c++是否在需要时将引用隐式转换为值？

Question

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;
}

我不明白这是如何编译的。具体来说，distinct.push_back(c) .如我所见，我正在将引用插入 vector ，而它的类型实际上是 vector<Circle> (不是 vector<Circle&>)

为什么要编译这段代码？引用是否隐式取消引用？我希望在正确的方向上得到一些指导 - 谢谢。

相关的，为什么这不编译:

我已将返回更改为 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;
}

错误信息:

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)
    ^

Answer 1

std::vector::push_back制作其参数的拷贝并将该拷贝保存在 vector 中。

您正在使用的重载签名是:

void push_back(const Circle& value);

你正在通过它 c其类型为 Circle& .这种类型的引用实际上是无关紧要的。它将首先被删除，一个类型为 Circle 的左值被删除。指的是 c 的实例指的是遗迹。那么value将从这个左值初始化。自类型 value是相同类型的左值引用，这意味着 value将绑定(bind)到传递的左值所指的对象。

所以 value将被初始化为引用与 c 相同的对象做过。由于添加了const，类型略有不同, 但添加 const不是问题。

内部push_back将复制 value 的对象指的是通过复制构造并将该新对象保存为新的 vector 元素。存储在 vector 中的对象将是 c 的拷贝引用。

---

您不能在 std::vector 中存储引用, 根本不允许。里面存的东西永远是值类型。引用在 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一个类中，以便引用可以用作值类型，例如保存在 std::vector .

---

向函数传递的是引用类型还是非引用类型的左值并不重要，因为在进行任何进一步分析之前，引用限定符已从类型中删除，并且左值已调整为引用实体引用的引用。

例如如果你有

void f(int a);
void g(int& b);

int c = 42;
int& d = c;

f(c);
f(d);
g(c);
g(d);

两者都是f(c)和 f(d)将初始化 a值为 42和两个g(c)和 g(d)将初始化 b引用变量 c .

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(重载决策除外)。使用引用的名称与使用变量本身的名称完全相同。