C++中的cout、cerr、clog使用及说明

#include <iostream>
using namespace std;

int main() {
    int num = 42;
    double pi = 3.14159;
    string name = "Alice";

    cout << "Hello, World!" << endl;
    cout << "Number: " << num << endl;
    cout << "Pi: " << pi << endl;
    cout << "Name: " << name << endl;

    return 0;
}

Hello, World!
Number: 42
Pi: 3.14159
Name: Alice

#include <iostream>
#include <iomanip>
using namespace std;

int main() {
    double value = 123.456789;

    // 设置宽度和填充字符
    cout << setw(10) << setfill('*') << 42 << endl;  // 输出：********42

    // 设置精度
    cout << fixed << setprecision(3) << value << endl;  // 输出：123.457

    // 科学计数法
    cout << scientific << value << endl;  // 输出：1.234568e+02

    // 布尔值以文字形式输出
    cout << boolalpha << true << endl;  // 输出：true

    return 0;
}

int a = 10, b = 20;
cout << "a = " << a << ", b = " << b << endl;  // 输出：a = 10, b = 20

#include <iostream>
#include <fstream>
using namespace std;

int main() {
    ofstream file("output.txt");
    streambuf* oldBuf = cout.rdbuf();  // 保存原来的缓冲区

    cout.rdbuf(file.rdbuf());  // 将输出重定向到文件
    cout << "This will be written to the file." << endl;

    cout.rdbuf(oldBuf);  // 恢复原来的输出
    cout << "This will be written to the console." << endl;

    file.close();
    return 0;
}

#include <iostream>
using namespace std;

int main() {
    ifstream file("nonexistent.txt");
    if (!file.is_open()) {
        cerr << "Error: Could not open file!" << endl;
        return 1;
    }

    // 其他操作
    return 0;
}

// 模拟一个长时间运行的进程
for (int i = 0; i < 1000000; ++i) {
    if (i % 100000 == 0) {
        cerr << "Processing iteration " << i << endl;  // 立即显示
    }
    // 处理逻辑
}

ofstream errorFile("errors.log");
streambuf* oldBuf = cerr.rdbuf();
cerr.rdbuf(errorFile.rdbuf());

cerr << "This error will be logged to errors.log" << endl;

cerr.rdbuf(oldBuf);  // 恢复

#include <iostream>
using namespace std;

void log(const string& message) {
    clog << "[LOG] " << message << endl;
}

int main() {
    log("Starting application...");
    
    // 程序逻辑
    
    log("Application terminated successfully.");
    return 0;
}

clog << "This is a log message.";  // 可能不会立即显示
clog << flush;  // 手动刷新缓冲区

ofstream logFile("app.log");
clog.rdbuf(logFile.rdbuf());

clog << "Logging to file..." << endl;  // 写入文件

class MyBuffer : public streambuf {
protected:
    int overflow(int c) override { //override C++11 特性，显式声明该函数重写基类的虚函数，提高代码安全性
        if (c != traits_type::eof()) { //获取流特性中定义的 EOF（文件结束符）值
            // 自定义处理逻辑
            cout << "*" << static_cast<char>(c) << "*";
        }
        return traits_type::not_eof(c);
    }
};

// 使用自定义缓冲区
MyBuffer buf;
ostream customOut(&buf);
customOut << "Test" << endl;  // 输出：*T*e*s*t*

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

mutex coutMutex;

void worker(int id) {
    lock_guard<mutex> lock(coutMutex);
    cout << "Thread " << id << " is working." << endl;
}

int main() {
    thread t1(worker, 1);
    thread t2(worker, 2);

    t1.join();
    t2.join();
    return 0;
}

class StreamRedirect {
public:
    StreamRedirect(ostream& stream, streambuf* newBuf)
        : stream(stream), oldBuf(stream.rdbuf()) {
        stream.rdbuf(newBuf);
    }

    ~StreamRedirect() {
        stream.rdbuf(oldBuf);
    }

private:
    ostream& stream;
    streambuf* oldBuf;
};

// 使用示例
ofstream file("output.txt");
{
    StreamRedirect redirect(cout, file.rdbuf());
    cout << "Redirected output" << endl;  // 写入文件
}  // 离开作用域时自动恢复