdup(strg))
{
}
bool operator == (const Base& rhs)
{
return n == rhs.n && strcpy(m_var, rhs.m_var);
}
void write(std::ostream& out)
{
out.write((char*)&n, sizeof(n));
out.write(m_var, strlen(m_var));
out.write("\n", 1);
}
int n;
char* m_var = nullptr;
~Base()
{
delete m_var;
}
};
int main(int, char**)
{
Base b1(10, "Hello Word!");
{
std::ofstream out("testfile.bin");
b1.write(out);
}
std::ifstream in("testfile.bin");
Base b2(in);
if (b1 == b2)
{
std::cout << "read ok!" << std::endl;
}
else
{
std::cout << "read failed!" << std::endl;
}
return 0;
}

Answer 2

4 years later but I was having a similar problem and found this post.
You can read char by char as you mention, with a loop.
int i;
for(i=0;;i++){
cout<<i;
in_file.read(&buffer[i],sizeof(char));
if buffer[i]=='\n') break;
}
The only problem with the code I came up is it saves the '\n'. But you can and should replace it to the NULL char '\0' after having found the new line '\n'.
(buffer[i]=='\0')
Please correct me if I am mistaken.