5 ms·

I'm not a C++ programmer; what is the difference between a const pointer and a reference?

thrownaway2424 13y ago
A const pointer could be NULL, among many other differences.
sirmarksalot 13y ago
The syntax is different, but in terms of what you can do, the only difference I know is that references can be reassigned. Both give you the guarantee that it's not pointing at the NULL address. In the case of const pointers, that's because you can't reassign it, and you'd never deliberately assign a const pointer to NULL. In the case of references, it's because there's actually no such thing as a null reference in C++. That's not the case in Java and C#, which allow "null" to be a right-hand value for all types.
- verroq 13y ago
  >the only difference I know is that references can be reassigned. Uhh no. Neither the const pointer nor the reference can be reassigned. And you have it the wrong way around. References cannot be null, pointers can. The other difference is that if you try and dynamic_cast a reference and it failed, it throws an exception where as a pointer just returns nullptr. >That's not the case in Java and C#, which allow "null" to be a right-hand value for all types. That's also wrong, primitives are not nullable.
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  - sirmarksalot 13y ago
    Sorry, somehow I managed to forget that primitives existed for that brief moment. I was reacting to the realization that "null" is a bizarre special case for the type system. I was wrong on reinitializing references as well. Thanks for pointing that out. As far as const pointers, I meant that they were unlikely to be null, since they would have to be initialized to null. Of course, then I realized that you could easily assign them to another null pointer that you didn't realize was null. Not a good day for me, but I don't think it warrants that kind of snarkiness. To my credit, please reread the part where I said "there is actually no such thing as a null reference."
    - verroq 13y ago
      Well there is Nullable<T> for C#. But I haven't touched that language in ages.
  - mmorett 13y ago
    The best part of your reply is in showing how he failed his own interview, figuratively speaking of course. In real life, the candidate doesn't get to explain things the day after they were turned down because of bad answers. I truly believe he knows his stuff, but even he made a mistake. He, however, didn't have to come up with his answer while people are staring at him.
seanmcdirmid 13y ago
A reference looks like a value but is actually a point (de-references are automatic). Most high-level languages prefer references over pointers as they are harder to screw up. E.g. myref = otherref + 10 is equivalent to * myptr = * otherptr + 10 Now, most languages also have a notion of values that are neither pointers nor references, and primitives generally are expressed as values, but hopefully you get the point.
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krob 13y ago
Pointer allows you to dereference a memory address. A reference is the physical memory address variable. If you have a variable and you want to pass-by-reference, making any changes to the original variable effect the originally allocated value. You use a pointer for instance if you want to dereference a physical memory address as part of an array of allocated memory addresses. // reference example int value = 0; int value1 = &value; value1 = 2; // value should equal 2 // pointer example int value[5] = {1,2,3,4,5}; *value += 1;value++; *value += 1;value++; *value += 1;value++; *value += 1;value++; *value += 1; // should now be {2,3,4,5,6}
- dnc 13y ago
  I'm not c++ programmer, so please correct me if I'm wrong, but I think that the reference example is not correct. Shouldn't it be as bellow? // reference example int value = 0; int& value1 = value; value1 = 2; // value should equal 2
  - thrownaway2424 13y ago
    Yes. And this subthread nicely proves the parent: At least half of people who claim to program in C++ can't.
  - Tyr42 13y ago
    That is more correct, yes.

A const pointer could be NULL, among many other differences.

The syntax is different, but in terms of what you can do, the only difference I know is that references can be reassigned. Both give you the guarantee that it's not pointing at the NULL address. In the case of const pointers, that's because you can't reassign it, and you'd never deliberately assign a const pointer to NULL. In the case of references, it's because there's actually no such thing as a null reference in C++. That's not the case in Java and C#, which allow "null" to be a right-hand value for all types.

verroq 13y ago
>the only difference I know is that references can be reassigned. Uhh no. Neither the const pointer nor the reference can be reassigned. And you have it the wrong way around. References cannot be null, pointers can. The other difference is that if you try and dynamic_cast a reference and it failed, it throws an exception where as a pointer just returns nullptr. >That's not the case in Java and C#, which allow "null" to be a right-hand value for all types. That's also wrong, primitives are not nullable.
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- sirmarksalot 13y ago
  Sorry, somehow I managed to forget that primitives existed for that brief moment. I was reacting to the realization that "null" is a bizarre special case for the type system. I was wrong on reinitializing references as well. Thanks for pointing that out. As far as const pointers, I meant that they were unlikely to be null, since they would have to be initialized to null. Of course, then I realized that you could easily assign them to another null pointer that you didn't realize was null. Not a good day for me, but I don't think it warrants that kind of snarkiness. To my credit, please reread the part where I said "there is actually no such thing as a null reference."
  - verroq 13y ago
    Well there is Nullable<T> for C#. But I haven't touched that language in ages.
- mmorett 13y ago
  The best part of your reply is in showing how he failed his own interview, figuratively speaking of course. In real life, the candidate doesn't get to explain things the day after they were turned down because of bad answers. I truly believe he knows his stuff, but even he made a mistake. He, however, didn't have to come up with his answer while people are staring at him.

seanmcdirmid 13y ago

A reference looks like a value but is actually a point (de-references are automatic). Most high-level languages prefer references over pointers as they are harder to screw up. E.g. myref = otherref + 10 is equivalent to * myptr = * otherptr + 10 Now, most languages also have a notion of values that are neither pointers nor references, and primitives generally are expressed as values, but hopefully you get the point.

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krob 13y ago

Pointer allows you to dereference a memory address. A reference is the physical memory address variable. If you have a variable and you want to pass-by-reference, making any changes to the original variable effect the originally allocated value. You use a pointer for instance if you want to dereference a physical memory address as part of an array of allocated memory addresses. // reference example int value = 0; int value1 = &value; value1 = 2; // value should equal 2 // pointer example int value[5] = {1,2,3,4,5}; *value += 1;value++; *value += 1;value++; *value += 1;value++; *value += 1;value++; *value += 1; // should now be {2,3,4,5,6}

dnc 13y ago
I'm not c++ programmer, so please correct me if I'm wrong, but I think that the reference example is not correct. Shouldn't it be as bellow? // reference example int value = 0; int& value1 = value; value1 = 2; // value should equal 2
- thrownaway2424 13y ago
  Yes. And this subthread nicely proves the parent: At least half of people who claim to program in C++ can't.
- Tyr42 13y ago
  That is more correct, yes.