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Why are we allowed to use const with reference types if we may only assign null to them?


Why are we allowed to use const with reference types if we may only assign null to them?

By : user2951487
Date : November 18 2020, 11:13 AM
it fixes the issue From MSDN
code :
class Foo
{
    // This is not a good idea to expose a public non-pure field
    public static readonly StringBuilder BarBuilder = new StringBuilder();
    public Foo(){
    }
}


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Why is a non-const reference to const allowed if first initialised with non-const?

Why is a non-const reference to const allowed if first initialised with non-const?


By : 李军良
Date : March 29 2020, 07:55 AM
will be helpful for those in need You aren't rebinding r to reference ci. You're assigning the referencee of r (i) the value of ci.
Is it allowed to static_cast between different types of const?

Is it allowed to static_cast between different types of const?


By : user2102157
Date : March 29 2020, 07:55 AM
it helps some times I have rarely seen static_cast between top level const so far. , From [expr.static.cast]
code :
const int a = 42; // test case 1, const obj
const double b = static_cast<double>(a); // Works just as well.
const double b = a; // Of course this is fine too
void foo(int*) { }
void foo(int const*) { }

int main()
{
  int a = 42;

  foo(&a);
  foo(static_cast<int const*>(&a));
}
Why are you not allowed to bind a rvalue reference to a non const reference but you are allowed to call non const member

Why are you not allowed to bind a rvalue reference to a non const reference but you are allowed to call non const member


By : user3374966
Date : March 29 2020, 07:55 AM
Hope that helps Function arguments are subject to type conversions, which create a temporary rvalue. This would give confusing results if the function were passed a non-const reference to that temporary, when you expect it modify the argument. This potential confusion is a justification for not allowing temporaries to bind to non-const lvalue references.
There is no corresponding confusion for other reference types; const references leave the object unmodified as expected, and rvalue references are specifically intended to bind to rvalues. Neither does the issue arise for member functions, since type conversions aren't applied to the object being called on.
Why is it allowed to pass R-Values by const reference but not by normal reference?

Why is it allowed to pass R-Values by const reference but not by normal reference?


By : WOS
Date : March 29 2020, 07:55 AM
To fix the issue you can do as the title says why is it allowed to pass R-Values(literals) by constant reference but not normal reference , For your final question:
C++: const not allowed in universal (forwarding) reference

C++: const not allowed in universal (forwarding) reference


By : llegareta
Date : March 29 2020, 07:55 AM
it helps some times As far as I can see the section of answer that you have quoted is accurate but misleading.
First it's important to clarify that an rvalue reference and a forwarding reference are not the same thing, they just share the same notation &&. Whether this is a good thing is up for debate.
code :
template <typename T>
void foo(T&&); // deduced type == forwarding reference

void foo(int&&); // explicit type == rvalue reference
template <typename T>
void foo(const T&&); // const rvalue reference despite deduced type
struct Non_POD
{
    Non_POD(int i) : m_i(i) { }
    int m_i;
};

Non_POD foo() { return {0}; }

const Non_POD const_foo() { return {0}; }

template <typename T>
void bar(const T&& val)
{
    std::cout << "Accepts: const rvalue ref. ";
    if constexpr (std::is_rvalue_reference_v<decltype(val)>)
    {
        std::cout << "Val is rvalue reference.\n";
    }
    else if constexpr (std::is_lvalue_reference_v<decltype(val)>)
    {
        std::cout << "Val is lvalue reference.\n";
    }
    else
    {
        std::cout << "Val is lvalue.\n";
    }

    std::cout << std::endl;
}

int main()
{
    bar(foo());
    bar(const_foo());
    Non_POD x(0);
    //bar(x); // error
}
struct Non_POD
{
    Non_POD(int i) : m_i(i) { }
    int m_i;
};

Non_POD foo() { return {0}; }

const Non_POD const_foo() { return {0}; }

template <typename T>
void bar(const T&& val)
{
    std::cout << "Accepts: const rvalue ref. ";
    if constexpr (std::is_rvalue_reference_v<decltype(val)>)
    {
        std::cout << "Val is rvalue reference.\n";
    }
    else if constexpr (std::is_lvalue_reference_v<decltype(val)>)
    {
        std::cout << "Val is lvalue reference.\n";
    }
    else
    {
        std::cout << "Val is lvalue.\n";
    }

    std::cout << std::endl;
}

template <typename T>
void bar(T&& val)
{
    std::cout << "Accepts: forwarding ref. ";
    if constexpr (std::is_rvalue_reference_v<decltype(val)>)
    {
        std::cout << "Val is rvalue reference.\n";
    }
    else if constexpr (std::is_lvalue_reference_v<decltype(val)>)
    {
        std::cout << "Val is lvalue reference.\n";
    }
    else
    {
        std::cout << "Val is lvalue.\n";
    }

    std::cout << std::endl;
}

int main()
{
    Non_POD x(0);
    const Non_POD cx(0);

    bar(x);
    bar(cx);
    bar(Non_POD(0));
    bar(foo());
    bar(const_foo());
}
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