I am confusing with the "'iter' was not declared in this scope" error.
#include <vector>
using std::vector;
int main()
{
vector<int> vec{1,2,3,4,5,6};
for(std::size_t i,vector<int>::iterator iter=vec.begin();iter!=vec.end();++i,++iter)
{
//do something
}
}
Just like you can do
int a = 10, b = 20;
A for loop's first section does the same thing. Since you can't do
int a = 10, double b = 20;
The same thing applies to the for loop.
Technically you can have two different types as you can declare an type and a pointer to that same type on the same line like
int i = 0, *b = nullptr;
And that is also valid to do in a for loop.
In C++17, there is a feature known as a structured binding declaration that allows you do to do this. For example:
for (auto [iter, i] = std::tuple{vec.begin(), 0u}; iter != vec.end(); ++iter, ++i)
{
/* ... */
}
While this nearly matches the syntax you were hoping for, the tuple part is not very readable, so I would just declare one of the variables outside of the loop.
It can, but both variables need to be the same type.
The common thing to do, when you need to maintain an index as well as an iterator is to write
{ // extra scoping block
std::size_t i = 0; // indeed, you need to initialise i
for(vector<int>::iterator iter = vec.begin(); iter != vec.end(); ++i,++iter)
{
// do something
}
}
