Boost::tuple
一 Boost::tuple
很多的時(shí)候我們經(jīng)常需要為我們的函數(shù)返回多個(gè)值，一般的做法是通過(guò)傳入非常量的指針或引用，但是這樣的話(huà)可能可讀性就要差一些，使用者可能需要確切的文檔才能確定到底哪個(gè)是返回值，為了更好的可讀性，我們可以使用class或struct來(lái)封裝我們要返回的多個(gè)值，然后返回封裝struct或class，但是使用這種方法的弊端就是增加的程序的代碼量，最好的解決辦法其實(shí)我們可以通過(guò)一種匿名的struct或class來(lái)解決這個(gè)問(wèn)題。
Boost::tuple就為我們提供了一種類(lèi)似于匿名struct的方法為我們解決函數(shù)的多個(gè)返回值的問(wèn)題。既增強(qiáng)了代碼的可讀性有不增加代碼量。其實(shí)在STL中已經(jīng)有這樣的特例，std::pair其實(shí)就是boost::tuple的2個(gè)參數(shù)的特例，對(duì)boost::tuple你可以綁定更多的參數(shù)，或者你可以迭代實(shí)現(xiàn)無(wú)限多參數(shù)的情況。
二 源碼剖析
頭文件: "boost/tuple/tuple.hpp"，它包含了 tuple 類(lèi)模板及庫(kù)的核心部分。
頭文件: "boost/tuple/tuple_io.hpp"，包含了對(duì) tuple 的輸入輸出操作符。
頭文件: "boost/tuple/tuple_comparison.hpp"，包含了 tuple 的關(guān)系操作符。
為了方便使用，Tuple 庫(kù)中有些名字位于名字空間 boost：如 tuple, make_tuple, tie, 和 get.
函數(shù)說(shuō)明：
1）構(gòu)造函數(shù)
2）拷貝構(gòu)造函數(shù)
3）t.get<N>()或get<N>(t) ，取得第N個(gè)值
4）make_tuple ，生成tuple
5）tie ， 生成都是ref的tuple
6) 重載比較運(yùn)算符 ，可以直接用來(lái)比較
7）重載輸入輸出運(yùn)算符 ，可以直接使用IO
8）get_head()和get_tail（）函數(shù)，用來(lái)取得值
9）length<>和element<>用來(lái)得到tuple的size和第N個(gè)的值類(lèi)型
10）如果使用boost::TR1,則還可以使用std::tr1::tuple_size（），std::tr1::tuple_element（），分別用來(lái)得到tuple的size和第N個(gè)值的類(lèi)型。
三 實(shí)例
1）tuple的構(gòu)造，拷貝構(gòu)造函數(shù)，get成員函數(shù)，get全局函數(shù)，make_tuple全局函數(shù)。
#include <string>
#include <iostream>
#include "boost/tuple/tuple.hpp"
boost::tuples::tuple<int,double> get_values()
{
return boost::make_tuple(6,12.0);
}
class base
{
public:
virtual ~base() {};
virtual void test()
{
std::cout << "base::test()\n";
}
};
class derived : public base
{
public:
virtual void test() { std::cout << "derived::test()\n"; }
};
void main()
{
// test for constructor
boost::tuple<int,double,std::string> triple(42,3.14,"My first tuple!");
boost::tuple<short,int,long> another;
boost::tuple<int,int,double> another2(10);
// test for make_tuple , ref and cref function
int plain=42;
int& ref=plain;
const int& cref=ref;
boost::tuples::tuple<int> plaint(plain);
plaint = boost::make_tuple(plain);
plaint = boost::make_tuple(ref);
plaint = boost::make_tuple(cref);
boost::tuples::tuple<int&> reft(ref);
boost::make_tuple(boost::ref(plain));
boost::make_tuple(boost::ref(ref));
boost::make_tuple(boost::ref(cref));
boost::tuples::tuple<const int&> creft(cref);
boost::make_tuple(boost::cref(plain));
boost::make_tuple(boost::cref(ref));
boost::make_tuple(boost::cref(cref));
// test for get function
boost::tuple<int,double,std::string> triple2(42,3.14,"The amazing tuple!");
int i=boost::tuples::get<0>(triple2);
double d=triple2.get<1>();
std::string s=boost::get<2>(triple2);
// test for function return tuple
boost::tuples::tuple<int,double> value = get_values();
// test for copy constructor
boost::tuple<int,std::string,derived> tup1(-5,"Tuples");
boost::tuple<unsigned int,std::string,base> tup2;
tup2=tup1;
tup2.get<2>().test();
std::cout << "Interesting value: " << tup2.get<0>() << '\n';
const boost::tuple<double,std::string,base> tup3(tup2);
//tup3.get<0>()=3.14; // error, because tup3 is const
boost::tuples::tuple<int,int,double> tuple1(10,30,20.000);
int head = tuple1.get_head();
int tailhead = tuple1.get_tail().get_head();
double tail = tuple1.get_tail().get_tail().get_head();
// for TR1
/**//*boost::tuples::tuple<double, char, int> tuplesize;
std::tr1::tuple_size();
std::tr1::tuple_element();*/
}
2）使用tie函數(shù)模版來(lái)生成對(duì)ref的綁定的tuple，tuple的比較使用，tuple的輸入輸出：
#include <string>
#include <iostream>
#include <vector>
#include <algorithm>
#include "boost/tuple/tuple.hpp"
#include "boost/tuple/tuple_comparison.hpp"
#include "boost/tuple/tuple_io.hpp"
template <int Index>
class element_less
{
public:
template <typename Tuple>
bool operator()(const Tuple& lhs,const Tuple& rhs) const
{
return boost::get<Index>(lhs)<boost::get<Index>(rhs);
}
};
int main()
{
// Tiers are tuples, where all elements are of non-const reference types.
// They are constructed with a call to the tie function template
int i; char c; double d;
boost::tie(i, c, d) = boost::make_tuple(1,'a', 5.5);
std::cout << i << " " << c << " " << d << std::endl;
// test ignore
char ch;
boost::tie(boost::tuples::ignore, ch) = std::make_pair(1, 'a');
std::cout << ch << std::endl;
// test for comparison
boost::tuple<int,std::string> tup1(11,"Match?");
boost::tuple<short,std::string> tup2(12,"Match?");
std::cout << std::boolalpha;
std::cout << "Comparison: tup1 is less than tup2\n";
std::cout << "tup1==tup2: " << (tup1==tup2) << '\n';
std::cout << "tup1!=tup2: " << (tup1!=tup2) << '\n';
std::cout << "tup1<tup2: " << (tup1<tup2) << '\n';
std::cout << "tup1>tup2: " << (tup1>tup2) << '\n';
std::cout << "tup1<=tup2: " << (tup1<=tup2) << '\n';
std::cout << "tup1>=tup2: " << (tup1>=tup2) << '\n';
tup2.get<0>()=boost::get<0>(tup1); //tup2=tup1 also works
std::cout << "\nComparison: tup1 equals tup2\n";
std::cout << "tup1==tup2: " << (tup1==tup2) << '\n';
std::cout << "tup1!=tup2: " << (tup1!=tup2) << '\n';
std::cout << "tup1<tup2: " << (tup1<tup2) << '\n';
std::cout << "tup1>tup2: " << (tup1>tup2) << '\n';
std::cout << "tup1<=tup2: " << (tup1<=tup2) << '\n';
std::cout << "tup1>=tup2: " << (tup1>=tup2) << '\n';
//test tuple using in the container
typedef boost::tuple<short,int,long,float,double,long double> num_tuple;
std::vector<num_tuple> vec;
vec.push_back(num_tuple(6,2));
vec.push_back(num_tuple(7,1));
vec.push_back(num_tuple(5));
std::sort(vec.begin(),vec.end(),element_less<1>());
std::cout << "\nAfter sorting: " << vec[0].get<0>() << '\n' << vec[1].get<0>() << '\n' << vec[2].get<0>() << '\n\n';
// test for io
boost::tuple<float, int, std::string> a(1.0f, 2, std::string("Howdy folks!"));
std::cout << std::endl << a << std::endl;
boost::tuple<int, int, int> ii;
std::cin >> ii;
std::cout << boost::tuples::set_open('[') << boost::tuples::set_close(']')<< boost::tuples::set_delimiter(':');
std::cout << ii << std::endl;
boost::tuples::tuple<int,int,double> tuple1;
int head = tuple1.get_head();
double tail = tuple1.get_tail();
}
四 注意
1）函數(shù) make_tuple 類(lèi)似于 std::make_pair. 缺省情況下，make_tuple 設(shè)置元素類(lèi)型為非const, 非引用的，即是最簡(jiǎn)單的、根本的參數(shù)類(lèi)
型。
2）為了使一個(gè) tuple 的元素設(shè)為引用類(lèi)型，你要使用函數(shù) boost::ref, 它來(lái)自另一個(gè)名為 Boost.Ref 的 Boost 庫(kù)。
3）如果元素需要是 const 引用的，就使用來(lái)自 Boost.Ref 的 boost::cref。
4）如果你要使綁定的每個(gè)元素變量都為ref，則可以使用tie函數(shù)。
五 參考
1）Beyond the C++ Standard Library: An Introduction to Boost
2）boost在線(xiàn)document