总结C++98/11/17表达式类别-创新互联

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目标

以下代码能否编译通过，能否按照期望运行？

#include 
#include 

namespace cpp98
{

struct A { };
A func() { return A(); }

int main()
{
 int i = 1;
 i = 2;
 // 3 = 4;
 const int j = 5;
 // j = 6;
 i = j;
 func() = A();
 return 0;
}

}

namespace cpp11
{

#define is_lvalue(x) std::is_lvalue_reference::value
#define is_prvalue(x) !std::is_reference::value
#define is_xvalue(x) std::is_rvalue_reference::value
#define is_glvalue(x) (is_lvalue(x) || is_xvalue(x))
#define is_rvalue(x) (is_xvalue(x) || is_prvalue(x))

void func();
int non_reference();
int&& rvalue_reference();
std::pair make();

struct Test
{
 int field;
 void member_function()
 {
 static_assert(is_lvalue(field), "");
 static_assert(is_prvalue(this), "");
 }
 enum Enum
 {
 ENUMERATOR,
 };
};

int main()
{
 int i;
 int&& j = std::move(i);
 Test test;

 static_assert(is_lvalue(i), "");
 static_assert(is_lvalue(j), "");
 static_assert(std::is_rvalue_reference::value, "");
 static_assert(is_lvalue(func), "");
 static_assert(is_lvalue(test.field), "");
 static_assert(is_lvalue("hello"), "");

 static_assert(is_prvalue(2), "");
 static_assert(is_prvalue(non_reference()), "");
 static_assert(is_prvalue(Test{3}), "");
 static_assert(is_prvalue(test.ENUMERATOR), "");

 static_assert(is_xvalue(rvalue_reference()), "");
 static_assert(is_xvalue(make().first), "");

 return 0;
}

}

namespace reference
{

int&& rvalue_reference()
{
 int local = 1;
 return std::move(local);
}

const int& const_lvalue_reference(const int& arg)
{
 return arg;
}

int main()
{
 auto&& i = rvalue_reference(); // dangling reference
 auto&& j = const_lvalue_reference(2); // dangling reference
 int k = 3;
 auto&& l = const_lvalue_reference(k);
 return 0;
}

}

namespace auto_decl
{

int non_reference() { return 1; }
int& lvalue_reference() { static int i; return i; }
const int& const_lvalue_reference() { return lvalue_reference(); }
int&& rvalue_reference() { static int i; return std::move(i); }

int main()
{
 auto [s1, s2] = std::pair(2, 3);
 auto&& t1 = s1;
 static_assert(!std::is_reference::value);
 static_assert(std::is_lvalue_reference::value);

 int i1 = 4;
 auto i2 = i1;
 decltype(auto) i3 = i1;
 decltype(auto) i4{i1};
 decltype(auto) i5 = (i1);
 static_assert(!std::is_reference::value, "");
 static_assert(!std::is_reference::value, "");
 static_assert(!std::is_reference::value, "");
 static_assert(std::is_lvalue_reference::value, "");

 auto n1 = non_reference();
 decltype(auto) n2 = non_reference();
 auto&& n3 = non_reference();
 static_assert(!std::is_reference::value, "");
 static_assert(!std::is_reference::value, "");
 static_assert(std::is_rvalue_reference::value, "");

 auto l1 = lvalue_reference();
 decltype(auto) l2 = lvalue_reference();
 auto&& l3 = lvalue_reference();
 static_assert(!std::is_reference::value, "");
 static_assert(std::is_lvalue_reference::value, "");
 static_assert(std::is_lvalue_reference::value, "");

 auto c1 = const_lvalue_reference();
 decltype(auto) c2 = const_lvalue_reference();
 auto&& c3 = const_lvalue_reference();
 static_assert(!std::is_reference::value, "");
 static_assert(std::is_lvalue_reference::value, "");
 static_assert(std::is_lvalue_reference::value, "");

 auto r1 = rvalue_reference();
 decltype(auto) r2 = rvalue_reference();
 auto&& r3 = rvalue_reference();
 static_assert(!std::is_reference::value, "");
 static_assert(std::is_rvalue_reference::value, "");
 static_assert(std::is_rvalue_reference::value, "");

 return 0;
}

}

namespace cpp17
{

class NonMoveable
{
public:
 int i = 1;
 NonMoveable(int i) : i(i) { }
 NonMoveable(NonMoveable&&) = delete;
};

NonMoveable make(int i)
{
 return NonMoveable{i};
}

void take(NonMoveable nm)
{
 return static_cast(nm);
}

int main()
{
 auto nm = make(2);
 auto nm2 = NonMoveable{make(3)};
 // take(nm);
 take(make(4));
 take(NonMoveable{make(5)});
 return 0;
}

}

int main()
{
 cpp98::main();
 cpp11::main();
 reference::main();
 auto_decl::main();
 cpp17::main();
}

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