C ++入门的问题(C++ Primer Questions)

 我目前正在通过Lahoie，Lippman和Moo第五版的C ++ Primer，并且一直在努力解决一些问题。 
 
 首先，我只是想确认，在使用任何cctype函数时，我必须确保包含标题，对吗？ 因为，最初，我忘了包含它，但它仍然运行。 这真让我困惑。 
 
 此外，我正在浏览一个不同的问题（我会去的），并发现另一个问题哈哈！ 当使用cctype任何东西时，我是否应该std::/write using std::将其写为std::/write using std::例如，如果我使用std::tolower在每个实例处写入std::tolower /为其写入using语句。 这是有道理的，因为它确实表示它们是“在std命名空间中定义的”，但我没有意识到并一直在写它，没有问题。 我猜size_t类似，对吧？ 
 
 说到size_t ，我有一个问题。 这是我的代码： 
 
// Exercise Section 3.5.2., Exercise 3.30

#include <iostream>
#include <cstddef>

using std::cout; using std::endl;

int main()
{
    constexpr size_t num_size = 10;
    int num[num_size] = {};

    for (size_t n = 0; n < num_size; ++n) {
        num[n] = n;
        cout << num[n] << endl;
    }
    return 0;
}
 
 因此，代码应该定义一个10个int的数组，并为每个元素赋予与其在数组中的位置相同的值。 
 
 它运行正常，但在num[n]=n部分收到错误。 它说Implicit conversion loses integer precision: size_t (aka 'unsigned long') to int 。 
 
 我明白这意味着什么，但我的问题是，本书说“当我们使用变量来为数组下标时，我们通常应该定义该变量来输入size_t ”。 我已经做到了这一点，它给出了这个错误。 它确实运行良好，但它似乎可能导致错误的那种事情。 
 
 PS在这段代码中，就像我上面提到的那样，我应该using std::size_t吗？ 

I am currently going through the 5th edition of C++ Primer by Lahoie, Lippman and Moo and have been struggling with a few things.
 
Firstly, I just wanted to confirm, when using any of the cctype functions, I have to make sure I include the header, right? Because, initially, I forgot to include it and yet it still ran. That's really confused me. 
 
Also, I was browsing for a different problem (which I'll get to) and found another issue haha! When using anything from cctype, are I supposed to write it as std::/write using std::, e.g. if I use the tolower either write std::tolower at every instance/write a using statement for it. This would make sense as it did say that they are "defined in the std namespace" but I didn't realise and have been writing it without and haven't had an issue. And I'm guessing similar for size_t, right?
 
Speaking of size_t, I have an issue. This is my code:
 
// Exercise Section 3.5.2., Exercise 3.30

#include <iostream>
#include <cstddef>

using std::cout; using std::endl;

int main()
{
    constexpr size_t num_size = 10;
    int num[num_size] = {};

    for (size_t n = 0; n < num_size; ++n) {
        num[n] = n;
        cout << num[n] << endl;
    }
    return 0;
}
 
So the code is supposed define an array of 10 ints and give each element the same value as its position in the array.
 
It runs correctly, but I am receiving an error at the num[n]=n part. It says Implicit conversion loses integer precision: size_t (aka 'unsigned long') to int.
 
I understand what this means, but my issue is that the book says "when we use a variable to subscript an array, we normally should define that variable to type size_t". I have done this and it gives this error. It does run fine but it seems like that sort of thing that can lead to errors.
 
P.S. In this code, like I asked above, should I have using std::size_t?

原文：https://stackoverflow.com/questions/24855637