[C++] decltype

개요

• 전달된 식의 값 종류에 따른 타입을 컴파일 타임에 추론하여 치환
• 값 종류
  • l-value
    • 이동이 불가능한 l-value
    • T&로 추론
  • pr-value
    • 순수 r-value
    • T로 추론
  • x-value
    • 이동이 가능한 l-value
    • T&&로 추론
• auto의 경우 정확하게 추론하지 않음

예제

• 코드

   #include <cstring>
   #include <iostream>
   #include <string>
      
   using namespace std;
      
   template <typename T> constexpr string type_name() {
   	const string s = __PRETTY_FUNCTION__;
   	const int prefixSize = s.find("[with T = ") + strlen("[with T = ");
      
   	return string(s.data() + prefixSize, s.find(';') - prefixSize);
   }
      
   template <typename T1, typename T2>
   void add(T1 t1, T2 t2, decltype(t1 + t2) &result) {
   	result = t1 + t2;
   }
   template <typename T1, typename T2>
   auto add(T1 t1, T2 t2) -> decltype(t1 + t2) {
   	return t1 + t2;
   }
      
   int main() {
   	int a = 1;
   	auto b1 = a;
   	decltype(a) b2 = a;
   	cout << type_name<decltype(b1)>() << endl;
   	cout << type_name<decltype(b2)>() << endl;
      
   	cout << "------ 1" << endl;
      
   	int &c = a;
   	auto d1 = c;
   	decltype(c) d2 = c;
   	cout << type_name<decltype(d1)>() << endl;
   	cout << type_name<decltype(d2)>() << endl;
      
   	cout << "------ 2" << endl;
      
   	int &&e = 3;
   	auto f1 = e;
   	decltype(e) f2 = 4;
   	cout << type_name<decltype(f1)>() << endl;
   	cout << type_name<decltype(f2)>() << endl;
      
   	cout << "------ 3" << endl;
      
   	int g = 1, h = 2;
   	auto i1 = g + h;
   	decltype(g + h) i2 = g + h;
   	cout << type_name<decltype(i1)>() << endl;
   	cout << type_name<decltype(i2)>() << endl;
      
   	cout << "------ 4" << endl;
      
   	int j = 1;
   	auto k1 = j;
   	decltype((j)) k2 = g;
   	cout << type_name<decltype(k1)>() << endl;
   	cout << type_name<decltype(k2)>() << endl;
      
   	cout << "------ 5" << endl;
      
   	const int l = 1;
   	auto m1 = l;
   	decltype(l) m2 = l;
   	cout << type_name<decltype(m1)>() << endl;
   	cout << type_name<decltype(m2)>() << endl;
      
   	cout << "------ 6" << endl;
      
   	int result = 0;
   	add(1, 2, result);
   	cout << result << endl;
      
   	cout << type_name<decltype(add(1, 2))>() << endl;
   	cout << type_name<decltype(add(1, 1.1))>() << endl;
      
   	return 0;
   }
  

• 실행 결과

   int
   int
   ------ 1
   int
   int&
   ------ 2
   int
   int&&
   ------ 3
   int
   int
   ------ 4
   int
   int&
   ------ 5
   int
   const int
   ------ 6
   3
   int
   double