C++ 指南 · 6/6

Key Concepts

本章目前仅提供英文版。

Four ideas separate idiomatic modern C++ from "C with classes": RAII, smart pointers, move semantics and templates.

RAII

RAII (Resource Acquisition Is Initialization) means an object acquires a resource in its constructor and releases it in its destructor. A local object's destructor always runs when it leaves scope, even if an exception passes through, so resources owned this way cannot leak.

#include <cstdio>
#include <stdexcept>

class File {
public:
    File(const char* path, const char* mode) : handle_{std::fopen(path, mode)} {
        if (!handle_) throw std::runtime_error("could not open file");
    }
    ~File() { std::fclose(handle_); }      // closed automatically at end of scope

    File(const File&) = delete;            // no copies: never close the same handle twice
    File& operator=(const File&) = delete;

    std::FILE* get() const noexcept { return handle_; }
private:
    std::FILE* handle_;
};

void write_log() {
    File f{"log.txt", "w"};
    std::fputs("start\n", f.get());
    // even if something below throws, f's destructor closes the file
}

The standard library works the same way: std::vector and std::string own memory, std::ifstream owns a file, std::lock_guard owns a mutex lock. Modern code rarely needs a bare new or delete.

Smart pointers

Heap objects should be owned by a smart pointer from <memory>.

Type	Ownership	How to create
`std::unique_ptr<T>`	Sole owner. Movable, not copyable	`std::make_unique<T>(...)` (C++14)
`std::shared_ptr<T>`	Shared ownership via a reference count	`std::make_shared<T>(...)`
`std::weak_ptr<T>`	Non-owning observer, used to break reference cycles	Created from a `shared_ptr`

#include <iostream>
#include <memory>
#include <string>
#include <vector>

struct Shape {
    virtual ~Shape() = default;
    virtual double area() const = 0;
};

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

int main() {
    std::vector<std::unique_ptr<Shape>> shapes;
    shapes.push_back(std::make_unique<Circle>(1.0));
    shapes.push_back(std::make_unique<Circle>(2.0));
    for (const auto& s : shapes) std::cout << s->area() << '\n';
    // every Circle is destroyed when shapes goes away

    auto config = std::make_shared<std::string>("debug");
    auto another = config;                       // reference count is now 2
    std::cout << config.use_count() << '\n';     // 2
}

Default to std::unique_ptr; use std::shared_ptr only when ownership is genuinely shared. A function that merely uses an object should take a reference (such as const Shape&) or a plain pointer, not a smart pointer.

Move semantics

C++11 added move operations, which transfer a resource instead of duplicating it. A temporary, or an object marked with std::move, can hand over its internal buffer, so an expensive copy becomes a few pointer assignments. std::move itself moves nothing; it is a cast meaning "you may move from this".

#include <iostream>
#include <string>
#include <utility>
#include <vector>

int main() {
    std::string big(1'000'000, 'x');
    std::vector<std::string> list;

    list.push_back(big);              // copy: big is untouched
    list.push_back(std::move(big));   // move: big's buffer is handed over
    // big is now valid but unspecified; assign a new value before relying on it
    big = "reused";

    std::cout << list.size() << '\n'; // 2
}

When returning a local, write return x;, not return std::move(x);: the compiler already applies copy elision or an implicit move. And if a class doesn't manage a resource directly, let the compiler generate its copy and move operations (the Rule of Zero).

Templates and generic programming

Templates are functions and classes parameterized by type; the compiler generates a concrete version for each type you use. The standard containers and algorithms are all templates.

#include <concepts>
#include <iostream>
#include <string>
#include <utility>
#include <vector>

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

template <typename T>
class Stack {
public:
    void push(T value) { items_.push_back(std::move(value)); }
    T pop() { T top = std::move(items_.back()); items_.pop_back(); return top; }
    bool empty() const { return items_.empty(); }
private:
    std::vector<T> items_;
};

// C++20 concepts: only integer types are accepted
template <std::integral T>
T half(T value) { return value / 2; }

int main() {
    std::cout << max_of(3, 7) << '\n';                                    // 7
    std::cout << max_of(std::string{"a"}, std::string{"b"}) << '\n';      // b
    Stack<int> s;
    s.push(1);
    s.push(2);
    std::cout << s.pop() << ' ' << half(10) << '\n';                      // 2 5
    // half(1.5);  // error: double does not satisfy std::integral
}

Template definitions must be visible where they are used, so they usually live in headers. C++20 concepts name a template's requirements and turn pages of instantiation errors into a short, readable message.

Summary

RAII: give every resource an owning object and let its destructor clean up.
Create heap objects with std::make_unique, and use std::shared_ptr only for genuinely shared ownership.
std::move only permits a move; reassign a moved-from object before relying on it.
Templates give type-independent code; concepts state what it needs.