Can I declare variables of different types in the

问题:

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• Is it possible to declare two variables of different types in a for loop? 6 answers

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?

回答1:

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 = { }.

回答2:

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...

回答3:

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;

回答4:

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

回答5:

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.