Question

我使用istringstream逐字逐句阅读string。但是，当我的条件失败时，我需要能够在读取上一个单词之前将istringstream还原为。我的示例代码有效，但我想知道是否有更直接的方法来使用流来完成此任务。

std::string str("my string");
std::istringstream iss(str);

std::ostringstream ossBackup << iss.rdbuf(); // Writes contents of buffer and in the process changes the buffer
std::string strBackup(ossBackup.str());      // Buffer has been saved as string

iss.str(strBackup);                          // Use string to restore iss's buffer
iss.clear();                                 // Clear error states
iss >> word;                                 // Now that I have a backup read the 1st word ("my" was read)

// Revert the `istringstream` to before the previous word was read.
iss.str(strBackup);                         // Restore iss to before last word was read
iss.clear();                                // Clear error states
iss >> word;                                // "my" was read again

Answer 1

如果您愿意，可以使用tellg()和seekg()保存并恢复您的职位：

#include <string>
#include <sstream>

int main()
{
    std::istringstream iss("some text");

    std::string word;

    // save the position
    std::streampos pos = iss.tellg();

    // read a word
    if(iss >> word)
        std::cout << word << '\n';

    iss.clear(); // clear eof or other errors
    iss.seekg(pos); // move to saved position

    while(iss >> word)
        std::cout << word << '\n';

}

Answer 2

这实际上只能保证适用于stringstream，但你可以反复调用unget()，直到你到达空格字符：

#include <iostream>
#include <sstream>

template <int n>
std::istream& back(std::istream& is)
{
    bool state = is.good();

    auto& f = std::use_facet<std::ctype<char>>(is.getloc());
    for (int i = 0; i < n && is; ++i)
        while (is.unget() && !f.is(f.space, is.peek()));

    if (state && !is)
        is.clear();
    return is;
}

int main()
{
    std::stringstream iss("hello world");
    std::string str;

    std::cout << "Value Before: ";
    iss >> str;

    std::cout << str << std::endl;
    iss >> back<1>; // go back one word

    std::cout << "Value after: ";
    iss >> str;

    std::cout << str;
}

Live Demo

如何保存和恢复std :: istringstream的缓冲区？

2 个答案: