c++ - Goto before variable initialization 导致编译错误

Question

考虑这段代码(VS2008):

void WordManager::formatWords(std::string const& document)
{
    document_ = document;
    unsigned int currentLineNo = 1;

    size_t oldEndOfLine = 0;
    size_t endOfLine    = document_.find('\n');
    while(endOfLine != std::string::npos)
    {
        std::string line = document_.substr(oldEndOfLine, (endOfLine - oldEndOfLine));
        if(line.size() < 2)
        {
            oldEndOfLine = endOfLine + 1;
            endOfLine    = document_.find('\n', oldEndOfLine);
            continue;
        }

        std::vector<std::string> words = Utility::split(line);
        for(unsigned int i(0); i < words.size(); ++i)
        {
            if(words[i].size() < 2)
                continue;
            Utility::trim(words[i], WordManager::delims);
            Utility::normalize(words[i], WordManager::replace, WordManager::replaceWith);

            if(ruleOne(words[i]) && ruleTwo(words[i]))
            {
                std::set<Word>::iterator sWIter(words_.find(Word(words[i])));

                if(sWIter == words_.end())
                    words_.insert(Word(words[i])).first->addLineNo(currentLineNo);
                else
                    sWIter->addLineNo(currentLineNo);
            }
        }
        ++currentLineNo;

        oldEndOfLine = endOfLine + 1;
        endOfLine    = document_.find('\n', oldEndOfLine);
    }
}

如果它很重要:这是家庭作业中的代码，用于过滤和修改文档中的单词。 document 保存文档(之前从文件中读取)

我想引入一个恶意的 goto，因为我认为在这种情况下它实际上更干净:

void WordManager::formatWords(std::string const& document)
{
    document_ = document;
    unsigned int currentLineNo = 1;

    size_t oldEndOfLine = 0;
    size_t endOfLine    = document_.find('\n');
    while(endOfLine != std::string::npos)
    {
        std::string line = document_.substr(oldEndOfLine, (endOfLine - oldEndOfLine));
        // HERE!!!!!!
        if(line.size() < 2)
            goto SkipAndRestart;

        std::vector<std::string> words = Utility::split(line);
        for(unsigned int i(0); i < words.size(); ++i)
        {
            if(words[i].size() < 2)
                continue;
            Utility::trim(words[i], WordManager::delims);
            Utility::normalize(words[i], WordManager::replace, WordManager::replaceWith);

            if(ruleOne(words[i]) && ruleTwo(words[i]))
            {
                std::set<Word>::iterator sWIter(words_.find(Word(words[i])));

                if(sWIter == words_.end())
                    words_.insert(Word(words[i])).first->addLineNo(currentLineNo);
                else
                    sWIter->addLineNo(currentLineNo);
            }
        }

SkipAndRestart:
        ++currentLineNo;

        oldEndOfLine = endOfLine + 1;
        endOfLine    = document_.find('\n', oldEndOfLine);
    }
}

此时此刻，这是否是一个好的设计选择是无关紧要的。编译器报错 error C2362: initialization of 'words' is skipped by 'goto SkipAndRestart'

我不明白这个错误。为什么跳过单词初始化很重要，而且会出错？这正是我想要发生的，我不想让它做更多的工作，只是重新启动该死的循环。 continue 宏不会做或多或少完全相同的事情吗？

Answer 1

您正在跳过 words 数组的构造:

    if(line.size() < 2)
        goto SkipAndRestart;
    std::vector<std::string> words = Utility::split(line);
    // ...
SkipAndRestart:

您可能在 SkipAndRestart: 标签之后使用了 words，这将是一个问题。你不在你的情况下使用它，但是 words 变量直到它被引入的范围的 end 才会被破坏，所以就对于编译器而言，该变量仍在标签处使用。

你可以通过将 words 放在它自己的范围内来避免这种情况:

    if(line.size() < 2)
        goto SkipAndRestart;
    {
        std::vector<std::string> words = Utility::split(line);
        // ...
    }
SkipAndRestart:

请注意，continue 语句跳转到循环的末尾，在您实际上不能放置标签的位置。这是在循环内的任何局部变量被破坏之后，但在跳回到循环顶部之前的一个点。