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 — t.niese
– t.niese, Commented May 9, 2019 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
– Coral Kashri, Commented May 9, 2019 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
– Lightness Races in Orbit, Commented May 9, 2019 at 14:55

vahancho · Accepted Answer · 2019-05-09 14:12:09Z

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 9, 2019 at 14:12

vahancho

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2 Comments

NathanOliver Over a year ago

This is probably the best option as long as you want them all to be the same type. +1.

Lightness Races in Orbit Over a year ago

I'm not a big fan of the type hiding tbh, but aList and bList are lexically near so I guess it's okay

t.niese · Accepted Answer · 2019-05-09 14:24:52Z

2

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";
}

answered May 9, 2019 at 14:24

t.niese

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Comments

NathanOliver · Accepted Answer · 2019-05-09 14:11:54Z

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 9, 2019 at 14:11

NathanOliver

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Comments

doctorlove · Accepted Answer · 2019-05-09 14:13:07Z

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 9, 2019 at 14:13

doctorlove

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Comments

Coral Kashri · Accepted Answer · 2019-05-09 15:22:54Z

1

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;...) ...
// ...

edited May 9, 2019 at 15:22

answered May 9, 2019 at 14:11

Coral Kashri

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Comments

andreee · Accepted Answer · 2019-05-09 15:04:08Z

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 9, 2019 at 15:04

andreee

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