For 循环在它自己的花括号内

2022-01-04 00:00:00 for-loop scope c++

我遇到过这种 for 循环布局:

I have come across this for-loop layout:

#include <iostream>
int main()
{
    {
        for (int i = 0; i != 10; ++i)
        {
            std::cout << "delete i->second;" << std::endl;
        }
    }

    {
        for (size_t i = 0; i < 20; ++i)
        {
            std::cout << "delete m_indices[i];" << std::endl;
        }
    }
    return 0;
}

我想知道这层额外的牙套是做什么用的?这在我们的代码库中出现了几次.

I was wondering what this extra layer of braces is for? This appears a few times in our code base.

推荐答案

很久很久以前，VS6 存在并且很流行.然而，它不符合许多 C++ 标准；这在当时是合理的，因为它是在标准正式发布之前(同年)发布的；然而，据我所知，它确实遵守了标准草案.

Once upon a time, many moons ago, VS6 existed and was popular. It failed however to conform to a number of C++ standards; which was reasonable at the time as it was released just before (on the same year) the standard was officially released; it did however adhere to the draft of the standard as far as I'm aware.

草案和官方标准之间变化的一个标准是第一部分中创建的 for 循环变量的生命周期；导致以下代码无法编译

One of the standards that changed between the draft and the official standard, was the lifetime of for loop variables created in the first section; leading to the following code failing to compile

{
    for (int i=0; i<1; ++i){}
    for (int i=0; i<2; ++i){}
}

因为 i 被第二个 for 循环重新定义.

because i was redefined by the second for loop.

虽然其他编译器也有这个bug；我强调 VS6 版本，因为它在标准发布后的几年内仍然是 Visual Studio 的唯一版本，但从未针对此特定问题发布更新；这意味着它产生了更显着的影响.

While other compilers also suffered this bug; I highlight the VS6 one because it remained the only version of visual studio for a number of years after the release of the standard, but never released an update for this particular issue; meaning that it had a more significant impact.

对此的解决方案是强制整个 for 循环进入其自己的范围，如您所示.

A solution to this is to force the whole for loop into its own scope as you have shown.

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