轉載自https://blog.csdn.net/fengbingchun/article/details/72835446
std::tuple是類似pair的模板。每個pair的成員類型都不相同，但每個pair都恰好有兩個成員。不同std::tuple類型的成員類型也不相同，但一個std::tuple可以有任意數量的成員。每個確定的std::tuple類型的成員數目是固定的，但一個std::tuple類型的成員數目可以與另一個std::tuple類型不同。
但我們希望將一些數據組合成單一對象，但又不想麻煩地定義一個新數據結構來表示這些數據時，std::tuple是非常有用的。我們可以將std::tuple看作一個”快速而隨意”的數據結構。
當我們定義一個std::tuple時，需要指出每個成員的類型。當我們創建一個std::tuple對象時，可以使用tuple的默認構造函數，它會對每個成員進行值初始化；也可以為每個成員提供一個初始值，此時的構造函數是explicit的，因此必須使用直接初始化方法。類似make_pair函數，標准庫定義了make_tuple函數，我們還可以使用它來生成std::tuple對象。類似make_pair，make_tuple函數使用初始值的類型來推斷tuple的類型。
一個std::tuple類型的成員數目是沒有限制的，因此，tuple的成員都是未命名的。要訪問一個tuple的成員，就要使用一個名為get的標准庫函數模板。為了使用get，我們必須指定一個顯式模板實參，它指出我們想要訪問第幾個成員。我們傳遞給get一個tuple對象，它返回指定成員的引用。get尖括號中的值必須是一個整型常量表達式。與往常一樣，我們從0開始計數，意味着get<0>是第一個成員。
為了使用tuple_size或tuple_element，我們需要知道一個tuple對象的類型。與往常一樣，確定一個對象的類型的最簡單方法就是使用decltype。
std::tuple的關系和相等運算符的行為類似容器的對應操作。這些運算符逐對比較左側tuple和右側tuple的成員。只有兩個tuple具有相同數量的成員時，我們才可以比較它們。而且，為了使用tuple的相等或不等運算符，對每對成員使用==運算符必須都是合法的；為了使用關系運算符，對每對成員使用<必須都是合法的。由於tuple定義了<和==運算符，我們可以將tuple序列傳遞給算法，並且可以在無序容器中將tuple作為關鍵字類型。
std::tuple的一個常見用途是從一個函數返回多個值。
std::tuple是一個模板，允許我們將多個不同類型的成員捆綁成單一對象。每個tuple包含指定數量的成員，但對一個給定的tuple類型，標准庫並未限制我們可以定義的成員數量上限。
std::tuple中元素是被緊密地存儲的(位於連續的內存區域)，而不是鏈式結構。
std::tuple實現了多元組，這是一個編譯期就確定大小的容器，可以容納不同類型的元素。多元組類型在當前標准庫中被定義為可以用任意數量參數初始化的類模板。每一模板參數確定多元組中一元素的類型。所以，多元組是一個多類型、大小固定的值的集合。
下面是從其他文章中copy的測試代碼，詳細內容介紹可以參考對應的reference：
#include "tuple.hpp"
#include <iostream>
#include <tuple>
#include <string>
#include <functional>
#include <utility>
//////////////////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/tuple/tuple/
int test_tuple_4()
{
{ // tuple::tuple: Constructs a tuple object. This involves individually constructing its elements,
// with an initialization that depends on the constructor form invoke
std::tuple<int, char> first; // default
std::tuple<int, char> second(first); // copy
std::tuple<int, char> third(std::make_tuple(20, 'b')); // move
std::tuple<long, char> fourth(third); // implicit conversion
std::tuple<int, char> fifth(10, 'a'); // initialization
std::tuple<int, char> sixth(std::make_pair(30, 'c')); // from pair / move
std::cout << "sixth contains: " << std::get<0>(sixth);
std::cout << " and " << std::get<1>(sixth) << '\n';
}
{ // std::tuple::operator=: Each of the elements in the tuple object is assigned its corresponding element
std::pair<int, char> mypair(0, ' ');
std::tuple<int, char> a(10, 'x');
std::tuple<long, char> b, c;
b = a; // copy assignment
c = std::make_tuple(100L, 'Y'); // move assignment
a = c; // conversion assignment
c = std::make_tuple(100, 'z'); // conversion / move assignment
a = mypair; // from pair assignment
a = std::make_pair(2, 'b'); // form pair /move assignment
std::cout << "c contains: " << std::get<0>(c);
std::cout << " and " << std::get<1>(c) << '\n';
}
{ // std::tuple::swap: Exchanges the content of the tuple object by the content of tpl,
// which is another tuple of the same type (containing objects of the same types in the same order)
std::tuple<int, char> a(10, 'x');
std::tuple<int, char> b(20, 'y');
a.swap(b);
std::cout << "a contains: " << std::get<0>(a);
std::cout << " and " << std::get<1>(a) << '\n';
std::swap(a, b);
std::cout << "a contains: " << std::get<0>(a);
std::cout << " and " << std::get<1>(a) << '\n';
}
{ // std::relational operators: Performs the appropriate comparison operation between the tuple objects lhs and rhs
std::tuple<int, char> a(10, 'x');
std::tuple<char, char> b(10, 'x');
std::tuple<char, char> c(10, 'y');
if (a == b) std::cout << "a and b are equal\n";
if (b != c) std::cout << "b and c are not equal\n";
if (b<c) std::cout << "b is less than c\n";
if (c>a) std::cout << "c is greater than a\n";
if (a <= c) std::cout << "a is less than or equal to c\n";
if (c >= b) std::cout << "c is greater than or equal to b\n";
}
return 0;
}
////////////////////////////////////////////////
// reference: https://msdn.microsoft.com/en-us/library/bb982771.aspx
int test_tuple_3()
{
typedef std::tuple<int, double, int, double> Mytuple;
Mytuple c0(0, 1, 2, 3);
// display contents " 0 1 2 3"
std::cout << " " << std::get<0>(c0);
std::cout << " " << std::get<1>(c0);
std::cout << " " << std::get<2>(c0);
std::cout << " " << std::get<3>(c0);
std::cout << std::endl;
Mytuple c1;
c1 = c0;
// display contents " 0 1 2 3"
std::cout << " " << std::get<0>(c1);
std::cout << " " << std::get<1>(c1);
std::cout << " " << std::get<2>(c1);
std::cout << " " << std::get<3>(c1);
std::cout << std::endl;
std::tuple<char, int> c2(std::make_pair('x', 4));
// display contents " x 4"
std::cout << " " << std::get<0>(c2);
std::cout << " " << std::get<1>(c2);
std::cout << std::endl;
Mytuple c3(c0);
// display contents " 0 1 2 3"
std::cout << " " << std::get<0>(c3);
std::cout << " " << std::get<1>(c3);
std::cout << " " << std::get<2>(c3);
std::cout << " " << std::get<3>(c3);
std::cout << std::endl;
typedef std::tuple<int, float, int, float> Mytuple2;
Mytuple c4(Mytuple2(4, 5, 6, 7));
// display contents " 4 5 6 7"
std::cout << " " << std::get<0>(c4);
std::cout << " " << std::get<1>(c4);
std::cout << " " << std::get<2>(c4);
std::cout << " " << std::get<3>(c4);
std::cout << std::endl;
return (0);
}
///////////////////////////////////////////////////
// reference: http://zh.cppreference.com/w/cpp/utility/tuple
static std::tuple<double, char, std::string> get_student(int id)
{
if (id == 0) return std::make_tuple(3.8, 'A', "Lisa Simpson");
if (id == 1) return std::make_tuple(2.9, 'C', "Milhouse Van Houten");
if (id == 2) return std::make_tuple(1.7, 'D', "Ralph Wiggum");
throw std::invalid_argument("id");
}
int test_tuple_2()
{
auto student0 = get_student(0);
std::cout << "ID: 0, "
<< "GPA: " << std::get<0>(student0) << ", "
<< "grade: " << std::get<1>(student0) << ", "
<< "name: " << std::get<2>(student0) << '\n';
double gpa1;
char grade1;
std::string name1;
std::tie(gpa1, grade1, name1) = get_student(1);
std::cout << "ID: 1, "
<< "GPA: " << gpa1 << ", "
<< "grade: " << grade1 << ", "
<< "name: " << name1 << '\n';
return 0;
}
///////////////////////////////////////////////
// reference: http://www.cplusplus.com/reference/tuple/
static void print_pack(std::tuple<std::string&&, int&&> pack)
{
std::cout << std::get<0>(pack) << ", " << std::get<1>(pack) << '\n';
}
static void fun(int &a)
{
a = 15;
}
int test_tuple_1()
{
{ // std::tuple_element: class template, Class designed to access the type of the Ith element in a tuple.
// It is a simple class with a single member type, tuple_element::type,
// defined as an alias of the type of the Ith element in a tuple of type T.
auto mytuple = std::make_tuple(10, 'a');
std::tuple_element<0, decltype(mytuple)>::type first = std::get<0>(mytuple);
std::tuple_element<1, decltype(mytuple)>::type second = std::get<1>(mytuple);
std::cout << "mytuple contains: " << first << " and " << second << '\n';
}
{ // std::tuple_size: Class template designed to access the number of elements in a tuple
std::tuple<int, char, double> mytuple(10, 'a', 3.14);
std::cout << "mytuple has ";
std::cout << std::tuple_size<decltype(mytuple)>::value;
std::cout << " elements." << '\n';
}
{ // std::forward_as_tuple: function template, Constructs a tuple object with rvalue references
// to the elements in args suitable to be forwarded as argument to a function.
std::string str("John");
print_pack(std::forward_as_tuple(str + " Smith", 25));
print_pack(std::forward_as_tuple(str + " Daniels", 22));
}
{ // std::get: funtion template, Returns a reference to the Ith element of tuple tpl.
std::tuple<int, char> mytuple(10, 'a');
std::get<0>(mytuple) = 20;
std::cout << "mytuple contains: ";
std::cout << std::get<0>(mytuple) << " and " << std::get<1>(mytuple);
std::cout << std::endl;
}
{ // std::make_tuple: function template, Constructs an object of the appropriate tuple type
// to contain the elements specified in args
auto first = std::make_tuple(10, 'a'); // tuple < int, char >
const int a = 0; int b[3]; // decayed types:
auto second = std::make_tuple(a, b); // tuple < int, int* >
auto third = std::make_tuple(std::ref(a), "abc"); // tuple < const int&, const char* >
std::cout << "third contains: " << std::get<0>(third);
std::cout << " and " << std::get<1>(third);
std::cout << std::endl;
}
{ // std::tie: function template, Constructs a tuple object whose elements are references
// to the arguments in args, in the same order
// std::ignore: object, This object ignores any value assigned to it. It is designed to be used as an
// argument for tie to indicate that a specific element in a tuple should be ignored.
int myint;
char mychar;
std::tuple<int, float, char> mytuple;
mytuple = std::make_tuple(10, 2.6, 'a'); // packing values into tuple
std::tie(myint, std::ignore, mychar) = mytuple; // unpacking tuple into variables
std::cout << "myint contains: " << myint << '\n';
std::cout << "mychar contains: " << mychar << '\n';
}
{ // std::tuple_cat: function template, Constructs an object of the appropriate tuple type
// to contain a concatenation of the elements of all the tuples in tpls, in the same order
std::tuple<float, std::string> mytuple(3.14, "pi");
std::pair<int, char> mypair(10, 'a');
auto myauto = std::tuple_cat(mytuple, std::tuple<int, char>(mypair));
std::cout << "myauto contains: " << '\n';
std::cout << std::get<0>(myauto) << '\n';
std::cout << std::get<1>(myauto) << '\n';
std::cout << std::get<2>(myauto) << '\n';
std::cout << std::get<3>(myauto) << '\n';
}
{ // tuple::tuple: A tuple is an object capable to hold a collection of elements.
// Each element can be of a different type.
std::tuple<int, char> foo(10, 'x');
auto bar = std::make_tuple("test", 3.1, 14, 'y');
std::get<2>(bar) = 100; // access element
int myint; char mychar;
std::tie(myint, mychar) = foo; // unpack elements
std::tie(std::ignore, std::ignore, myint, mychar) = bar; // unpack (with ignore)
mychar = std::get<3>(bar);
std::get<0>(foo) = std::get<2>(bar);
std::get<1>(foo) = mychar;
std::cout << "foo contains: ";
std::cout << std::get<0>(foo) << ' ';
std::cout << std::get<1>(foo) << '\n';
}
{
std::tuple<int, char> foo{ 12, 'a' };
std::cout << std::get<0>(foo) << "\n"; // 12
fun(std::get<0>(foo));
std::cout << std::get<0>(foo) << "\n"; // 15
}
return 0;
}