This question already has an answer here:
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?
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 = { }.
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...
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;
You can only declare one type in for statement. So the second code is the usable one.
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.
来源：https://stackoverflow.com/questions/8644707/can-i-declare-variables-of-different-types-in-the-initialization-of-a-for-loop