rvalue Reference not matching

Question

I have this exercise in which we try various combinations of rvalue and lvalue references using a template class, I am getting two assertion errors; if someone could guide.

#include <assert.h>

typedef int& IntLRef;
typedef IntLRef& IntLLRef;
typedef IntLRef&& IntLRRef;

typedef int&& IntRRef;
typedef IntRRef& IntRLRef;
typedef IntRRef&& IntRRRef;

template<typename T, typename U>
struct IsSameType
{
  static const bool value = false;
};

template<typename T>
struct IsSameType <T, T>
{
    static const bool value = true;
};
static_assert(IsSameType<IntLRef, IntLLRef>::value, "LRef DIF LLRef"); static_assert(IsSameType<IntLRef, IntLRRef>::value, "LRef DIF LRRef"); static_assert(IsSameType<IntLLRef, IntLRRef>::value, "LLRef DIF LRRef");

static_assert(IsSameType<IntRRef, IntRLRef>::value, "RRef DIF RLRef"); static_assert(IsSameType<IntRRef, IntRRRef>::value, "RRef DIF RRRef"); static_assert(IsSameType<IntRLRef, IntRRRef>::value, "RLRef DIF RRRef");

int main();

I am getting assertion error :

rvalue_ex3.cpp:34:48: error: static assertion failed: RRef DIF RLRef
   34 |   static_assert(IsSameType<IntRRef, IntRLRef>::value, "RRef DIF RLRef");
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
rvalue_ex3.cpp:36:49: error: static assertion failed: RLRef DIF RRRef
   36 |   static_assert(IsSameType<IntRLRef, IntRRRef>::value, "RLRef DIF RRRef");
      |                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
akm009@a

I need to make modfications to assert it as true and understand why it is failing

its not clear what you are asking or what you want to achieve. You can assert `! IsSameType<...` , but I suppose thats not what you want, what do you want? — 463035818_is_not_an_ai, Nov 28 '22 at 10:33
Btw: Your `IsSameType` functionality is already available via the standard library, including a variable template: https://en.cppreference.com/w/cpp/types/is_same E.g. `IsSameType::value` could be replaced with `std::is_same_v` Also you should include `cassert` instead of `assert.h` when using C++ and you do not need this include for `static_assert` to be available in C++... — fabian, Nov 28 '22 at 10:45
Note: the special properties of the `main` function make a declaration of the `main` function pointless in 99.99% of all cases. It must not be called anywhere in your program, so knowing the exact signature is usually of no benefit to you... — fabian, Nov 28 '22 at 10:53

score 5 · Accepted Answer · answered Nov 28 '22 at 10:41

This is called reference collapsing:

It is permitted to form references to references through type manipulations in templates or typedefs, in which case the reference collapsing rules apply: rvalue reference to rvalue reference collapses to rvalue reference, all other combinations form lvalue reference

(emphasis added)

Which means that IntRRef which is int&& is not the same as IntRLRef because the latter is defined as IntRRef& which is an lvalue reference to an rvalue reference and thus collapses to an lvalue reference: int&.

rvalue Reference not matching

1 Answers1