C ++中std :: cin对象的规则是什么？ [英] What are the rules of the std::cin object in C++?

问题描述

我正在写一个小程序，用于我个人使用实践学习C ++和其功能，一个MLA引文生成器（我正在写一个大型文件与几十个引文）。

I am writing a small program for my personal use to practice learning C++ and for its functionality, an MLA citation generator (I'm writing a large paper with tens of citations).

由于缺乏一个更好的方法来做（我不明白类或使用其他.cpp文件在你的主，所以不要打扰告诉我，我当我有更多的时间，我会工作），我为每种类型的引文写一个功能。如果我得到更多的时间，我可能会把它分解为每个重用代码的函数。

For lack of a better way to do it (I don't understand classes or using other .cpp files inside your main, so don't bother telling me, I'll work on that when I have more time), I am writing a function for each type of citation. I might break this down into a function for each reused code if I get more time.

我的问题是：std :: cin对象如何工作？我目前正在使用std :: cin >>读取字符串，我希望是单个单词，getline（std :: cin，string）用于带空格的字符串。我没有得到正确的输出，虽然。我只想知道std :: cin的工作原理，以及为什么我意外地跳过一些输入（例如，它跳过webPage，而不是给我一个机会输入它）。

My question is: how does the std::cin object work? I am currently reading in with std::cin >> for the strings I expect to be single words, and getline(std::cin, string) for the strings with spaces. I am not getting the right output, though. I just want to know how std::cin works and why I keep unexpectedly skipping over some some inputs (for instance, it skips over webPage instead of giving me a chance to input into it).

void webCit()
{
std::cout << "Leave any unknowns blank.\n";

std::cout << "Author last name: ";
std::string lastName;
std::cin >> lastName;

if (lastName.size() != 0)
{
    lastName = lastName + ", ";    
}


std::cout << "Author first name: ";
std::string firstName;
std::cin >> firstName;

if (firstName.size() != 0)
{
    firstName = firstName + ". ";
}


std::cout << "Article title: ";
std::string articleTitle;
getline(std::cin, articleTitle);

if (articleTitle.size() != 0)
{
    articleTitle = "\"" + articleTitle + ".\" ";
}

std::cout << "Title of web page: ";
std::string pageTitle;
std::cin >> pageTitle;

if(pageTitle.size() != 0)
{
    pageTitle = pageTitle + ". ";
}

std::cout << "Publication date: ";
std::string publicationDate;
getline(std::cin, publicationDate);

if(publicationDate.size() != 0)
{
    publicationDate = publicationDate + ". ";

} 

std::cout << "Web address: ";
std::string webAddress;
getline(std::cin, webAddress);

webAddress = "<" + webAddress + ">. ";

std::cout << "Date accessed: ";
std::string dateAccessed;
getline(std::cin, dateAccessed);

if(dateAccessed.size() != 0)
{
    dateAccessed = dateAccessed + ". ";
}

std::string citation =
    lastName + firstName + articleTitle + pageTitle + publicationDate + webAddress + dateAccessed;

std::cout << citation; //TEST; remove after
}

编辑：I / O

Leave any unknowns blank.
Author last name: Hooked
Author first name: Jerbear
Article title: Title of web page: title
Publication date: Web address: www.win.com
Date accessed: 4/29/09
Hooked, Jerbear. Title. <www.win.com>. 4/29/09.

正如你所看到的，出现了问题，因为我的输入被跳过了。 >

As you can see, something is going wrong, because my input is getting skipped over.

推荐答案

这里发生的是 std :: cin> firstName; 只读取但不包括第一个空白字符，其中包括换行符（或'\\\
' getline（std :: cin，articleTitle）; ，'\\\
' code>仍然是 std :: cin 中的下一个字符，而 getline（）会立即返回。 / p>



What is happening here is that std::cin >> firstName; only reads up to but not including the first whitespace character, which includes the newline (or '\n') when you press enter, so when it gets to getline(std::cin, articleTitle);, '\n' is still the next character in std::cin, and getline() returns immediately.

// cin = "Bloggs\nJoe\nMan of Steel, Woman of Kleenex\n"
std::cin >> lastName;
std::cin >> firstName;
// firstName = "Joe", lastName = "Bloggs", cin = "\nMan of Steel, Woman of Kleenex\n"
getline(std::cin, articleTitle);
// articleTitle = "", cin = "Man of Steel, Woman of Kleenex\n"

$ b b

添加 std :: cin>> std :: ws '（ ws 意思 w hite s

Adding 'std::cin >> std::ws' (ws meaning whitespace) before your calls to getline() fixes the problem:

std::cin >> firstName >> std::ws;
getline(std::cin, articleTitle);

但是，如果你在参数中做到这一点， >

But it is easier to see where you missed it if you do it in the argument:

std::cin >> firstName;
getline(std::cin >> std::ws, articleTitle);

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