Re-declaring object in a for loop - C++

问题:

I do have a question on variable re-declaration in loops.

Why declaring an object in a foor loop doesn't trigger the redeclaration error?

Do the object get destroyed and recreated at each iteration of the loop?

I'm inserting a sample code

class DataBlock {
    int id;
    string data;
public:
    DataBlock(int tid=0,const string &tdata=""){
        id=tid;
        data=tdata;
    }
}

int main(int argc, char *argv[]){
    ifstream file;
    int temp_id;        //temporary hold the the id read from the file
    string temp_data;   //temporary hold the data read from the file

    set <DataBlock> s;

    //check for command line input here
    file.open(argv[1]);

    //check for file open here
    file >> temp_id >> temp_data;
    while (!file.eof()){
        DataBlock x(temp_id,temp_data);   //Legit, But how's the workflow?
        s.insert(x);
        file >> temp_id >> temp_data;
    }
    file.close();
    return 0;
}

回答1:

Why declaring an object in a foor loop doesn't trigger the redeclaration error?

A redeclaration error happens when you declare the same name twice (or more) in the same scope. That looks like

int i = 5;
int i = 6; // uh oh, you already declared i

In your loop you don't have that, you just have

loop
{
    int i = 5;
}

So no redeclaration.

You could also have

int i = 5
{
    int i = 6;
    std::cout << i;
}

And not have a redeclaration error as the variables are in different scopes and you can have the same variable in multiple scopes. I this case 6 would be print as that i is the i that is in scope.

Do the object get destroyed and recreated at each iteration of the loop?

Yes. Think of a loop as a function that gets called multiple times. When you enter the body of a loop/function the variables declared in it get constructed¹ and when you reach the end of the loop/function the variables are destroyed.

^{1: it is a little more complicated then that but we don't need to get into all those details in this answer}

回答2:

Why declaring an object in a foor loop doesn't trigger the redeclaration error?

No it does not.

Each time the for loop iterates, a new scope is entered and the objects created in the previous one are destructed and their storage allocation are freed.

for (int i=0 ; i<2 ; ++i) {
    MyClass c;
}

will be as if:

{
    int i=0;
    {
        MyClass c; // c(0)
    } // c destructed, storage allocation freed
    ++i;
    {
        MyClass c; // c(1)
    } // c destructed, storage allocation freed
    ++i;
}

c(0) and c(1) do share the same name, but in scopes without overlapping. Everything is fine.