C++ cin char read symbol-by-symbol

Question

I need to read symbol-by-symbol. But I don't know how to read until end of input. As exemple test system will cin>>somecharvariable m times. I have to read symbol-by-symbol all characters. Only m times. How I can do it?

Answer 1

If you want formatted input character-by-character, do this:

char c;
while (infile >> c)
{
  // process character c
}

If you want to read raw bytes, do this:

char b;
while (infile.get(b))
// while(infile.read(&b, 1)   // alternative, compare and profile
{
  // process byte b
}

In either case, infile should be of type std::istream & or similar, such as a file or std::cin.

Answer 2

There are several ways to read one character at a time until you have read them all, and none of them is necessarily the best.

Personally, I’d go with the following code:

char c;
while (cin.get(c)) {
    // Process c here.
}

If you only need to read m characters, consider using a for loop:

char c;
for (unsigned int i = 0; i < m && cin.get(c); ++i) {
    // Process c here.
}

This runs the loop as long as two conditions are fulfilled: (1) less than m characters have been read, and (2) there are still characters to read.

However, both solutions have a drawback: they are relatively inefficient. It’s more efficient to read the m characters in one go.

So first allocate a big enough buffer to store m chars and then attempt to read them:

std::vector<char> buffer(m);
cin.read(&m[0], m);
unsigned total_read = cin.gcount();

Notice the last line – this will tell you whether m characters have been actually read.