1. 引入標頭檔案

#include <tuple>

2. std::tuple初始化

std::tuple<int,std::string,float> t1(10,"Test",3.14);

這裡要注意，不是所有的C++ 11編譯器都支援copy-list-initialization的方式。如下程式碼所示。

std::tuple<int,int> foo_tuple() 
{
 return {1,-1}; // Error until N4387
 return std::tuple<int,int>{1,-1}; // Always works
 return std::make_tuple(1,-1); // Always works
}

3. 列印std::tuple

列印std::tuple可以將它的元素逐個打印出來，不過非常繁瑣，我們可以通過如下通用的列印函式，幫助我們一次性的將tuple的所有要素打印出來。

#include <iostream>
#include <tuple>
#include <string>
 
// helper function to print a tuple of any size
template<class Tuple,std::size_t N>
struct TuplePrinter {
 static void print(const Tuple& t) 
 {
  TuplePrinter<Tuple,N-1>::print(t);
  std::cout << "," << std::get<N-1>(t);
 }
};
 
template<class Tuple>
struct TuplePrinter<Tuple,1> {
 static void print(const Tuple& t) 
 {
  std::cout << std::get<0>(t);
 }
};
 
template<typename... Args,std::enable_if_t<sizeof...(Args) == 0,int> = 0>
void print(const std::tuple<Args...>& t)
{
 std::cout << "()\n";
}
 
template<typename... Args,std::enable_if_t<sizeof...(Args) != 0,int> = 0>
void print(const std::tuple<Args...>& t)
{
 std::cout << "(";
 TuplePrinter<decltype(t),sizeof...(Args)>::print(t);
 std::cout << ")\n";
}
// end helper function
 
int main()
{
 std::tuple<int,3.14);
 print(t1);
}

輸出：

(10,Test,3.14)

4、合併多個std::tuple

std::tuple_cat函式可以將多個std::tuple合併為一個tuple。

int main()
{
 std::tuple<int,3.14);
 int n = 7;
 auto t2 = std::tuple_cat(t1,std::make_tuple("Foo","bar"),t1,std::tie(n));
 n = 42;
 print(t2);
}

輸出：

(10,3.14,Foo,bar,10,42)

5. std::tuple的解包(unpack)

std::tie能夠將std::tuple包含的要素解包(unpack)成單個的物件。

#include <iostream>
#include <tuple>
#include <string>

int main() {
 auto info = std::make_tuple(3.8,'A',"Lisa Simpson");
 
 double score = 0.0;
 char grade;
 std::string name;
 std::tie(score,grade,name) = info;

 std::cout << "score:" << score << ",grade:" << grade << ",name:" << name << std::endl;

return 0;
}

輸出:

score:3.8,grade:A,name:Lisa Simpson

std::tie還支援std::pair物件的解包(unpack)。

#include <iostream>
#include <tuple>
#include <string>
#include <utility>

int main() {
 auto info = std::make_pair(3.8,"Lisa Simpson");
 
 double score = 0.0;
 std::string name;
 std::tie(score,name:Lisa Simpson



當我們不關注tuple中的某個元素時，可以使用std::ignore忽略該元素。

#include <iostream>
#include <tuple>
#include <string>
#include <utility>

int main() {
 auto info = std::make_pair(3.8,std::ignore) = info;

 std::cout << "score:" << score << ",name:" << name << std::endl;

return 0;
}

輸出:

score:3.8,name:



參考材料


https://en.cppreference.com/w/cpp/utility/tuple/tuple_cat
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