c++11中关于什么是lvalue, 什么是rvalue, 什么是xvalue, 什么是prvalue, 什么是…
一直搞得我晕头转向的, 今天下定决心一定要把它搞定, 写了一个程序来判断lvalue, rvalue, prvalue等
1 namespace test{ 2 3 template <typename T> 4 struct is_lvalue_reference { 5 const static bool value = false; 6 }; 7 8 template <typename T> 9 struct is_lvalue_reference<T&> { 10 const static bool value = true; 11 }; 12 13 template <typename T> 14 struct is_rvalue_reference { 15 const static bool value = false; 16 }; 17 18 template <typename T> 19 struct is_rvalue_reference<T&&> { 20 const static bool value = true; 21 }; 22 23 template <typename T> 24 struct is_xvalue { 25 const static bool value = (!is_lvalue_reference<T>::value) && is_rvalue_reference<T>::value; 26 }; 27 28 template <typename T> 29 struct is_prvalue { 30 const static bool value = !(is_lvalue_reference<T>::value || is_rvalue_reference<T>::value); 31 }; 32 33 template <typename T> 34 struct is_lvalue { 35 const static bool value = !(is_xvalue<T>::value || is_prvalue<T>::value); 36 }; 37 38 template <typename T> 39 struct is_rvalue { 40 const static bool value = (is_xvalue<T>::value || is_prvalue<T>::value); 41 }; 42 43 44 template <typename T> 45 struct is_glvalue{ 46 const static bool value = (is_xvalue<T>::value || is_lvalue<T>::value); 47 }; 48 49 }
测试验证:
1 struct Foo {}; 2 Foo funRetFoo(); 3 Foo& funRetFooLRef(); 4 Foo&& funRetFooRRef(); 5 6 7 TEST(TypeTraits, isRvalue){ 8 9 10 //base type 11 EXPECT_FALSE(::test::is_lvalue_reference<int>::value); 12 EXPECT_FALSE(::test::is_rvalue_reference<int>::value); 13 EXPECT_FALSE(::test::is_lvalue<int>::value); 14 EXPECT_FALSE(::test::is_xvalue<int>::value); 15 EXPECT_TRUE(::test::is_prvalue<int>::value); 16 EXPECT_FALSE(::test::is_glvalue<int>::value); 17 EXPECT_TRUE(::test::is_rvalue<int>::value); 18 19 // return obj 20 EXPECT_FALSE(::test::is_lvalue_reference<decltype(funRetFoo())>::value); 21 EXPECT_FALSE(::test::is_rvalue_reference<decltype(funRetFoo())>::value); 22 EXPECT_FALSE(::test::is_lvalue<decltype(funRetFoo())>::value); 23 EXPECT_FALSE(::test::is_xvalue<decltype(funRetFoo())>::value); 24 EXPECT_TRUE(::test::is_prvalue<decltype(funRetFoo())>::value); 25 EXPECT_FALSE(::test::is_glvalue<decltype(funRetFoo())>::value); 26 EXPECT_TRUE(::test::is_rvalue<decltype(funRetFoo())>::value); 27 28 29 // return ref obj 30 EXPECT_TRUE(::test::is_lvalue_reference<decltype(funRetFooLRef())>::value); 31 EXPECT_FALSE(::test::is_rvalue_reference<decltype(funRetFooLRef())>::value); 32 EXPECT_TRUE(::test::is_lvalue<decltype(funRetFooLRef())>::value); 33 EXPECT_FALSE(::test::is_xvalue<decltype(funRetFooLRef())>::value); 34 EXPECT_FALSE(::test::is_prvalue<decltype(funRetFooLRef())>::value); 35 EXPECT_TRUE(::test::is_glvalue<decltype(funRetFooLRef())>::value); 36 EXPECT_FALSE(::test::is_rvalue<decltype(funRetFooLRef())>::value); 37 38 // return rref obj 39 EXPECT_FALSE(::test::is_lvalue_reference<decltype(funRetFooRRef())>::value); 40 EXPECT_TRUE(::test::is_rvalue_reference<decltype(funRetFooRRef())>::value); 41 EXPECT_FALSE(::test::is_lvalue<decltype(funRetFooRRef())>::value); 42 EXPECT_TRUE(::test::is_xvalue<decltype(funRetFooRRef())>::value); 43 EXPECT_FALSE(::test::is_prvalue<decltype(funRetFooRRef())>::value); 44 EXPECT_TRUE(::test::is_glvalue<decltype(funRetFooRRef())>::value); 45 EXPECT_TRUE(::test::is_rvalue<decltype(funRetFooRRef())>::value); 46 47 48 int lvalue; 49 // 模拟=号左边 50 EXPECT_TRUE(::test::is_lvalue_reference<decltype(*&lvalue)>::value); 51 EXPECT_FALSE(::test::is_rvalue_reference<decltype(*&lvalue)>::value); 52 EXPECT_TRUE(::test::is_lvalue<decltype(*&lvalue)>::value); 53 EXPECT_FALSE(::test::is_xvalue<decltype(*&lvalue)>::value); 54 EXPECT_FALSE(::test::is_prvalue<decltype(*&lvalue)>::value); 55 EXPECT_TRUE(::test::is_glvalue<decltype(*&lvalue)>::value); 56 EXPECT_FALSE(::test::is_rvalue<decltype(*&lvalue)>::value); 57 58 59 //operator++() 60 EXPECT_FALSE(::test::is_lvalue_reference<decltype(lvalue++)>::value); 61 EXPECT_FALSE(::test::is_rvalue_reference<decltype(lvalue++)>::value); 62 EXPECT_FALSE(::test::is_lvalue<decltype(lvalue++)>::value); 63 EXPECT_FALSE(::test::is_xvalue<decltype(lvalue++)>::value); 64 EXPECT_TRUE(::test::is_prvalue<decltype(lvalue++)>::value); 65 EXPECT_FALSE(::test::is_glvalue<decltype(lvalue++)>::value); 66 EXPECT_TRUE(::test::is_rvalue<decltype(lvalue++)>::value); 67 68 //operator++(int) 69 EXPECT_TRUE(::test::is_lvalue_reference<decltype(++lvalue)>::value); 70 EXPECT_FALSE(::test::is_rvalue_reference<decltype(++lvalue)>::value); 71 EXPECT_TRUE(::test::is_lvalue<decltype(++lvalue)>::value); 72 EXPECT_FALSE(::test::is_xvalue<decltype(++lvalue)>::value); 73 EXPECT_FALSE(::test::is_prvalue<decltype(++lvalue)>::value); 74 EXPECT_TRUE(::test::is_glvalue<decltype(++lvalue)>::value); 75 EXPECT_FALSE(::test::is_rvalue<decltype(++lvalue)>::value); 76 77 }
