C++ 入门 + 进阶训练

#include <iostream>
using namespace std;          // 引入标准命名空间

int main() {
    cout << "Hello, C++!" << endl;   // 输出到标准输出
    int x;
    cin >> x;                         // 从标准输入读
    cout << "x = " << x << "\n";
    return 0;
}

g++ -std=c++17 -Wall -Wextra hello.cpp -o hello
./hello

#include <iostream>
using namespace std;

void swap(int& a, int& b) {     // 引用传参，直接改外部
    int t = a; a = b; b = t;
}

int main() {
    int x = 1, y = 2;
    swap(x, y);
    cout << x << " " << y << "\n";   // 2 1
    return 0;
}

#include <iostream>
using namespace std;

int add(int a, int b = 0) {        // 默认参数（从右往左给）
    return a + b;
}
int add(double a, double b) {       // 函数重载：同名不同参数
    return (int)(a + b);
}
inline int square(int x) { return x * x; }  // 建议内联，减少小函数调用开销

int main() {
    cout << add(3) << "\n";      // 3  （b 用默认值 0）
    cout << add(3, 4) << "\n";   // 7
    return 0;
}

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

int main() {
    string s = "hello";
    s += " world";          // 拼接
    s.append("!");
    cout << s << " len=" << s.size() << "\n";   // hello world! len=12
    cout << s.substr(0, 5) << "\n";              // hello
    if (s.find("world") != string::npos) cout << "found\n";
    return 0;
}

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

class Student {
private:
    int id;
    string name;
public:
    void set(int i, string n) { id = i; name = n; }
    void show() const { cout << id << " " << name << "\n"; }
    int getId() const { return id; }   // const 成员函数
};

int main() {
    Student s;
    s.set(1, "XiaoMing");
    s.show();
    return 0;
}

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

class Buffer {
    char* data;
    int len;
public:
    Buffer(int n) : len(n) {            // 初始化列表
        data = new char[n];
    }
    ~Buffer() { delete[] data; }        // 析构：释放资源

    // 拷贝构造（深拷贝）
    Buffer(const Buffer& o) : len(o.len) {
        data = new char[len];
        memcpy(data, o.data, len);
    }
    // 拷贝赋值
    Buffer& operator=(const Buffer& o) {
        if (this != &o) {               // 防自赋值
            delete[] data;
            len = o.len;
            data = new char[len];
            memcpy(data, o.data, len);
        }
        return *this;
    }
};

int main() {
    Buffer a(10);
    Buffer b = a;     // 调用拷贝构造（深拷贝，互不影响）
    return 0;
}

#include <iostream>
using namespace std;

class Animal {
public:
    void eat() { cout << "eating\n"; }
protected:
    int legs = 4;                 // 子类可见，对外不可见
};

class Dog : public Animal {       // 公有继承 = is-a
public:
    void bark() { cout << "wang, legs=" << legs << "\n"; }
};

int main() {
    Dog d;
    d.eat();     // 继承自 Animal
    d.bark();
    return 0;
}

#include <iostream>
using namespace std;

class Shape {
public:
    virtual double area() const = 0;   // 纯虚函数 → 抽象类，不能实例化
    virtual ~Shape() = default;        // 多态基类：析构必须 virtual
};

class Circle : public Shape {
    double r;
public:
    Circle(double r) : r(r) {}
    double area() const override { return 3.14159 * r * r; }
};

int main() {
    Shape* s = new Circle(2.0);
    cout << s->area() << "\n";   // 12.56... 调的是 Circle::area
    delete s;                    // 虚析构保证调用 Circle 析构
    return 0;
}

#include <iostream>
using namespace std;

class Vec {
    int x, y;
public:
    Vec(int x, int y) : x(x), y(y) {}
    Vec operator+(const Vec& o) const { return Vec(x + o.x, y + o.y); }
    bool operator==(const Vec& o) const { return x == o.x && y == o.y; }
    friend ostream& operator<<(ostream& os, const Vec& v) {
        return os << "(" << v.x << "," << v.y << ")";
    }
};

int main() {
    Vec a(1, 2), b(3, 4);
    cout << (a + b) << " " << (a == b) << "\n";   // (4,6) 0
    return 0;
}

#include <iostream>
using namespace std;

template <typename T>
T maxof(T a, T b) { return a > b ? a : b; }

template <typename T>
class Box {
    T v;
public:
    Box(T v) : v(v) {}
    T get() const { return v; }
};

int main() {
    cout << maxof(3, 7) << "\n";          // 7
    cout << maxof(3.14, 2.7) << "\n";     // 3.14
    Box<string> b("hi");
    cout << b.get() << "\n";               // hi
    return 0;
}

#include <iostream>
#include <vector>
#include <map>
#include <algorithm>
using namespace std;

int main() {
    vector<int> v = {5, 2, 9, 1};
    sort(v.begin(), v.end());                  // 算法 + 迭代器
    for (int x : v) cout << x << " ";          // 范围 for：1 2 5 9
    cout << "\n";

    map<string, int> m;                        // 有序键值对（红黑树）
    m["alice"] = 90;
    m["bob"] = 80;
    if (m.count("alice")) cout << m["alice"] << "\n";

    auto it = find(v.begin(), v.end(), 5);     // 算法 find
    if (it != v.end()) cout << "found 5\n";
    return 0;
}

#include <iostream>
using namespace std;

int main() {
    int* p = new int(42);
    cout << *p << "\n";
    delete p;                 // 配 new（不是 free）
    p = nullptr;

    int* arr = new int[10];   // 数组用 delete[]
    delete[] arr;
    return 0;
}

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

int main() {
    unique_ptr<int> u = make_unique<int>(10);  // 独占所有权
    cout << *u << "\n";                        // 离开作用域自动 delete

    shared_ptr<int> s1 = make_shared<int>(20); // 引用计数共享
    shared_ptr<int> s2 = s1;                   // 计数 +1
    cout << s1.use_count() << "\n";            // 2
    return 0;                                  // 计数归零自动释放
}

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

int divide(int a, int b) {
    if (b == 0) throw runtime_error("div by zero");
    return a / b;
}

int main() {
    try {
        cout << divide(10, 0) << "\n";
    } catch (const exception& e) {        // 按基类引用捕获
        cerr << "error: " << e.what() << "\n";
    }
    return 0;
}

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

int main() {
    vector<int> v = {5, 2, 9, 1, 7};
    sort(v.begin(), v.end(), [](int a, int b) { return a > b; }); // 降序
    for (int x : v) cout << x << " ";
    cout << "\n";

    int base = 3;
    auto f = [base](int x) { return x + base; };  // 值捕获 base
    cout << f(10) << "\n";                        // 13
    return 0;
}

#include <iostream>
#include <vector>
#include <utility>
using namespace std;

int main() {
    vector<int> a = {1, 2, 3};
    vector<int> b = move(a);   // 把 a 的资源移动给 b，a 变为空（可析构状态）
    cout << "b=" << b.size() << " a=" << a.size() << "\n";

    auto it = b.begin();       // auto 自动推导类型
    for (auto& x : b) x *= 2;  // 范围 for + 引用修改
    return 0;
}