Can I declare variables of different types in the initialization of a for loop?

Question

Why does this C++ code not compile under VS2010:

for ( int a = 0, short b = 0; a < 10; ++a, ++b ) {}

while this one does:

short b = 0;
for ( int a = 0; a < 10; ++a, ++b ) {}

Is the declaration of two variables of different types inside the for-loop initializer prohibited? If so, how can you work around it?

Looks like you say. Using the same type works: `for ( int a = 0, b = 0; a < 10; ++a, ++b ) {}` Hmm. Never noticed that before. — juergen d, Dec 27 '11 at 12:41
Yes this is the "double loop" mechanism that came from the C language. Quite rare, though. Not very readable. — Raveline, Dec 27 '11 at 12:45

Johannes Schaub - litb · Accepted Answer · 2011-12-27T13:04:00.680

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Yes, that is prohibited. Just as otherwise you cannot declare variables of differing types in one declaration statement (edit: modulo the declarator modifiers that @MrLister mentions). You can declare structs

for (struct { int a = 0; short b = 0; } d; d.a < 10; ++d.a, ++d.b ) {}

C++03 code:

for (struct { int a; short b; } d = { 0, 0 }; d.a < 10; ++d.a, ++d.b ) {}

Of course when all are 0, you can omit the initializers altogether and write = { }.

edited Dec 27 '11 at 13:04

answered Dec 27 '11 at 12:41

Johannes Schaub - litb

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I hate the fact that you have shared this. I bet we will spend the next year answering silly variations on the usage of this. When in reality we never want to see it again. – Martin York Dec 27 '11 at 12:48
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@Loki: This wasn't exactly the first time the `for struct` hack appeared on SO. – fredoverflow Dec 27 '11 at 14:38
@Fred http://stackoverflow.com/a/889001/34509 – Johannes Schaub - litb Dec 27 '11 at 15:27
Not working with MSVC12. Workaround for 2 variable declarations: `for (auto i = std::make_pair(myVec1Iter.begin(), myVec2Iter.begin()); ... ; ...) {...}` – user1056903 Aug 09 '16 at 01:08

Mr Lister · Answer 2 · 2011-12-28T12:31:33.727

Nothing to do with the for loop. This also doesn't compile if you write int a = 0, short b = 0; outside of any loop.
So the answer is: it is always forbidden to declare two variables of different types in a single statement.

Edit: Oh, for the pedantic, I do realise that you can declare a base type and a pointer type in the same statement, for instance an int and an int pointer, so those would be different types, yes.
Hm, that makes me think. In a 32 bit environment, a pointer would be 4 bytes, just like an int, so you could use short a = 0, *b = 0; and then cast b to an int. Hm...

score 1 · Answer 3 · answered Dec 27 '11 at 12:51

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What is prohibited is the ending of a statement with a comma as you do in int a = 0, short ...

If you want to use this notation then bothe variable muss have the same type int i = 0, s = 0;

answered Dec 27 '11 at 12:51

A4L

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score 0 · Answer 4 · answered Dec 27 '11 at 12:41

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You can only declare one type in for statement. So the second code is the usable one.

answered Dec 27 '11 at 12:41

RonzyFonzy

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score 0 · Answer 5 · edited May 03 '13 at 17:34

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You can't declare a variable into the while condition of the for (the comprobation step).

for ( a = 0, short b = 0; a < 10; ++a, ++b ) {}

that's neither works.

edited May 03 '13 at 17:34

Shafik Yaghmour

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answered Dec 27 '11 at 12:42

Leandro Bardelli

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Can I declare variables of different types in the initialization of a for loop?

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