C++ T&&；（双符号）以C+表示的平均值+；11?
我一直在研究C++11的一些新特性，我注意到其中一个是在声明变量时使用了双符号，如C++ T&&；（双符号）以C+表示的平均值+；11?,c++,c++11,rvalue-reference,c++-faq,perfect-forwarding,C++,C++11,Rvalue Reference,C++ Faq,Perfect Forwarding,我一直在研究C++11的一些新特性，我注意到其中一个是在声明变量时使用了双符号，如T&&var 首先，这个野兽叫什么？我希望谷歌能允许我们搜索这样的标点符号 这到底是什么意思 乍一看，它似乎是一个双重引用（就像C风格的双指针T**var），但我很难想到它的用例。它声明了一个（标准提案文档） 下面是对右值的介绍 下面是微软标准库之一的右值引用的精彩深入介绍 注意：有关MSDN的链接文章（“VC10中的右值引用：C++0x功能，第2部分”）非常清楚地介绍了右值引用，但对右值引用做了一些陈述，这些陈述
T&&varT**varT&& r = T();
foo(foo const& other)
{
this->length = other.length;
this->ptr = new int[other.length];
copy(other.ptr, other.ptr + other.length, this->ptr);
}
template <typename T, typename A1>
std::unique_ptr<T> factory(A1&& a1)
{
return std::unique_ptr<T>(new T(std::forward<A1>(a1)));
}
foo(foo&& other)
{
this->length = other.length;
this->ptr = other.ptr;
other.length = 0;
other.ptr = nullptr;
}
std:：movef1f2foo f1((foo())); // Move a temporary into f1; temporary becomes "empty"
foo f2 = std::move(f1); // Move f1 into f2; f1 is now "empty"
template <typename T, typename A1>
std::unique_ptr<T> factory(A1& a1)
{
return std::unique_ptr<T>(new T(a1));
}
TTTT&&TAA&auto p1 = factory<foo>(foo()); // calls foo(foo&&)
auto p2 = factory<foo>(*p1); // calls foo(foo const&)
auto p1=factory（foo（））；//调用foo（foo&&）
自动p2=工厂（*p1）；//调用foo（foo const&）
- 对于重载解析，左值比左值引用更喜欢绑定，右值比右值引用更喜欢绑定。因此，为什么临时人员更喜欢调用移动构造函数/移动赋值操作符而不是复制构造函数/赋值操作符
- 右值引用将隐式绑定到右值和隐式转换结果的临时值。i、 e.
float f=0f；int&&i=f格式良好，因为float可以隐式转换为int；该引用将是转换结果的临时引用 - 命名的右值引用是左值。未命名的右值引用是右值。这对于理解为什么在：
foo&&r=foo（）中需要调用
非常重要；foo f=std:：move（r）std:：move
std::string s;
std::string another(s); // calls std::string(const std::string&);
std::string more(std::string(s)); // calls std::string(const std::string&);
std::string s;
std::string another(s); // calls std::string(const std::string&);
std::string more(std::string(s)); // calls std::string(std::string&&);
std::string s;
std::string& ref = s;
(std::string&)& anotherref = ref; // usually expressed via template
(std::string&)&& ref // ref is std::string&
(const std::string&)&& ref // ref is const std::string&
(std::string&&)&& ref // ref is std::string&&
(const std::string&&)&& ref // ref is const std::string&&
template<typename A, typename B> auto min(A&& aref, B&& bref) {
// for example, if you pass a const std::string& as first argument,
// then A becomes const std::string& and by extension, aref becomes
// const std::string&, completely maintaining it's type information.
if (std::forward<A>(aref) < std::forward<B>(bref))
return std::forward<A>(aref);
else
return std::forward<B>(bref);
}
// template
template<class T> foo(T&& t) { ... }
// auto
auto&& t = ...;
// typedef
typedef ... T;
T&& t = ...;
// decltype
decltype(...)&& t = ...;
void foo(X& x); // lvalue reference overload
void foo(X&& x); // rvalue reference overload
X x;
X foobar();
foo(x); // argument is lvalue: calls foo(X&)
foo(foobar()); // argument is rvalue: calls foo(X&&)
#include <cstring>
class Sample {
int *ptr; // large block of memory
int size;
public:
Sample(int sz=0) : ptr{sz != 0 ? new int[sz] : nullptr}, size{sz}
{
if (ptr != nullptr) memset(ptr, 0, sz);
}
// copy constructor that takes lvalue
Sample(const Sample& s) : ptr{s.size != 0 ? new int[s.size] :\
nullptr}, size{s.size}
{
if (ptr != nullptr) memcpy(ptr, s.ptr, s.size);
std::cout << "copy constructor called on lvalue\n";
}
// move constructor that take rvalue
Sample(Sample&& s)
{ // steal s's resources
ptr = s.ptr;
size = s.size;
s.ptr = nullptr; // destructive write
s.size = 0;
cout << "Move constructor called on rvalue." << std::endl;
}
// normal copy assignment operator taking lvalue
Sample& operator=(const Sample& s)
{
if(this != &s) {
delete [] ptr; // free current pointer
size = s.size;
if (size != 0) {
ptr = new int[s.size];
memcpy(ptr, s.ptr, s.size);
} else
ptr = nullptr;
}
cout << "Copy Assignment called on lvalue." << std::endl;
return *this;
}
// overloaded move assignment operator taking rvalue
Sample& operator=(Sample&& lhs)
{
if(this != &s) {
delete [] ptr; //don't let ptr be orphaned
ptr = lhs.ptr; //but now "steal" lhs, don't clone it.
size = lhs.size;
lhs.ptr = nullptr; // lhs's new "stolen" state
lhs.size = 0;
}
cout << "Move Assignment called on rvalue" << std::endl;
return *this;
}
//...snip
};
template<class T> class unique_ptr {
//...snip
unique_ptr(unique_ptr&& __u) noexcept; // move constructor
};
std::unique_ptr<int[] pt1{new int[10]};
std::unique_ptr<int[]> ptr2{ptr1};// compile error: no copy ctor.
// So we must first cast ptr1 to an rvalue
std::unique_ptr<int[]> ptr2{std::move(ptr1)};
std::unique_ptr<int[]> TakeOwnershipAndAlter(std::unique_ptr<int[]> param,\
int size)
{
for (auto i = 0; i < size; ++i) {
param[i] += 10;
}
return param; // implicitly calls unique_ptr(unique_ptr&&)
}
// Now use function
unique_ptr<int[]> ptr{new int[10]};
// first cast ptr from lvalue to rvalue
unique_ptr<int[]> new_owner = TakeOwnershipAndAlter(\
static_cast<unique_ptr<int[]>&&>(ptr), 10);
cout << "output:\n";
for(auto i = 0; i< 10; ++i) {
cout << new_owner[i] << ", ";
}
output:
10, 10, 10, 10, 10, 10, 10, 10, 10, 10,
// first cast ptr from lvalue to rvalue
unique_ptr<int[]> new_owner = TakeOwnershipAndAlter(std::move(ptr),0);