flat/flatmaprealloc.cpp

The following code example is taken from the book
C++23 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2026
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <print>
#include <string>
#include <memory>
#include <flat_map>

void printCapaAndAddrOf(const auto& fm, const std::string& key)
{
  std::print("{}\n", fm);
  std::print("      size: {}, capa: {}\n", fm.size(), fm.keys().capacity());

  auto&& posBeg = fm.begin();             // print address of first element (if any)
  if (posBeg != fm.end()) {
    std::print("      begin: {:15}", *posBeg);
    const void* addr = &(posBeg->first);
    std::print("at: {}\n", addr);    
  }

  auto&& posFirst = fm.find(key);         // print element address of passed key (if any)
  if (posFirst != fm.end()) {
    std::print("      {:5}: {:15}", key, *posFirst);
    const void* addr = &(posFirst->first);
    std::print("at: {}\n", addr);    
  }
  std::print("\n");
}

int main()
{
  std::flat_map<std::string, double> fm;
  auto beg0 = fm.begin();
  printCapaAndAddrOf(fm, "");

  const std::string firstKey = "elem0";   // insert an element we later search
  fm.emplace(firstKey, 1);
  auto beg1 = fm.begin();
  printCapaAndAddrOf(fm, firstKey);

  fm.emplace("zzlast", 2);                // insert a new last element
  auto beg2 = fm.begin();
  printCapaAndAddrOf(fm, firstKey);

  fm.emplace("aafirst", 3);               // insert a new first element
  auto beg3 = fm.begin();
  printCapaAndAddrOf(fm, firstKey);

  fm.emplace("ccsecond", 4);              // insert a new second element
  auto beg4 = fm.begin();
  printCapaAndAddrOf(fm, firstKey);

  std::println("beg0 == beg1: {}", beg0 == beg1);
  std::println("beg1 == beg2: {}", beg1 == beg2);
  std::println("beg2 == beg3: {}", beg2 == beg3);
  std::println("beg3 == beg4: {}", beg3 == beg4);
}