Getting frequencies of words (ASCII)

main.cpp

#include <iostream>
#include <fstream>
#include <string>
#include <functional>
#include <algorithm>
#include <vector>

class dictionary {
public:
    using value_t = std::pair<std::string, size_t>;
    std::function<bool(const value_t&, const value_t&)> cmp = [](const value_t& lhs, const value_t& rhs) -> bool {
        if (lhs.second == rhs.second) {
            return lhs.first < rhs.first;
        }

        return lhs.second > rhs.second;
    };

    void push(const std::string& value) {
        auto found = std::find_if(
            container.begin(),
            container.end(),
            [&value](const value_t& v) {
                return v.first == value; 
            });

        if (found == std::end(container)) {
            container.emplace_back(value, 1);
        } else {
            found->second++;
        }
    }

    void sort() {
        std::sort(container.begin(), container.end(), cmp);
    }

    std::vector<value_t> to_vector() const {
        return container;
    }

private:
    std::vector<value_t> container;
};


void apply_to_words(std::istream& input, const std::function<void(std::string&)>& callback) {
    std::string buffer;
    while (!input.eof()) {
        char c = input.get();

        if (std::isalpha(c)) {
            buffer.push_back(std::tolower(c));
        } else if (!buffer.empty()) {
            callback(buffer);
            buffer.clear();
        }
    }

    if (!buffer.empty()) {
        callback(buffer);
    }
}

std::vector<std::pair<std::string, size_t>> count_words(std::istream& input) {
    dictionary dict;

    apply_to_words(input, [&dict](std::string& word) { dict.push(word); });

    dict.sort();
    return dict.to_vector();
}

int main(int argc, char** argv) {
    if (argc < 3) {
        std::cout << "usage: freq <input file> <output file>" << std::endl;
        return EXIT_FAILURE;
    }

    std::ifstream input(argv[1]);
    if (!input.is_open()) {
        std::cerr << "Cannot open the input file";
        return 1;
    }

    std::ofstream output(argv[2]);
    if (!output.is_open()) {
        std::cerr << "Cannot open the output file";
        return 1;
    }

    auto result = count_words(input);

    return EXIT_SUCCESS;
}