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[英]Should the Copy-and-Swap Idiom become the Copy-and-Move Idiom in C++11?
[英]ambiguous overload for ‘operator=’ with c++11 std::move and copy and swap idiom
我收到以下錯誤:
[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&&)
編譯以下代碼時:
#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;
}
使用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
有人可以幫助我理解為什么在這種情況下使用復制和交換習語時會出現歧義嗎?
用於過載解決目的的功能
Test& operator=(Test other);
Test& operator=(Test&& other);
是相等的,因為用於分別轉換為Test和Test &&的隱式轉換序列是相等的。 前者並不是更好,因為直接引用綁定也被認為是身份轉換。
當面對2個同樣好的匹配的模糊性時,編譯器會給出錯誤。 你可能想要這個:
#ifdef COPY_AND_SWAP
Test& operator=(Test other);
#else
Test& operator=(const Test& other);
Test& operator=(Test&& other);
#endif
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