Lifetime of C++ Temporaries

temporaries.cc

// A little exercise to demonstrate the lifetime of temporaries in
// C++. Inspired by the comment in this video
// https://youtu.be/xu7q8dGvuwk?t=3156 at CppCon2017 by Titus Winters.
//
// Two things to note:
//
// - Temporaries (rvalues) can have their lifetimes extended by
//   binding them to a const reference. But the lifetime is extended
//   only to the lifetime of the const reference itself, no more (see
//   the Split(..) function below).
//
// - References to temporaries that have been destructed can cause
//   undefined behavior upon using them to make illegal memory
//   accesses (see the commented out code in the main function).

#include <iostream>
#include <vector>
#include <string>

// a string view
using string_view = std::pair<const char*, const char*>;
std::ostream& operator<<(std::ostream& output, const string_view s) {
  output << std::string(s.first, s.second - s.first);
  return output;
}

// a joiner
template<typename T>
std::string Join(T v) {
  return std::string(v);
}
template<typename T, typename... Trest>
std::string Join(T first, Trest... rest) {
  return first + Join(rest...);
}

// a splitter that returns references into the input string
std::vector<string_view> Split(const std::string& s,
			       const std::string& delimiter) {
  std::vector<string_view> result;
  const char *beg = s.c_str(), *end = s.c_str() + s.size();
  const char* cur = beg;
  auto loc = s.find(delimiter);
  const char* nxt = (loc == std::string::npos) ? end : beg + loc;
  while (cur < end) {
    result.push_back(std::make_pair(cur, nxt));
    cur = nxt + delimiter.size();
    auto loc = s.find(delimiter, cur - beg);
    nxt = (loc == std::string::npos) ? end : beg + loc;
  }
  return result;
}

int main() {
  {
    auto intermediate = Join("fun", " ", "little", " ", "experiment!",
  			      "fun", " ", "little", " ", "experiment!",
  			      "fun", " ", "little", " ", "experiment!");
    auto result = Split(intermediate, " ");
    for (const string_view elements : result) {
      std::cout << elements << std::endl;
    }
  }

  // this causes undefined behavior!
  // {
  //   auto result = Split(Join("fun", " ", "little", " ", "experiment!",
  // 			     "fun", " ", "little", " ", "experiment!",
  // 			     "fun", " ", "little", " ", "experiment!"), " ");
  //   for (const string_view elements : result) {
  //     std::cout << elements << std::endl;
  //   }
  // }
  return 0;
}