C++20 STL Cookbook/

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Create a Disk Usage Counter

Create a Disk Usage Counter

Learn to create a disk usage counter.

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This is a simple utility that totals the size of every file in a directory and its sub-directories. It runs on both POSIX/Unix and Windows file systems.

How to do it

This recipe is a utility to report the size of every file in a directory and its sub-directories, along with a total. We'll re-use some of the functions we've used elsewhere in this chapter:

  • We start with a few convenience aliases:

namespace fs = std::filesystem;
using dit = fs::directory_iterator;
using de = fs::directory_entry;

  • We also use our format specialization for fs::path objects:

template<>
struct std::formatter<fs::path>: std::formatter<std::string> {
    template<typename FormatContext>
    auto format(const fs::path& p, FormatContext& ctx) {
        return format_to(ctx.out(), "{}", p.string());
    }
};

  • For reporting the size of the directory, we'll use this make_commas() function:

string make_commas(const uintmax_t& num) {
    string s{ std::to_string(num) };
    for(long l = s.length() - 3; l > 0; l -= 3) {
        s.insert(l, ",");
    }
    return s;
}

We've used this before. It inserts a comma before every third character from the end.

  • To sort our directory, we'll need a lowercase string function:

string strlower(string s) {
    auto char_lower = [](const char& c) -> char {
        if(c >= 'A' && c <= 'Z') return c + ('a' –'A');
        else return c;
    };
    std::transform(s.begin(), s.end(), s.begin(),char_lower);
    return s;
}

  • We need a comparison predicate for sorting directory_entry objects by the lowercase of the path name:

bool dircmp_lc(const de& lhs, const de& rhs) {
    const auto lhstr{ lhs.path().string() };
    const auto rhstr{ rhs.path().string() };
    return strlower(lhstr) < strlower(rhstr);
}
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