清除 cout 缓冲区 (c++)

Question

您好，我正在编写一个小程序来实现 shell，但我运行遇到了一个不寻常的问题。出于某种原因，我无法清除 std::cout 缓冲区。该程序不会打印出消息。我知道一个简单的解决方案是切换到 std::cerr，但是有没有办法让消息用 cout 打印？我尝试过的事情：

std::cout.flush()
在将任何内容写入标准输出后插入 std::endl。
正在将 std::flush 插入输出流
std::cout.setf(std::ios::unitbuf); 我发现应该取消缓冲输出。

非常感谢任何帮助，这是我的代码：

int main()
{
    //Tryed this to unbuffer cout, no luck.
    std::cout.setf(std::ios::unitbuf);

    std::string input;

    //Print out shell prompt and read in input from keyboard.
    std::cout << "myshell> ";   
    std::getline(std::cin, input);

    //**********************************************************************
    //Step 1) Read in string and parse into tokens.
    //**********************************************************************

    char * buf = new char[input.length() + 1];
    strcpy(buf, input.c_str());

    int index = 0;
    char * command[256]; 

    command[index] = std::strtok(buf, " ");    //Get first token.
    std::cout << command[index] << std::endl;

    while (command[index] != NULL)
    {
        ++index;
        command[index] = std::strtok(NULL," ");    //Get remaining tokens.
        std::cout << command[index] << std::endl;
    }   

    std::cout.flush(); //No luck here either

    //HERE IS WHERE MY PROBLEM IS.
    std::cout << index << " items were added to the command array" << std::endl;

    delete[] buf;

    return 0;   
}

Answer 1

问题是您在 while 循环的最后一次迭代中将 NULL 发送到 cout，这会导致 UB，并且在您的情况下会干扰 cout。在向 cout 发送任何内容之前检查 NULL 就可以了：

if (command[index] != NULL) {
    std::cout << command[index] << std::endl;
}

Answer 2

如果您需要了解您的流发生了什么，请记住它们可以携带状态信息 (the iostate, which I recommend you read about)。以下代码可能有助于跟踪您的错误：

 try {
      std::cout.exceptions(std::cout.failbit);          
 } catch(const std::ios_base::failure& e) {
      std::cerr << "stream error: " << e.what() << std::endl;
      std::cout.clear();
 }

 // continue working with cout, because std::cout.clear() removed
 // failbit

或者，更简单：

if(not std::cout) {
    // address your error (if it is recoverable)
 }

这就是您的代码的样子：

#include <cstring>
#include <string>
#include <iostream>
int main()
{
    //Tryed this to unbuffer cout, no luck.
    std::cout.setf(std::ios::unitbuf);

    std::string input;

    //Print out shell prompt and read in input from keyboard.
    std::cout << "myshell> ";   
    std::getline(std::cin, input);

    //**********************************************************************
    //Step 1) Read in string and parse into tokens.
    //**********************************************************************

    char * buf = new char[input.length() + 1];
    strcpy(buf, input.c_str());

    int index = 0;
    char * command[256]; 

    command[index] = std::strtok(buf, " ");    //Get first token.
    std::cout << command[index] << std::endl;

    while (command[index] != NULL)
    {
        ++index;
        command[index] = std::strtok(NULL," ");    //Get remaining tokens.
        std::cout << command[index] << std::endl;
    }

    // I added from here...
    if(not std::cout) {
      std::cerr << "cout is messed up... fixing it..." << std::endl;
      std::cout.clear();
    }
    // ... to here.

    std::cout.flush(); //No luck here either

    //HERE IS WHERE MY PROBLEM IS.
    std::cout << index << " items were added to the command array" << std::endl;

    delete[] buf;

    return 0;   
}

结果：

$ ./a.out
myshell> 1 2 3
1
2
3
cout is messed up... fixing it...
3 items were added to the command array

清除 cout 缓冲区 (c++)

Clearing the cout buffer (c++)

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flush

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