"And C++ is explicitly a multi-paradigm language so there can be affordances for...

JoeAltmaier · on Dec 26, 2023

Then use such a paradigm; it is explictly made possible.

grumpyprole · on Dec 26, 2023

This is essentially the same argument as "one can write secure software in C++". If it's not enforced by the language and tooling, one can't safely assume anything.

JoeAltmaier · on Dec 26, 2023

Use mechanisms that are safe.

You can write an incorrect program in any language. Some kind of self-discipline is important and necessary.

erik_seaberg · on Dec 26, 2023

We just spent half a century proving that discipline never completely solved the problem. We can’t afford platforms in which a tiny mistake anywhere causes unpredictable failures in unrelated and correct code, and we can certainly afford the lifetime and bounds checks that prevent this.

angiosperm · on Dec 26, 2023

So, use modern features that don't encourage mistakes. Most C bugs come from mistakes with pointers; in good modern C++ you rarely see pointers, and can afford to focus all the attention needed where they do appear.

nuancebydefault · on Dec 26, 2023

I've never seen a C++ codebase that rarely uses pointers. Any good example?

gumby · on Dec 27, 2023

I just glanced at Lohmann-JSON which just uses them for the json elements themselves (there’s a comment that says “raw pointers to save space” and spdlog and fmtlib which appear to use them only to interface to C APIs and *this. That was just a quick glance at some libs we use.

I have seen them used in the PIMPL idiom, which is a rather controlled case.

angiosperm · on Dec 27, 2023

Here is a program that solves the New York Times puzzle "Letter Boxed". Its only uses of pointers are in processing main()'s arguments, which are provided via pointer, and mapping an input file, which Posix provides via a pointer. It does use std::string_view, which has pointers in it; the point is that you don't operate directly on the pointers, so cannot have bugs caused by misusing the pointers.

Try: "g++ -std=c++20 lb.cc && ./a.out iodhuamplrnc words".

  #include <array>
  #include <bit>  // popcount
  #include <iostream>
  #include <utility>
  #include <string_view>
  #include <vector>
  #include <unistd.h>
  #include <sys/mman.h>
  #include <sys/types.h>
  #include <sys/stat.h>
  #include <fcntl.h>
  
  struct Word {
    std::string_view str;
    unsigned bits{};
  
    static unsigned index(unsigned c) { return c - 'a'; };  // 'a'..'z' -> 0..25; others -> >25
    Word(std::string_view s, unsigned accept, long long sides) : str(s) {
      long long prev_side{-1}, side;  //  Values are 0..3. First letter gets a free pass.
      for (char c: s) {
        const auto ix = index(c);
        if (ix < 26 && (accept & (1u << ix)) && (side = ((sides >> 2*ix) & 0x3)) != prev_side) {
          bits |= (1u << ix), prev_side = side;
        } else { str = {}; return; }}}
  };
  
  int main(int ac, char** av) {
    auto usage = [av](){ std::cerr << "usage: " << av[0] << " abcdefghijkl [<wordlist>]\n"; };
    if (!(ac == 2 || ac == 3)) { return usage(), 1; }
    const std::array prefixes = { "", "/usr/share/dict/", "/usr/share/dict/american-english-" };
    const auto name = (ac == 3) ? av[2] : prefixes.back() + std::string{"large"};
    std::string_view file;
    for (auto pfx = 0u; pfx != prefixes.size(); ++pfx) {
      int fd = ::open((prefixes[pfx] + name).c_str(), O_RDONLY);
      std::size_t file_size = ::lseek(fd, 0, SEEK_END);
      void const* addr = ::mmap(nullptr, file_size, PROT_READ, MAP_SHARED, fd, 0);
      if (addr != MAP_FAILED) { file = {static_cast<char const*>(addr), file_size}; break; }
    }
    if (file.empty()) { return std::cerr << av[0] << ": cannot read " << name  << '\n', 3; }
  
    auto target_sum = 0u; auto sides_sum = 0ll;
    for (unsigned i{}, ix{}; i < 12 && (ix = Word::index(av[1][i])) < 26; ++i)
      { target_sum |= (1u << ix), sides_sum |= (i/3ll << 2*ix); }
    if (std::popcount(target_sum) != 12 || av[1][12] != '\0') { return usage(), 3; }
    const auto target = target_sum; const auto sides = sides_sum;
  
    const auto candidates = [target,sides,file] { std::array<std::vector<Word>, 26> candidates;
      for (std::string_view in = file, next; !(next = in.substr(0, in.find('\n'))).empty();
          in.remove_prefix(next.size() + (next.size() < in.size()))) {  // Skip past '\n' if present
        if (const Word word{next, target, sides}; word.str.size() >= 3)
          { candidates.at(Word::index(word.str.front())).push_back(word); }
      }
      return candidates;
    }();
    std::array<std::vector<std::array<std::string_view, 2>>, 100> answers; // radix sort by length
    for (auto const& firsts: candidates) for (const auto first: firsts) {
      for (const auto second: candidates.at(Word::index(first.str.back()))) {
        if ((first.bits | second.bits) == target) {
          answers.at(first.str.size() + second.str.size()).push_back({first.str, second.str}); }}
    }
    for (auto const& of_len_N: answers) for (const auto answer: of_len_N)
      { std::cout << answer[0] << ' ' << answer[1] << '\n'; }
  }

erik_seaberg · on Dec 28, 2023

I don’t have a modern implementation handy, but according to https://en.cppreference.com/w/cpp/string/basic_string_view/d... a string_view depends on its data() pointer, and the lifetime of that buffer doesn’t seem to have guarantees.

angiosperm · on Dec 28, 2023

Thus, the word "view". It refers to storage being managed by the creator of the string_view object, in this case a file mapped using mmap and not unmapped until after program termination.

protomolecule · on Dec 28, 2023

"so cannot have bugs caused by misusing the pointers"

std::string_view lets you have some of the bugs that are caused by misusing pointers.