C ++指针和内存释放

Question

I'm less used to rigid type-checking and lower level stuff such as deallocation. Recently, while attempting to write something along the lines of

// Product is a struct I defined earlier, nothing fancy
vector<string, Product>::iterator it;
// Do some stuff with the iterator...
// And now I know I won't be using the iterator again
map<string, Product>::iterator it;

However, this yielded the error 'duplicate declaration of local variable "it"'. So my question then is simply, why did this happen? I researched memory deconstruction, but so far as I'm aware it's apparently impossible to deconstruct iterators because they're actually pointers? Does C++ really not allow you to use the same name for different variables if you know you won't need the old one again, or normally would you release them into memory?

Answer 1

Since it is in the same scope , it cannot be re-declared: variable shadowing does not apply in the same scope.

One way round this is to use scoping blocks:

// Product is a struct I defined earlier, nothing fancy
{
    vector<string, Product>::iterator it;
    // Do some stuff with the iterator...
    // And now I know I won't be using the iterator again
}

{
    map<string, Product>::iterator it;
    // However, this yielded the error 'duplicate declaration 
    // Not any more!
}

In C++ you actually only need one of the scoping blocks but that could be construed as an obfuscating asymmetry. Java attempts to sort this out but fails miserably in that you can remove the second pair but not the first!

Answer 2

It doesn't matter what language you use, if you give two different variables the same name in the same scope (and they are meant to be different things) you will just [edit] confuse everyone who comes along after you (including in this case yourself). And that counts double for people who have to maintain your code.

Get out of that habit as soon as possible. It doesn't matter what the language offers you. Just don't do it.