https://www.reddit.com/r/cpp_questions/comments/2xlzcc/can_someone_please_eli5_how_stdmap_works_in_the/

main.cpp

#include <vector>
#include <iostream>
#include <fstream>

std::vector<unsigned> calculate_frequencies(std::istream& stream) {
	auto frequencies = std::vector<unsigned>(0x100);
	auto c = char{};
	while (stream.get(c)) {
		if(!std::isspace(c) and !std::ispunct(c)) {
			++frequencies.at(static_cast<unsigned char>(c));
		}
	}
	return frequencies;
}

int main(int argc, char** argv) {
	if (argc != 2) {
		std::cerr << "Usage: " << argv[0] << " <input-file>\n";
		return 1;
	}
	
	std::ifstream input{argv[1]};
	if(!input.is_open()) {
		std::cerr << "Could not open File " << argv[1] << '\n';
		return 2;
	}
	
	const auto frequencies = calculate_frequencies(input);
	
	std::ofstream output{"Freq.dat"};
	if(!output.is_open()) {
		std::cerr << "Could not open output (Freq.dat)\n";
		return 3;
	}
	
	for (auto i = std::size_t{}; i < 0x100; ++i) {
		if (frequencies.at(i)) {
			output << i << "  " << static_cast<char>(i) << "  " << frequencies.at(i) << '\n';
		}
	}
}