在for循环中重新声明对象-C ++ [英] Re-declaring object in a for loop - C++

问题描述

我确实对循环中的变量重新声明有疑问.

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

为什么在foor循环中声明对象不会触发重新声明错误?

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

对象是否在循环的每次迭代中都被破坏并重新创建?

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

我要插入示例代码

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;
}

推荐答案

为什么在foor循环中声明对象不会触发重新声明错误?

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

当您在同一范围内两次(或多次)声明同一名称时，发生重新声明错误.看起来像

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

在循环中您没有，而只有

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

loop
{
    int i = 5;
}

因此无需重新声明.

您也可以拥有

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

也没有重新声明错误，因为变量在不同的作用域中，并且您可以在多个作用域中使用相同的变量.在这种情况下，将打印6，因为i是范围内的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.

对象是否在每次循环迭代时都被销毁并重新创建?

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

是的.将循环视为多次调用的函数.当您输入循环/函数的主体时，在其中声明的变量将被构造 ¹ ，而当您到达循环/函数的末尾时，变量将被破坏.

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:比这稍微复杂一点，但是我们不需要在这个答案中深入了解所有这些细节}

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