C++ for loop with multiple pointer initializers

Question

In the for loop below:

struct Block
{
    Block(int d) : data(d), next(nullptr) {}

    int data;
    Block* next;
};

Block* aList = new Block(1);
Block* bList = new Block(2);

for (Block* a = aList, *b = bList; a != nullptr; a = a->next, b = b->next)
    if (aList->data != bList->data)
        cout << "Difference found.\n";

I don't like putting the * before b, but of course it's needed to distinguish Block from Block*. Is there another way to do this? for ((Block*) a, b... isn't a go.

Not exactly the same question, but still really close to be a duplicate [Declaring multiple object pointers on one line causes compiler error](https://stackoverflow.com/questions/13618282/declaring-multiple-object-pointers-on-one-line-causes-compiler-error) — t.niese, May 09 '19 at 14:12
@TopologicalSort when you edit the question and change it, please do it in a separate section (or open a new one if it totally another - witch is not the case here), so answers that were relevant to the original question, will stay so... — Coral Kashri, May 09 '19 at 14:37
@KorelK No, don't do that. We don't want a timeline of changes. We can see old revisions in the post's history. _Integrate updates into the flow of the post,_ as the OP has done here. — Lightness Races in Orbit, May 09 '19 at 14:55

score 5 · Answer 1 · answered May 09 '19 at 14:12

5

You might do it like this:

for (auto a = aList, b = bList; a != nullptr; a = a->next, b = b->next)
    if (aList->data != bList->data)
        cout << "Difference found.\n";

answered May 09 '19 at 14:12

vahancho

20,808
3
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2

This is probably the best option as long as you want them all to be the same type. +1. – NathanOliver May 09 '19 at 14:14
I'm not a big fan of the type hiding tbh, but `aList` and `bList` are lexically near so I guess it's okay – Lightness Races in Orbit May 09 '19 at 14:33

score 2 · Accepted Answer · answered May 09 '19 at 14:24

If you don't want to repeat the * then you could use using and create an alias BlockPtr which you use instead of Block*:

int main() {
  using BlockPtr = Block*;

  BlockPtr aList = new Block(1);
  BlockPtr bList = new Block(2);

  for (BlockPtr a = aList, b = bList; a != nullptr; a = a->next, b = b->next)
    if (aList->data != bList->data)
      cout << "Difference found.\n";
}

Or relay on auto:

int main() {

  auto aList = new Block(1);
  auto bList = new Block(2);

  for (auto a = aList, b = bList; a != nullptr; a = a->next, b = b->next)
    if (aList->data != bList->data)
      cout << "Difference found.\n";
}

Coral Kashri · Answer 3 · 2019-05-09T15:22:54.960

Yes, just use:

Block* a = aList, *b = bList

EDIT:

Option 1 - Using Boost

#include <boost/typeof/typeof.hpp>
/*
    ...
*/
for (BOOST_TYPEOF_KEYWORD(Block*) a = aList, b = bList;...)

Another option is to create a single variable of the type you want, and use it's type to initialize other variables (similar to auto):

Option 2

Block* aList = new Block(1);
Block* bList = new Block(2);

for (decltype(aList) a = aList, b = bList; ...) ...

Option 3 - Using Boost

#include <boost/typeof/typeof.hpp>
/*
    Like the first option
*/
for (BOOST_TYPEOF(aList) a = aList, b = bList;...) ...
// ...

score 1 · Answer 4 · answered May 09 '19 at 14:11

1

When declaring pointers, the * belongs to the name, not the type. That means you can make b a pointer like

for (Block *a = aList, *b = bList; a != nullptr; a = a->next, b = b->next)

answered May 09 '19 at 14:11

NathanOliver

171,901
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288
402

score 1 · Answer 5 · answered May 09 '19 at 14:13

1

You are trying to declare two pointers in an expression.

Block* a = aList, b = bList;

It happens to be part of a for loop, but just as

int * a, * b;

is two int pointers, you can use

Block* a = aList, * b = bList;

in your for loop.

answered May 09 '19 at 14:13

doctorlove

18,872
2
46
62

score 0 · Answer 6 · answered May 09 '19 at 15:04

0

What about just defining a type alias?

using BlockPtr = Block*;

for (BlockPtr a = aList, b = bList; a != nullptr; a = a->next, b = b->next)
    if (aList->data != bList->data)
        cout << "Difference found.\n";

answered May 09 '19 at 15:04

andreee

4,459
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C++ for loop with multiple pointer initializers

6 Answers6

Option 1 - Using Boost

Option 2

Option 3 - Using Boost