RosettaCodeData/Task/Letter-frequency/C++/letter-frequency.cpp

#include <fstream>
#include <iostream>

int main()
{
  std::ifstream input("filename.txt", std::ios_base::binary);
  if (!input)
  {
    std::cerr << "error: can't open file\n";
    return -1;
  }

  size_t count[256];
  std::fill_n(count, 256, 0);

  for (char c; input.get(c); ++count[uint8_t(c)]) // process input file
    ; // empty loop body

  for (size_t i = 0; i < 256; ++i)
  {
    if (count[i] && isgraph(i)) // non-zero counts of printable characters
    {
      std::cout << char(i) << " = " << count[i] << '\n';
    }
  }
}