Understanding Lvalues , Rvalues

// lvalue references

const int& lvr1{ 42 }; // value

 

int lv1{ 0 };

int& lvr2{ lv1 }; // lvalue (non-const)

 

constexpr int lv2{ 42 };

const int& lvr3{ lv2 }; // lvalue (const)

 

constexpr int f1(int x)

{

    return 6 * x;

}

const int& lvr4{ f1(7) }; // Function return value







-----

// rvalue references

const int&& rvr1{ 42 }; // value

 

// Enable next two statements to reveal compiler error

#if 0

int&& rvr2       {lv1}; // lvalue (non-const)

const int&& rvr3  {lv2}; // lvalue (const)

#else

int&& rvr2{ static_cast<int&&>(lv1) }; // rvalue (non-const)

const int&& rvr3{ static_cast<const int&&>(lv2) }; // rvalue (const)

#endif

const int&& rvr4{ f1(7) }; // Function return value

#include using namespace std;   // §8.3.2 // References are either form of: // T& D         lvalue reference // T&& D        rvalue reference // They are distinct types (differentiating overloaded functions)   // §8.5.3 // The initializer of an rvalue reference shall not be an lvalue   // lvalue references const int& lvr1{ 42 }; // value   int lv1{ 0 }; int& lvr2{ lv1 }; // lvalue (non-const)   constexpr int lv2{ 42 }; const int& lvr3{ lv2 }; // lvalue (const)   constexpr int f1(int x) {     return 6 * x; } const int& lvr4{ f1(7) }; // Function return value   const int& lvr5{ (lv1 + f1(7)) / 2 }; // expression   // rvalue references const int&& rvr1{ 42 }; // value   // Enable next two statements to reveal compiler error #if 0 int&& rvr2       {lv1}; // lvalue (non-const) const int&& rvr3  {lv2}; // lvalue (const) #else int&& rvr2{ static_cast<int&&>(lv1) }; // rvalue (non-const) const int&& rvr3{ static_cast<const int&&>(lv2) }; // rvalue (const) #endif const int&& rvr4{ f1(7) }; // Function return value const int&& rvr5{ (lv1 + f1(7)) / 2 }; // expression   int main() {     lv1 = 42;     // Print out the references     cout << lvr1 << " Value" << endl;     cout << lvr2 << " lvalue (non-const)" << endl;     cout << lvr3 << " lvalue (const)" << endl;     cout << lvr4 << " Function return value" << endl;          cout << rvr1 << " Value" << endl;     cout << rvr2 << " rvalue (const)" << endl;     cout << rvr3 << " rvalue (non-const)" << endl;     cout << rvr4 << " Function return value" << endl;       return 0; }

Output:

42 Value
42 lvalue (non-const)
42 lvalue (const)
42 Function return value

42 Value
42 rvalue (const)
42 rvalue (non-const)
42 Function return value