标准库是C++的重要组成部分，它提供了非常丰富的能力，包括输入输出操作、字符串处理、容器、算法、数值运算等多个方面。

下面介绍常见的部分。

命名空间std

标准库的组件都放在命名空间std中，避免命名冲突。

建议在代码中使用std::前缀，而不是using namespace std;，尤其是在大型项目中。

#include <iostream>

int main() {
    std::cout << "Hello, World!" << std::endl;
    return 0;
}

常用的标准库函数

比如数学函数，<cmath>头文件提供了许多数学函数。

计算平方根：sqrt

#include <iostream>
#include <cmath>

int main() {
    double number = 16.0;
    double squareRoot = sqrt(number);
    std::cout << "The square root of " << number << " is " << squareRoot << std::endl;
    return 0;
}

计算幂：pow

#include <iostream>
#include <cmath>

int main() {
    double base = 2.0;
    double exponent = 3.0;
    double result = pow(base, exponent);
    std::cout << base << " raised to the power of " << exponent << " is " << result << std::endl;
    return 0;
}

标准输入输出流库（iostream）

标准输入输出流

#include <iostream>

int main() {
    int number;
    std::cout << "Please enter a number: ";
    std::cin >> number;
    std::cout << "You entered: " << number << std::endl;
    return 0;
}

格式化输入输出

#include <iostream>
#include <iomanip>

int main() {
    double pi = 3.14159265;
    std::cout << "Pi is approximately " << std::fixed << std::setprecision(2) << pi << std::endl;
    return 0;
}

字符串库（string）

创建和初始化字符串

#include <iostream>
#include <string>

int main() {
    std::string str1 = "Hello, World!";
    std::string str2;
    str2 = "Hello again!";
    std::cout << str1 << std::endl;
    std::cout << str2 << std::endl;
    return 0;
}

字符串拼接

#include <iostream>
#include <string>

int main() {
    std::string greeting = "Hello";
    std::string name = "Alice";
    std::string message = greeting + " " + name + "!";
    std::cout << message << std::endl;
    return 0;
}

字符串查找和修改

#include <iostream>
#include <string>

int main() {
    std::string text = "Hello, World!";
    size_t found = text.find("World");
    if (found != std::string::npos) {
        std::cout << "Found 'World' at position: " << found << std::endl;
        text.replace(found, 5, "C++");
        std::cout << "Modified string: " << text << std::endl;
    }
    return 0;
}