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一文詳解C++17中的結(jié)構(gòu)化綁定

更新時(shí)間：2023年12月03日 15:17:51 作者：fengbingchun

C++17中的結(jié)構(gòu)化綁定(structured binding)是指將指定名稱綁定到初始化程序的子對(duì)象或元素,本文主要來(lái)和大家聊聊C++17中結(jié)構(gòu)化綁定的實(shí)現(xiàn),感興趣的小伙伴可以了解下

C++17中的結(jié)構(gòu)化綁定(structured binding):將指定名稱綁定到初始化程序的子對(duì)象或元素。簡(jiǎn)而言之，它們使我們能夠從元組或結(jié)構(gòu)中聲明多個(gè)變量。與引用一樣，結(jié)構(gòu)化綁定是現(xiàn)有對(duì)象的別名；與引用不同，結(jié)構(gòu)化綁定不必是引用類型(reference type)。

C++17中結(jié)構(gòu)化綁定的主要目的是使代碼干凈且易于理解。結(jié)構(gòu)化綁定允許你在單個(gè)聲明中將多個(gè)變量綁定到結(jié)構(gòu)化對(duì)象的元素，例如std::tuple或struct。

每個(gè)結(jié)構(gòu)化綁定都有一個(gè)引用類型,該類型是decltype在應(yīng)用于無(wú)括號(hào)的結(jié)構(gòu)化綁定時(shí)返回的類型:E表示初始化表達(dá)式的類型

(1).binding an array:標(biāo)識(shí)符的數(shù)量必須等于數(shù)組元素的數(shù)量;每個(gè)標(biāo)識(shí)符的引用類型是數(shù)組元素類型; 注意：如果數(shù)組類型是cv限定的，那么它的元素類型也是cv限定的

int arr1[3] = { 1, 2, 3 };
 
auto [x, y, z] = arr1; // 拷貝
std::cout << "x:" << x << ",y:" << y << ",z:" << z << "\n"; // x:1,y:2,z:3
x = 4;
std::cout << "arr1:" << arr1[0] << "\n"; // arr1:1
 
auto& [x2, y2, z2] = arr1; // 引用
std::cout << "x2:" << x2 << ",y2:" << y2 << ",z2:" << z2 << "\n"; // x2:1,y2:2,z2:3
x2 = 5;
std::cout << "arr1:" << arr1[0] << "\n"; // arr1:5
 
const auto& [x3, y3, z3] {arr1};
const auto& [x4, y4, z4](arr1);
std::cout << "x3:" << x3 << ",x4:" << x4 << "\n"; // x3:5,x4:5

(2).binding a tuple-like type:表達(dá)式std::tuple_size<E>::value必須是格式正確的整型常量表達(dá)式，并且標(biāo)識(shí)符的數(shù)量必須等于std::tuple_size<E>::value

std::tuple<std::string, int, float> foo{ "csdn", 8, 6.6 };
 
const auto& [x3, y3, z3] = foo;
std::cout << "x3:" << x3 << ",y3:" << y3 << ",z3:" << z3 << "\n"; // x3:csdn,y3:8,z3:6.6
 
auto& [x4, y4, z4] = foo;
x4 = "github";
y4 = 6;
z4 = 8.8;
std::cout << "foo:" << std::get<0>(foo) << "," << std::get<1>(foo) << "," << std::get<2>(foo) << "\n"; // foo:github,6,8.8
 
int a = 1, b = 2;
const auto& [x6, y6] = std::tie(a, b); // x6 and y6 are of type int&
x6 = 6;
auto [x7, y7] = std::tie(a, b); // x7 and y7 are still of type int&
std::cout << "x7:" << x7 << ",a:" << a << "\n"; // x7:6,a:6
x7 = 8;
std::cout << "x6:" << x6 << ",a:" << a << "\n"; // x6:8,a:8
if (&x6 != &x7)
    std::cout << "Error: &x6 != &x7\n"; // 不會(huì)執(zhí)行

(3).binding to data members:標(biāo)識(shí)符的數(shù)量必須等于非靜態(tài)數(shù)據(jù)成員的數(shù)量

namespace {
 
typedef struct Info {
    mutable std::string name;
    volatile int number;
} Info;
 
inline Info func()
{
    return Info{ "csdn", 666 };
}
 
typedef struct Data {
    int b{ 1 }, d{ 2 }, p{ 3 }, q{ 4 };
} Data;
 
} // namespace
 
const auto [x5, y5] = func();
std::cout << "x5:" << x5 << ",y5:" << y5 << "\n"; // x5:csdn,y5:666
x5 = "github";
//y5 = 888; //表達(dá)式必須是可修改的左值,需將const auto[x5, y5]調(diào)整為auto[x5, y5]
 
const auto [b1, d1, p1, q1] = Data{};
std::cout << "b1:" << b1 << ",d1:" << d1 << ",p1:" << p1 << ",q1:" << q1 << "\n"; // b1:1,d1:2,p1:3,q1:4
 
const auto [b2, d2, p2, q2] = Data{ 4, 3, 2, 1 };
std::cout << "b2:" << b2 << ",d2:" << d2 << ",p2:" << p2 << ",q2:" << q2 << "\n"; // b2:4,d2:3,p2:2,q2:1
 
Data s;
auto& [b3, d3, p3, q3] = s;
std::cout << "b3:" << b3 << ",d3:" << d3 << ",p3:" << p3 << ",q3:" << q3 << "\n"; // b3:1,d3:2,p3:3,q3:4
 
b3 = 4, d3 = 3, p3 = 2, q3 = 1;
std::cout << "s.b:" << s.b << ",s.d:" << s.d << ",s.p:" << s.p << ",s.q:" << s.q << "\n"; // s.b:4,s.d:3,s.p:2,s.q:1

注意：

(1).結(jié)構(gòu)化綁定無(wú)法受到約束；

(2).lambda表達(dá)式無(wú)法捕獲結(jié)構(gòu)化綁定(C++20中可以)；

(3).必須確保在適當(dāng)?shù)臅r(shí)刻使用引用，盡量減少不必要的拷貝；

(4).使用結(jié)構(gòu)化綁定時(shí)，就不能再使用std::tie創(chuàng)建虛擬變量；

(5).結(jié)構(gòu)化綁定中有一個(gè)隱藏的匿名對(duì)象，結(jié)構(gòu)化綁定時(shí)引入的新變量名其實(shí)都指向這個(gè)匿名對(duì)象的成員/元素；

(6).可以在結(jié)構(gòu)化綁定中使用修飾符，如const和引用，這些修飾符會(huì)作用在新的匿名實(shí)體上，而不是結(jié)構(gòu)化綁定引入的新的變量名上；

(7).理論上講，結(jié)構(gòu)化綁定適用于任何有public數(shù)據(jù)成員的結(jié)構(gòu)體、C風(fēng)格數(shù)組和"類似元組(tuple-like)的對(duì)象";

(8).在任何情況下，結(jié)構(gòu)化綁定中聲明的變量名的數(shù)量都必須和元素或數(shù)據(jù)成員的數(shù)量相同;

(9).使用結(jié)構(gòu)化綁定需要繼承時(shí)需要遵循一定的規(guī)則：所有的非靜態(tài)數(shù)據(jù)成員必須在同一個(gè)類中定義(也就是說(shuō)，這些成員要么是全部直接來(lái)自于最終的類，要么是全部來(lái)自同一個(gè)父類)；并且只有當(dāng)public成員的順序保證是固定的時(shí)候你才應(yīng)該使用結(jié)構(gòu)化綁定；

(10).結(jié)構(gòu)化綁定機(jī)制是可拓展的，你可以為任何類型添加對(duì)結(jié)構(gòu)化綁定的支持。標(biāo)準(zhǔn)庫(kù)中就為std::pair<>、std::tuple<>、std::array<>添加了支持；

const std::map<std::string, std::string> addrs{
    {"csdn", "https://blog.csdn.net/fengbingchun/"},
    {"github", "https://github.com/fengbingchun"}
};
 
for (const auto& [key, val] : addrs) {
    std::cout << "key: " << key << ", addr: " << val << "\n"; // key: csdn, addr: https://blog.csdn.net/fengbingchun/
                                                                // key: github, addr: https://github.com/fengbingchun
}
 
Info info{ "github", 888 };
auto&& [u, v] = std::move(info); // u和v指向的匿名實(shí)體是info的右值引用,同時(shí)info仍持有值
std::cout << "info.name:" << info.name << "\n"; // info.name:github
Info info2{ "csdn", 666 };
auto [u2, v2] = std::move(info2); // info2已失去了值
std::cout << "info2.name:" << info2.name << "\n"; // info2.name:

執(zhí)行結(jié)果如下圖所示：