Why c++ does not support multiple initializers in for loop? [duplicate]

Answer 1

If you want int and double both, in the initialization, then one way to do this is to define an anonymous struct! Yes, you can define struct in the for loop, as well. It seems to be a less known feature of C++:

#include <iostream>

int main()
{
    for( struct {int i; double j;} v = {0, 3.0}; v.i < 10; v.i++, v.j+=0.1)
       std::cout << v.i << "," << v.j << std::endl; 
}

Output:

0,3
1,3.1
2,3.2
3,3.3
4,3.4
5,3.5
6,3.6
7,3.7
8,3.8
9,3.9

Online demo : http://ideone.com/6hvmq

Answer 2

Because the syntax was already taken. In a variable declaration/definition, separating with commas adds new variables of the same type, not of different types. That syntax is available in for loops:

for ( std::vector<int>::const_iterator it = v.begin(), end = v.end();
      it != end; ++it ) {
   // do something here
}

Answer 3

In C++ grammar, different data types are separated with ; (if not function). In for loop, once the ; is found the meaning is changed. i.e.

for (<initializations>; <condition>; <next operation>)

Other reason is possibly to avoid complexity in an already complex grammar, this feature is not allowed.

If you want to declare variables in for loop scope, then you can always simulate that situation:

int main()
{
  {
    double j = 3.0;
    for (int i = 0; i < 10; i++, j+=0.1)
        cout << i << j << endl;
  }
    return 0;
}