c++是否在需要时隐式转换为值？
c++是否在需要时隐式转换为值？
提问于 2019-11-26 23:24:39
我有以下代码：
#include <stdio.h>
#include <math.h>
#include <array>
#include <unordered_set>
using namespace std;
class Circle {
public:
int x;
int y;
int r;
Circle() : x(0), y(0), r(0) {}
Circle(int x, int y, int r) {
this->x = x;
this->y = y;
this->r = r;
}
double area() {
return PI * pow(r, 2);
}
};
vector <Circle> filter_circles(vector <Circle> &circles) {
auto stringify = [](const pair<int, int> &p, string sep = "-") -> string {
return to_string(p.first) + sep + to_string(p.second);
};
unordered_set <string> circles_seen;
vector <Circle> distinct;
for (auto &c: circles) {
auto centre = stringify(make_pair(c.x, c.y));
if (circles_seen.find(centre) == circles_seen.end()) {
distinct.push_back(c);
} else {
circles_seen.insert(centre);
}
}
return distinct;
}我不明白这是怎么编出来的。具体来说，distinct.push_back(c).在我看来，当它的类型实际上是vector<Circle> (而不是vector<Circle&>)时，我将引用推到向量中。
为什么这段代码要编译？引用是否隐式取消引用？我想要一些正确方向的指导--谢谢。
与此相关，为什么这个不编译：
我已经将返回更改为vector<Circle&>
vector <Circle&> filter_circles(vector <Circle> &circles) {
auto stringify = [](const pair<int, int> &p, string sep = "-") -> string {
return to_string(p.first) + sep + to_string(p.second);
};
unordered_set <string> circles_seen;
vector <Circle> distinct;
for (auto &c: circles) {
auto centre = stringify(make_pair(c.x, c.y));
if (circles_seen.find(centre) == circles_seen.end()) {
distinct.push_back(c);
} else {
circles_seen.insert(centre);
}
}
return distinct;
}错误消息：
Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1785:16: error: 'pointer' declared as a pointer to a reference of type 'Circle &'
typedef _Tp* pointer;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1514:22: note: in instantiation of template class 'std::__1::allocator<Circle &>' requested here
typedef typename allocator_type::value_type value_type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:326:22: note: in instantiation of template class 'std::__1::allocator_traits<std::__1::allocator<Circle &> >' requested here
typedef typename __alloc_traits::size_type size_type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:465:15: note: in instantiation of template class 'std::__1::__vector_base<Circle &, std::__1::allocator<Circle &> >' requested here
: private __vector_base<_Tp, _Allocator>
^
dartboard_detector.cpp:57:18: note: in instantiation of template class 'std::__1::vector<Circle &, std::__1::allocator<Circle &> >' requested here
vector <Circle&> filter_circles(vector <Circle> &circles) {
^
In file included from dartboard_detector.cpp:2:
In file included from /usr/local/Cellar/opencv@2/2.4.13.7_5/include/opencv/cv.h:64:
In file included from /usr/local/Cellar/opencv@2/2.4.13.7_5/include/opencv2/core/core.hpp:53:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:644:
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1786:22: error: 'const_pointer' declared as a pointer to a reference of type 'Circle &'
typedef const _Tp* const_pointer;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1805:45: error: multiple overloads of 'address' instantiate to the same signature 'std::__1::allocator<Circle &>::const_pointer
(std::__1::allocator<Circle &>::const_reference) const noexcept' (aka 'int (Circle &) const noexcept')
_LIBCPP_INLINE_VISIBILITY const_pointer address(const_reference __x) const _NOEXCEPT
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1803:39: note: previous declaration is here
_LIBCPP_INLINE_VISIBILITY pointer address(reference __x) const _NOEXCEPT
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:951:22: error: implicit instantiation of undefined template 'std::__1::__pointer_traits_element_type<int, false>'
typedef typename __pointer_traits_element_type<pointer>::type element_type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1073:22: note: in instantiation of template class 'std::__1::pointer_traits<int>' requested here
typedef typename pointer_traits<_Ptr>::template rebind<void> type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1518:22: note: in instantiation of template class 'std::__1::__void_pointer<int, std::__1::allocator<Circle &>, false>' requested here
typedef typename __void_pointer<pointer, allocator_type>::type void_pointer;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:326:22: note: in instantiation of template class 'std::__1::allocator_traits<std::__1::allocator<Circle &> >' requested here
typedef typename __alloc_traits::size_type size_type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:465:15: note: in instantiation of template class 'std::__1::__vector_base<Circle &, std::__1::allocator<Circle &> >' requested here
: private __vector_base<_Tp, _Allocator>
^
dartboard_detector.cpp:57:18: note: in instantiation of template class 'std::__1::vector<Circle &, std::__1::allocator<Circle &> >' requested here
vector <Circle&> filter_circles(vector <Circle> &circles) {
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:741:8: note: template is declared here
struct __pointer_traits_element_type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1073:53: error: no member named 'rebind' in 'std::__1::pointer_traits<int>'
typedef typename pointer_traits<_Ptr>::template rebind<void> type;
~~~~~~~~~~~~~~~~~~~~~~ ^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1518:22: note: in instantiation of template class 'std::__1::__void_pointer<int, std::__1::allocator<Circle &>, false>' requested here
typedef typename __void_pointer<pointer, allocator_type>::type void_pointer;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:326:22: note: in instantiation of template class 'std::__1::allocator_traits<std::__1::allocator<Circle &> >' requested here
typedef typename __alloc_traits::size_type size_type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:465:15: note: in instantiation of template class 'std::__1::__vector_base<Circle &, std::__1::allocator<Circle &> >' requested here
: private __vector_base<_Tp, _Allocator>
^
dartboard_detector.cpp:57:18: note: in instantiation of template class 'std::__1::vector<Circle &, std::__1::allocator<Circle &> >' requested here
vector <Circle&> filter_circles(vector <Circle> &circles) {
^
In file included from dartboard_detector.cpp:2:
In file included from /usr/local/Cellar/opencv@2/2.4.13.7_5/include/opencv/cv.h:64:
In file included from /usr/local/Cellar/opencv@2/2.4.13.7_5/include/opencv2/core/core.hpp:53:
In file included from /Library/Developer/CommandLineTools/usr/include/c++/v1/algorithm:644:
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:851:22: error: type 'int' cannot be used prior to '::' because it has no members
typedef typename _Tp::template rebind<_Up> type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:955:26: note: in instantiation of template class 'std::__1::__pointer_traits_rebind<int, const void, false>' requested here
template <class _Up> using rebind = typename __pointer_traits_rebind<pointer, _Up>::type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1096:53: note: in instantiation of template type alias 'rebind' requested here
typedef typename pointer_traits<_Ptr>::template rebind<const void> type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/memory:1519:22: note: in instantiation of template class 'std::__1::__const_void_pointer<int, std::__1::allocator<Circle &>, false>' requested here
typedef typename __const_void_pointer<pointer, allocator_type>::type const_void_pointer;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:326:22: note: in instantiation of template class 'std::__1::allocator_traits<std::__1::allocator<Circle &> >' requested here
typedef typename __alloc_traits::size_type size_type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:465:15: note: in instantiation of template class 'std::__1::__vector_base<Circle &, std::__1::allocator<Circle &> >' requested here
: private __vector_base<_Tp, _Allocator>
^
dartboard_detector.cpp:57:18: note: in instantiation of template class 'std::__1::vector<Circle &, std::__1::allocator<Circle &> >' requested here
vector <Circle&> filter_circles(vector <Circle> &circles) {
^
In file included from dartboard_detector.cpp:2:
In file included from /usr/local/Cellar/opencv@2/2.4.13.7_5/include/opencv/cv.h:64:
In file included from /usr/local/Cellar/opencv@2/2.4.13.7_5/include/opencv2/core/core.hpp:60:
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:475:30: error: 'reference' is a protected member of 'std::__1::__vector_base<Circle &, std::__1::allocator<Circle &> >'
typedef typename __base::reference reference;
^
dartboard_detector.cpp:57:18: note: in instantiation of template class 'std::__1::vector<Circle &, std::__1::allocator<Circle &> >' requested here
vector <Circle&> filter_circles(vector <Circle> &circles) {
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:329:54: note: declared protected here
typedef value_type& reference;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:476:30: error: 'const_reference' is a protected member of 'std::__1::__vector_base<Circle &, std::__1::allocator<Circle &> >'
typedef typename __base::const_reference const_reference;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:330:54: note: declared protected here
typedef const value_type& const_reference;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:478:30: error: 'difference_type' is a protected member of 'std::__1::__vector_base<Circle &, std::__1::allocator<Circle &> >'
typedef typename __base::difference_type difference_type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:331:54: note: declared protected here
typedef typename __alloc_traits::difference_type difference_type;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:479:30: error: 'pointer' is a protected member of 'std::__1::__vector_base<Circle &, std::__1::allocator<Circle &> >'
typedef typename __base::pointer pointer;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:332:54: note: declared protected here
typedef typename __alloc_traits::pointer pointer;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:480:30: error: 'const_pointer' is a protected member of 'std::__1::__vector_base<Circle &, std::__1::allocator<Circle &> >'
typedef typename __base::const_pointer const_pointer;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:333:54: note: declared protected here
typedef typename __alloc_traits::const_pointer const_pointer;
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:688:15: error: 'data' declared as a pointer to a reference of type 'std::__1::vector<Circle &, std::__1::allocator<Circle &> >::value_type' (aka 'Circle &')
value_type* data() _NOEXCEPT
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:691:21: error: 'data' declared as a pointer to a reference of type 'std::__1::vector<Circle &, std::__1::allocator<Circle &> >::value_type' (aka 'Circle &')
const value_type* data() const _NOEXCEPT
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:708:36: error: multiple overloads of 'push_back' instantiate to the same signature 'void (Circle &)'
_LIBCPP_INLINE_VISIBILITY void push_back(value_type&& __x);
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:705:36: note: previous declaration is here
_LIBCPP_INLINE_VISIBILITY void push_back(const_reference __x);
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:725:14: error: multiple overloads of 'insert' instantiate to the same signature 'std::__1::vector<Circle &, std::__1::allocator<Circle &> >::iterator
(std::__1::vector<Circle &, std::__1::allocator<Circle &> >::const_iterator, Circle &)' (aka '__wrap_iter<int> (__wrap_iter<int>, Circle &)')
iterator insert(const_iterator __position, value_type&& __x);
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:722:14: note: previous declaration is here
iterator insert(const_iterator __position, const_reference __x);
^
dartboard_detector.cpp:72:12: error: no viable conversion from returned value of type 'vector<Circle>' to function return type 'vector<Circle &>'
return distinct;
^~~~~~~~
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:551:5: note: candidate constructor not viable: no known conversion from 'vector<Circle>' to 'const std::__1::vector<Circle &, std::__1::allocator<Circle &> > &' for 1st
argument
vector(const vector& __x);
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:558:5: note: candidate constructor not viable: no known conversion from 'vector<Circle>' to 'initializer_list<std::__1::vector<Circle &, std::__1::allocator<Circle &>
>::value_type>' (aka 'initializer_list<Circle &>') for 1st argument
vector(initializer_list<value_type> __il);
^
/Library/Developer/CommandLineTools/usr/include/c++/v1/vector:564:5: note: candidate constructor not viable: no known conversion from 'vector<Circle>' to 'std::__1::vector<Circle &, std::__1::allocator<Circle &> > &&' for 1st
argument
vector(vector&& __x)
^回答 3
Stack Overflow用户
发布于 2019-11-26 23:50:10
std::vector::push_back复制其参数并将其保存在向量中。
您正在使用的重载的签名是：
void push_back(const Circle& value);传递给它的是c，它的类型是Circle&。这种类型中的引用实际上是不相关的。它将首先被移除，类型为Circle的lvalue引用c所指的剩余实例。然后将从这个lvalue初始化value。由于value的类型是对同一类型的lvalue引用，这意味着value将绑定到传递的lvalue引用的对象。
因此，value将被初始化为引用与c相同的对象。由于添加了const，类型略有不同，但是添加const不是问题。
在内部，push_back将通过复制构造创建value引用的对象的副本，并将新对象保存为新的向量元素。存储在向量中的对象将是c引用的对象的副本。
不能将引用存储在std::vector中，这是完全不允许的。它中存储的东西始终是值类型。参考资料在C++中是特殊的。它们遵循与非引用类型不同的规则。有std::reference_wrapper，它允许将引用包装在类中，以便引用可以用作值类型，例如保存在std::vector中。
无论您是传递一个函数，引用类型还是非引用类型的lvalue都不重要，因为引用限定符在任何进一步分析之前都会从该类型中删除，并且lvalue将被调整为引用引用所引用的实体。
如果你有
void f(int a);
void g(int& b);
int c = 42;
int& d = c;
f(c);
f(d);
g(c);
g(d);f(c)和f(d)都将使用值42初始化a，而g(c)和g(d)都将通过引用变量c初始化b。
您是按值传递还是按引用传递取决于函数的签名，而不是调用站点(重载解析除外)。使用引用的名称与使用变量本身的名称完全相同。
Stack Overflow用户
发布于 2019-11-26 23:29:50
我不明白这是如何编译的。具体来说，distinct.push_back(c)。在我看来，当它的类型实际上是向量(而不是向量)时，我将引用推到向量中。
你对这种类型的理解是不正确的。你不能把引用推到向量中。
for (auto &c: circles)意味着c是对向量circles的单个元素的引用，即c是对Circle对象的引用。
vector <Circle> distinct;
for (auto &c: circles) {
auto centre = stringify(make_pair(c.x, c.y));
if (circles_seen.find(centre) == circles_seen.end()) {
distinct.push_back(c);
} else {
circles_seen.insert(centre);
}
}Stack Overflow用户
发布于 2019-11-27 00:12:47
看来你误解了推荐信。如果一个变量是一个引用，那只是它的初始化相关的insofaras :它必须绑定到一个已经存在的对象。然后是该对象的引用名的名称。而非引用变量则创建对象和该对象的变量名的名称。
初始化后不再有任何区别:我们有一个对象，并且该对象有一个名称。(对象可以有多个名称，也可以没有名称)。
当您编写distinct.push_back(c)时，它意味着推送名为c的对象。在创建c名称之前，推送不受该对象是否存在的影响。
你问题的最后一部分是一个单独的问题。有两个问题:第一，vector of references is not allowed，即使是这样，您也可以使用带有对象向量的return语句，对于任何已经不相同的X和Y，从vector<X>到vector<Y>的隐式转换都是不存在的。
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/59060864
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