C++程序线程休眠前字符串无法输出问题咨询

我正在开发一款文本游戏，初始阶段用户需要完成测试，测试结束后程序会评分并展示成绩。为了营造评分需要时间的效果，我想让程序暂停一会儿，但现在的问题是，程序会直接进入休眠状态，"Grading..."字符串根本没机会显示；可一旦注释掉线程休眠的代码，这个字符串就能正常显示了。

我的代码如下：

#include <iostream>
using namespace std;
int main(){
  char questionAnswer;
  int randomAnswer;
  string testAnswers, testKey;
   //create test answers
    for(int answer = 0; answer < 5; answer++){
      randomAnswer = rand()%4+1;
      switch(randomAnswer){
        case 1:
          testKey += 'A';
          break;
        case 2:
          testKey += 'B';
          break;
        case 3:
          testKey += 'C';
          break;
        case 4:
          testKey += 'D';
          break;
      }
    }
   //start test
    for(int question = 0; question < 5; question++){
      cout << "Question " << question+1 << ":\t";
      cin >> questionAnswer;
      while(toupper(questionAnswer) != 'A' && toupper(questionAnswer) != 'B' && toupper(questionAnswer) != 'C' && toupper(questionAnswer) != 'D'){
        cout << "\nInput Valid Answer:\t";
        cin >> questionAnswer;
      }
      testAnswers += toupper(questionAnswer);
      cout << endl;
    }
    cout << "\nGrading...";
    //pause fo dramatic effect
    this_thread::sleep_for(5s);
    cout << "Student Answers:\t" << testAnswers << endl;
    cout << "Test Answers:\t" << testKey << endl;
}

解决方案 #

问题根源是标准输出的缓冲区机制：cout默认采用行缓冲，只有遇到换行符\n、缓冲区填满，或者主动调用刷新操作时，才会把缓冲区里的内容输出到控制台。你的cout << "\nGrading...";没有触发缓冲刷新，内容还留在内存里，程序就直接进入休眠了。

三种解决办法： #

1. 用endl代替普通输出：endl会自动输出换行符并刷新缓冲区

   cout << "\nGrading..." << endl;
   

2. 手动调用刷新函数：在输出后直接调用flush()强制刷新

   cout << "\nGrading...";
   cout.flush();
   

3. 设置无缓冲模式：在程序开头添加一行代码，让cout实时输出内容

   cout.setf(ios::unitbuf);
   

额外的代码修复建议： #

• 使用this_thread::sleep_for需要包含头文件<thread>，否则编译会报错
• rand()生成的随机数默认是固定序列，需要用srand(time(nullptr))初始化随机种子，确保每次运行生成的测试答案不同

修改后的完整代码：

#include <iostream>
#include <thread>
#include <ctime>
using namespace std;

int main(){
  char questionAnswer;
  int randomAnswer;
  string testAnswers, testKey;

  // 初始化随机种子，确保每次生成不同答案
  srand(time(nullptr));

   // 生成测试答案
    for(int answer = 0; answer < 5; answer++){
      randomAnswer = rand()%4+1;
      switch(randomAnswer){
        case 1:
          testKey += 'A';
          break;
        case 2:
          testKey += 'B';
          break;
        case 3:
          testKey += 'C';
          break;
        case 4:
          testKey += 'D';
          break;
      }
    }
   // 开始测试
    for(int question = 0; question < 5; question++){
      cout << "Question " << question+1 << ":\t";
      cin >> questionAnswer;
      while(toupper(questionAnswer) != 'A' && toupper(questionAnswer) != 'B' && toupper(questionAnswer) != 'C' && toupper(questionAnswer) != 'D'){
        cout << "\nInput Valid Answer:\t";
        cin >> questionAnswer;
      }
      testAnswers += toupper(questionAnswer);
      cout << endl;
    }
    cout << "\nGrading..." << endl; // 用endl自动刷新缓冲区
    // 暂停营造评分耗时效果
    this_thread::sleep_for(5s);
    cout << "Student Answers:\t" << testAnswers << endl;
    cout << "Test Answers:\t" << testKey << endl;
    return 0;
}

内容的提问来源于stack exchange，提问作者Lucas Sintim