[C++] attribute

Updated: November 01, 2022

5 minute read

개요

타입, 객체, 코드 등에 대한 구현/정의 속성을 설명
문법
- C++11
  - [[ attribute-list ]]
- C++17
  - [[ using attribute-namespace : attribute-list ]]
표준 속성
- [[noreturn]]
  - C++11
  - 함수가 return 하지 않음을 지정
  - 함수가 반환되는 경우 동작이 정의되지 않음
  - 함수 선언에서 함수 이름에만 적용 가능
  - throw 사용하여 함수를 빠져나가야 함
- [[carries_dependency]]
  - C++11
  - indicates that dependency chain in release-consume std::memory_order propagates in and out of the function
- [[deprecated]] / [[deprecated(“reason”)]]
  - C++14
  - 사용은 허용되지만 권장되지 않음
  - 사용 시 컴파일 경고 발생
  - class/struct/union
    - class [[deprecated]] C;
  - typedef/using
    - [[deprecated]] typedef C* PC;
    - using PC [[deprecated]] = C*;
  - variable
    - [[deprecated]] int i;
  - non-static 데이터 멤버
    - class T { [[deprecated]] int i; };
  - 함수
    - [[deprecated]] void f();
  - namespace
    - namespace [[deprecated]] NS { int i; }
  - enum
    - enum [[deprecated]] E {};
    - enum { A [[deprecated]], B [[deprecated]] = 42 };
      - C++17
  - 템플릿
    - template <> class [[deprecated]] C<int> {};
- [[fallthrough]]
  - C++17
  - switch문에서 특정 라벨에 break가 없는게 의도적임을 지정
  - 컴파일 경고가 발생하지 않음
- [[nodiscard]] / [[nodiscard(“reason”)]]
  - [[nodiscard]]
    - C++17
  - [[nodiscard(“reason”)]]
    - C++20
  - 함수의 반환 값을 사용하지 않으면 컴파일 경고 발생
    - 해당 속성으로 선언된 함수
    - 해당 속성으로 선언된 enum, class 등을 반환하는 함수
- [[maybe_unused]]
  - C++17
  - 사용하지 않는 변수 등에 대한 경고를 억제
- [[likely]] / [[unlikely]]
  - C++20
  - 컴파일러가 적절한 최적화를 하도록 분기문에 실행 빈도를 지정
- [[no_unique_address]]
  - C++20
  - non-static 데이터 멤버가 다른 non-static 데이터 멤버와 구별되는 주소를 가질 필요가 없음을 지정
- [[assume]]
  - C++23
  - 표현식이 항상 true로 평가되도록 지정

예제

[[noreturn]]

코드

   #include <iostream>
        
   using namespace std;
        
   [[noreturn]] void func1() { throw "throw"; };
        
   void func2 [[noreturn]] () { throw "throw"; };
        
   int main() {
   	try {
   		func1();
   	} catch (...) {
   		cout << "~~~ func1()" << endl;
   	}
        
   	try {
   		func2();
   	} catch (...) {
   		cout << "~~~ func2()" << endl;
   	}
        
   	return 0;
   }

실행 결과
```
   ~~~ func1()
   ~~~ func2()
```

[[deprecated]] / [[deprecated(“reason”)]]

코드

   #include <iostream>
   #include <string>
        
   using namespace std;
        
   class [[deprecated]] Test1 {
   	public:
   		void Print() { cout << "Test1::Print()" << endl; }
   };
        
   class Test2 {
   	public:
   		[[deprecated]] int i;
   		[[deprecated]] void Print() { cout << "Test2::Print()" << endl; }
   };
        
   template <typename T> class Test3 {};
   template <> class [[deprecated]] Test3<int> {};
        
   [[deprecated]] int I = 0;
        
   int main() {
   	cout << I << endl;
        
   	Test1().Print();
        
   	Test2().i;
   	Test2().Print();
        
   	[[deprecated]] int i = 0;
   	cout << i << endl;
        
   	Test3<string>();
   	Test3<int>();
        
   	return 0;
   }

실행 결과

   main.cpp: In function ‘int main()’:
   main.cpp:23:17: warning: ‘I’ is deprecated [-Wdeprecated-declarations]
      23 |         cout << I << endl;
         |                 ^
   main.cpp:20:20: note: declared here
      20 | [[deprecated]] int I = 0;
         |                    ^
   main.cpp:25:15: warning: ‘Test1’ is deprecated [-Wdeprecated-declarations]
      25 |         Test1().Print();
         |               ^
   main.cpp:6:22: note: declared here
       6 | class [[deprecated]] Test1 {
         |                      ^~~~~
   main.cpp:27:17: warning: ‘Test2::i’ is deprecated [-Wdeprecated-declarations]
      27 |         Test2().i;
         |                 ^
   main.cpp:13:36: note: declared here
      13 |                 [[deprecated]] int i;
         |                                    ^
   main.cpp:27:17: warning: ‘Test2::i’ is deprecated [-Wdeprecated-declarations]
      27 |         Test2().i;
         |                 ^
   main.cpp:13:36: note: declared here
      13 |                 [[deprecated]] int i;
         |                                    ^
   main.cpp:27:17: warning: ‘Test2::i’ is deprecated [-Wdeprecated-declarations]
      27 |         Test2().i;
         |                 ^
   main.cpp:13:36: note: declared here
      13 |                 [[deprecated]] int i;
         |                                    ^
   main.cpp:28:22: warning: ‘void Test2::Print()’ is deprecated [-Wdeprecated-declarations]
      28 |         Test2().Print();
         |         ~~~~~~~~~~~~~^~
   main.cpp:14:37: note: declared here
      14 |                 [[deprecated]] void Print() { cout << "Test2::Print()" << endl; }
         |                                     ^~~~~
   main.cpp:31:17: warning: ‘i’ is deprecated [-Wdeprecated-declarations]
      31 |         cout << i << endl;
         |                 ^
   main.cpp:30:28: note: declared here
      30 |         [[deprecated]] int i = 0;
         |                            ^
   main.cpp:34:20: warning: ‘Test3’ is deprecated [-Wdeprecated-declarations]
      34 |         Test3<int>();
         |                    ^
   main.cpp:18:34: note: declared here
      18 | template <> class [[deprecated]] Test3<int> {};
         |                                  ^~~~~~~~~~
   0
   Test1::Print()
   Test2::Print()
   0

[[fallthrough]]

코드

   #include <iostream>
        
   using namespace std;
        
   int main() {
   	int i = 0;
        
   	switch (i) {
   	case 0:
   		cout << 0 << endl;
   		[[fallthrough]];
   	case 1:
   		cout << 1 << endl;
   		break;
   	}
        
   	return 0;
   }

실행 결과
```
   0
   1
```

[[nodiscard]] / [[nodiscard(“reason”)]]

코드

   class [[nodiscard]] Test {};
   Test func1() { return Test(); }
        
   [[nodiscard("reason")]] int func2() { return 1; };
        
   int main() {
   	func1();
        
   	func2();
        
   	return 0;
   }

실행 결과

   main.cpp: In function ‘int main()’:
   main.cpp:7:14: warning: ignoring returned value of type ‘Test’, declared with attribute ‘nodiscard’ [-Wunused-result]
       7 |         func1();
         |         ~~~~~^~
   main.cpp:2:6: note: in call to ‘Test func1()’, declared here
       2 | Test func1() { return Test(); }
         |      ^~~~~
   main.cpp:1:21: note: ‘Test’ declared here
       1 | class [[nodiscard]] Test {};
         |                     ^~~~
   main.cpp:9:14: warning: ignoring return value of ‘int func2()’, declared with attribute ‘nodiscard’: ‘reason’ [-Wunused-result]
       9 |         func2();
         |         ~~~~~^~
   main.cpp:4:29: note: declared here
       4 | [[nodiscard("reason")]] int func2() { return 1; };
         |                             ^~~~~

[[maybe_unused]]

코드

   [[maybe_unused]] void func([[maybe_unused]] int x, int y) {}
        
   int main() {
   	int i = 0;
   	[[maybe_unused]] double d = 0.0;
        
   	return 0;
   }

실행 결과

   main.cpp: In function ‘int main()’:
   main.cpp:4:13: warning: unused variable ‘i’ [-Wunused-variable]
       4 |         int i = 0;
         |             ^

[[likely]] / [[unlikely]]

코드

   #include <algorithm>
   #include <chrono>
   #include <iostream>
        
   using namespace std;
        
   constexpr long long fact_with_attributes(long long n) noexcept {
   	if (n > 1) [[likely]] {
   		return n * fact_with_attributes(n - 1);
   	} else [[unlikely]] {
   		return 1;
   	}
   }
        
   constexpr long long fact_no_attributes(long long n) noexcept {
   	if (n > 1) {
   		return n * fact_no_attributes(n - 1);
   	} else {
   		return 1;
   	}
   }
        
   int main() {
   	auto run = [](auto func) {
   		const auto start = chrono::high_resolution_clock::now();
        
   		for (int i = 0; i < 2000000; i++) {
   			for (int j = 0; j <= 20; j++) {
   				func(j);
   			}
   		}
        
   		const auto end = chrono::high_resolution_clock::now();
        
   		const chrono::duration<double> result = end - start;
   		cout << result.count() << endl;
   	};
        
   	run(fact_with_attributes);
   	run(fact_no_attributes);
        
   	return 0;
   }

실행 결과
```
   1.68545
   1.7237
```

[[no_unique_address]]

코드

   #include <iostream>
        
   using namespace std;
        
   class Test {};
        
   class Test1 {
   	private:
   		int i;
   		Test t;
        
   	public:
   		void Print() {
   			cout << &this->i << endl;
   			cout << &this->t << endl;
   		}
   };
        
   class Test2 {
   	private:
   		int i;
   		[[no_unique_address]] Test t;
        
   	public:
   		void Print() {
   			cout << &this->i << endl;
   			cout << &this->t << endl;
   		}
   };
        
   int main() {
   	cout << sizeof(int) << endl;
   	cout << sizeof(Test) << endl;
   	cout << sizeof(Test1) << endl;
   	cout << sizeof(Test2) << endl;
        
   	cout << "------" << endl;
        
   	Test1().Print();
        
   	cout << "------" << endl;
        
   	Test2().Print();
        
   	return 0;
   }

실행 결과

   4
   1
   8
   4
   ------
   0x7ffd4e7a40b4
   0x7ffd4e7a40b8
   ------
   0x7ffd4e7a40bc
   0x7ffd4e7a40bc

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[C++] attribute

개요

예제

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