モダンC++書き換えパターン集

1. ポインタ管理

// Before
Foo* f = new Foo();
f->DoSomething();
delete f;

// After
auto f = std::make_unique<Foo>();
f->DoSomething();  // delete不要

2. NULL → nullptr

// Before
Bar* b = NULL;

// After
Bar* b = nullptr;

3. #define 定数 → constexpr

// Before
#define MAX_SIZE 256

// After
constexpr int MaxSize = 256;

4. typedef → using

// Before
typedef std::map<int, std::string> MyMap;

// After
using MyMap = std::map<int, std::string>;

5. 古いfor文 → 範囲for

// Before
for (int i = 0; i < vec.size(); ++i) {
    vec[i].print();
}

// After
for (auto& item : vec) {
    item.print();
}

6. 明示的な型 → auto

// Before
std::map<int, std::string>::iterator it = myMap.find(3);

// After
auto it = myMap.find(3);

7. RAII（リソース管理）

// Before
FILE* fp = fopen("data.txt", "r");
if (fp) {
    // 読み込み処理
    fclose(fp); // 閉じ忘れるとリーク
}

// After
std::ifstream file("data.txt");
if (file.is_open()) {
    // 読み込み処理
} // 自動でクローズ

---

古いc++の例

#include <iostream>
#include <vector>
#include <cstring>
#include <cstdio>

#define SIZE 5
#define MSG "Hello World"

typedef struct Point {
    int x;
    int y;
} Point;

class Sample {
public:
    Sample(const char* msg) {
        strcpy(buf, msg);
    }
    void print() {
        std::cout << buf << std::endl;
    }
private:
    char buf[64];
};

int main() {
    int* arr = new int[SIZE];
    for (int i = 0; i < SIZE; ++i) {
        arr[i] = i * 10;
    }

    CArray<Point*> points;
    for (int i = 0; i < SIZE; ++i) {
        Point* p = new Point();
        p->x = i;
        p->y = i * 2;
        points.Add(p);
    }

    const char* str = NULL;
    if (str == NULL) {
        str = MSG;
    }

    typedef std::vector<int> IntVector;
    IntVector vec;
    for (int i = 0; i < SIZE; ++i) {
        vec.push_back(arr[i]);
    }

    char chars[] = "ABCDE";
    for (int i = 0; i < strlen(chars); ++i) {
        std::cout << chars[i] << std::endl;
    }

    FILE* fp = fopen("test.txt", "r");
    if (fp) {
        char line[256];
        while (fgets(line, sizeof(line), fp)) {
            std::cout << line;
        }
        fclose(fp);
    }

    Sample* s = new Sample(MSG);
    s->print();
    delete s;

    for (int i = 0; i < SIZE; ++i) {
        delete points[i];
    }
    delete[] arr;

    return 0;
}

モダンC++への書き換え例

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

constexpr int SIZE = 5;
constexpr auto MSG = "Hello World";

struct Point {
    int x;
    int y;
};

class Sample {
public:
    explicit Sample(const std::string& msg) : buf(msg) {}
    void print() const {
        std::cout << buf << std::endl;
    }
private:
    std::string buf;
};

int main() {
    auto arr = std::make_unique<std::array<int, SIZE>>();
    for (int i = 0; i < SIZE; ++i) {
        (*arr)[i] = i * 10;
    }

    std::vector<std::unique_ptr<Point>> points;
    for (int i = 0; i < SIZE; ++i) {
        auto p = std::make_unique<Point>();
        p->x = i;
        p->y = i * 2;
        points.push_back(std::move(p));
    }

    const char* str = nullptr;
    if (str == nullptr) {
        str = MSG;
    }

    using IntVector = std::vector<int>;
    IntVector vec;
    for (auto val : *arr) {
        vec.push_back(val);
    }

    std::string chars = "ABCDE";
    for (const auto& c : chars) {
        std::cout << c << std::endl;
    }

    std::ifstream ifs("test.txt");
    if (ifs) {
        std::string line;
        while (std::getline(ifs, line)) {
            std::cout << line << std::endl;
        }
    }

    auto s = std::make_unique<Sample>(MSG);
    s->print();

    return 0;
}

実際に書き換えてみよう↓