lang/add.cpp

The following code example is taken from the book
C++20 - The Complete Guide by Nicolai M. Josuttis, Leanpub, 2021
The code is licensed under a Creative Commons Attribution 4.0 International License.

---

// raw code

#include <iostream>
#include <vector>
#include <set>
#include <ranges>
#include <atomic>

// concept for container with push_back():
template<typename Coll>
concept SupportsPushBack = requires(Coll coll, Coll::value_type val) {
  coll.push_back(val);
};

// concept to disable narrowing conversions:
template<typename From, typename To>
concept ConvertsWithoutNarrowing =
    std::convertible_to<From, To> &&
    requires (From&& x) {
      { std::type_identity_t<To[]>{std::forward<From>(x)} }
          -> std::same_as<To[1]>;
    };


// add() for single value:
template<typename Coll, typename T>
requires ConvertsWithoutNarrowing<T, typename Coll::value_type>
void add(Coll& coll, const T& val)
{
  if constexpr (SupportsPushBack<Coll>) {
    coll.push_back(val);
  }
  else {
    coll.insert(val);
  }
}

// add() for multiple values:
template<typename Coll, std::ranges::input_range T>
requires ConvertsWithoutNarrowing<std::ranges::range_value_t<T>,
                                  typename Coll::value_type>
void add(Coll& coll, const T& val)
{
  if constexpr (SupportsPushBack<Coll>) {
    coll.insert(coll.end(),
                std::ranges::begin(val), std::ranges::end(val));
  }
  else {
    coll.insert(std::ranges::begin(val), std::ranges::end(val));
  }
}

int main()
{
  std::vector<int> iVec;
  add(iVec, 42);      // OK: calls push_back() for T being int

  std::set<int> iSet;
  add(iSet, 42);      // OK: calls insert() for T being int
  
  short s = 42;
  add(iVec, s);       // OK: calls push_back() for T being short
  
  long long ll = 42;
  //add(iVec, ll); // ERROR: narrowing
  //add(iVec, 7.7); // ERROR: narrowing
  
  std::vector<double> dVec;
  add(dVec, 0.7);      // OK: calls push_back() for floating-point types
  add(dVec, 0.7f);     // OK: calls push_back() for floating-point types
  //add(dVec, 7); // ERROR: narrowing

  // insert collections:
  add(iVec, iSet);     // OK: insert set elements into a vector
  add(iSet, iVec);     // OK: insert vector elements into a set

  // can even insert raw array:
  int vals[] = {0, 8, 18};
  add(iVec, vals);     // OK
  //add(dVec, vals); // ERROR: narrowing
}