8

我收到以下错误：

[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&&)

编译以下代码时：

#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;
}

编译时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

有人可以帮我理解为什么在这种情况下使用复制和交换习语时会出现歧义吗？

4

1 回答 1

11

出于重载解决目的，函数

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

是相等的，因为分别用于转换为 Test 和 Test&& 的隐式转换序列是相等的。前者并不好，因为直接引用绑定也被认为是一种身份转换。

当遇到 2 个同样好的匹配的歧义时，编译器会给出错误。你可能想要这个：

#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 回答