8

次のエラーが発生します。

[matt ~] g++ -std=c++11 main.cpp -DCOPY_AND_SWAP && ./a.out
main.cpp: In function ‘int main(int, const char* const*)’:
main.cpp:101:24: error: ambiguous overload for ‘operator=’ in ‘move = std::move<Test&>((* & copy))’
main.cpp:101:24: note: candidates are:
main.cpp:39:7: note: Test& Test::operator=(Test)
main.cpp:52:7: note: Test& Test::operator=(Test&&)

次のコードをコンパイルする場合：

#include <iostream>
#include <unordered_map>
class Test final {
public:
  typedef std::unordered_map<std::string, std::string> Map;
public:
  Test();
  explicit Test(Map&& map);
  ~Test();
  Test(const Test& other);
  Test(Test&& test);
#ifdef COPY_AND_SWAP
  Test& operator=(Test other);
#else
  Test& operator=(const Test& other);
#endif
  Test& operator=(Test&& other);
  size_t Size() const noexcept;
  friend void swap(Test& lhs, Test& rhs);
private:
  friend std::ostream& operator<<(std::ostream& stream, const Test& test);
private:
  Map map_;
};
Test::Test() : map_() {
  std::cerr << "Default constructor called" << std::endl;
};
Test::Test(const Test& other) : map_(other.map_) {
  std::cerr << "Copy constructor called" << std::endl;
};
Test::Test(Test&& other) : map_(std::move(other.map_)) {
  std::cerr << "Move constructor called" << std::endl;
};
Test::Test(Map&& map) : map_(std::move(map)) {
  std::cerr << "Map constructor called" << std::endl;
};
Test::~Test() {};
#ifdef COPY_AND_SWAP
Test& Test::operator=(Test other) {
  std::cerr << "Copy and swap assignment called" << std::endl;
  using std::swap;
  swap(this->map_, other.map_);
  return *this;
}
#else
Test& Test::operator=(const Test& other) {
  std::cerr << "Copy assignment called" << std::endl;
  this->map_ = other.map_;
  return *this;
}
#endif
Test& Test::operator=(Test&& other) {
  std::cerr << "Move assignment called" << std::endl;
  this->map_ = other.map_;
  other.map_.clear();
  return *this;
}
size_t Test::Size() const noexcept {
  return map_.size();
}
void swap(Test& lhs, Test& rhs) {
  using std::swap;
  swap(lhs.map_, rhs.map_);
}
std::ostream& operator<<(std::ostream& stream, const Test& test) {
  return stream << test.map_.size();
}
int main (const int argc, const char * const * const argv) {
  using std::swap;
  Test::Map map {
    {"some", "dummy"},
    {"data", "to"},
    {"fill", "up"},
    {"the", "map"}
  };
  std::cout << " map size(): " << map.size() << std::endl;
  std::cout << "Constructing" << std::endl;
  Test test(std::move(map));
  std::cout << " map.size(): " << map.size() << std::endl;
  std::cout << "test.Size(): " << test.Size() << std::endl;
  std::cout << "Copy construction" << std::endl;
  Test copy(test);
  std::cout << "copy.Size(): " << copy.Size() << std::endl;
  std::cout << "Move construction" << std::endl;
  Test move(std::move(copy));
  std::cout << "move.Size(): " << move.Size() << std::endl;
  std::cout << "copy.Size(): " << copy.Size() << std::endl;
  std::cout << "Swapping" << std::endl;
  swap(move, copy);
  std::cout << "move.Size(): " << move.Size() << std::endl;
  std::cout << "copy.Size(): " << copy.Size() << std::endl;
  std::cout << "Swapping back" << std::endl;
  swap(move, copy);
  std::cout << "move.Size(): " << move.Size() << std::endl;
  std::cout << "copy.Size(): " << copy.Size() << std::endl;
  std::cout << "Copy assignment" << std::endl;
  copy = test;
  std::cout << "test.Size(): " << test.Size() << std::endl;
  std::cout << "copy.Size(): " << copy.Size() << std::endl;
  std::cout << "Move assignment" << std::endl;
  move = std::move(copy);
  std::cout << "move.Size(): " << move.Size() << std::endl;
  std::cout << "copy.Size(): " << copy.Size() << std::endl;
 return 0;
}

でコンパイルした場合g++ -std=c++11 main.cpp && ./a.out：

[matt ~] g++ -std=c++11 main.cpp && ./a.out
 map size(): 4
Constructing
Map constructor called
 map.size(): 0
test.Size(): 4
Copy construction
Copy constructor called
copy.Size(): 4
Move construction
Move constructor called
move.Size(): 4
copy.Size(): 0
Swapping
move.Size(): 0
copy.Size(): 4
Swapping back
move.Size(): 4
copy.Size(): 0
Copy assignment
Copy assignment called
test.Size(): 4
copy.Size(): 4
Move assignment
Move assignment called
move.Size(): 4
copy.Size(): 0

この場合、コピーとスワップのイディオムを使用すると、あいまいさが発生する理由を誰かが理解するのを手伝ってもらえますか？

4

1 に答える 1

11

過負荷解決の目的で、関数

Test& operator=(Test other);
Test& operator=(Test&& other);

TestとTest&&にそれぞれ変換するために使用される暗黙の変換シーケンスが等しいため、は等しくなります。直接参照バインディングもID変換と見なされるため、前者は適切ではありません。

2つの同等に適切な一致のあいまいさに直面すると、コンパイラーはエラーを出します。あなたはおそらくこれが欲しいでしょう：

#ifdef COPY_AND_SWAP
  Test& operator=(Test other);
#else
  Test& operator=(const Test& other);
  Test& operator=(Test&& other);
#endif
于 2013-01-24T14:50:51.017 に答える