实验六 模板类和文件I/O

task3_1.cpp

#include <iostream>
#include <fstream>
#include <array>
#define N 5

int main() {
    using namespace std;

    array<int, N> x{ 97,98,99,100,101 };

    ofstream out;
    out.open("data1.dat", ios::binary);
    if (!out.is_open()) {
        cout << "fail to open data1.dat\n";
        return 1;
    }

    // 把从地址&x开始连续sizeof(x)个字节的数据块以字节数据方式写入文件data1.txt
    out.write(reinterpret_cast<char*>(&x), sizeof(x));
    out.close();
}

task3_2.cpp

#include <iostream>
#include <fstream>
#include <array>
#define N 5

int main() {
    using namespace std;
    array<int, N> x;

    ifstream in;
    in.open("data1.dat", ios::binary);
    if (!in.is_open()) {
        cout << "fail to open data1.dat\n";
        return 1;
    }

    // 从文件流对象in关联的文件data1.dat中读取sizeof(x)字节数据写入&x开始的地址单元
    in.read(reinterpret_cast<char *>(&x), sizeof(x));
    in.close();

    for (auto i = 0; i < N; ++i)
        cout << x[i] << ", ";
    cout << "\b\b \n";
}

 

对task3_2.cpp的代码稍做改动，把line8的数组类型从int类型修改成char类型，即： array<int, N> x; ----> 修改成: array<char, N> x;

其它不做任何改动，再次运行程序，观察运行结果是什么？尝试分析原因：为什么会是那样的结果？

#include <iostream>
#include <fstream>
#include <array>
#define N 5

int main() {
    using namespace std;
    array<char, N> x;

    ifstream in;
    in.open("data1.dat", ios::binary);
    if (!in.is_open()) {
        cout << "fail to open data1.dat\n";
        return 1;
    }

    // 从文件流对象in关联的文件data1.dat中读取sizeof(x)字节数据写入&x开始的地址单元
    in.read(reinterpret_cast<char *>(&x), sizeof(x));
    in.close();

    for (auto i = 0; i < N; ++i)
        cout << x[i] << ", ";
    cout << "\b\b \n";
}

发生上述现象是因为char和int类型所占用的字节数不同，从而导致在修改后从文件读出数据时是以char型读取，读取时是一个一个字节读取，从而会产生空格。

 

Vector.hpp

#pragma once
using namespace std;

template<typename T>
class Vector {
public:
    Vector(int n0) :n{ n0 } { x = new T[n]; }
    Vector(int n0, T t) :n{ n0 } {
        x = new T[n];
        for (int i = 0; i < n; i++)
            x[i] = t;
    }
    Vector(const Vector<T>& v) :n{ v.n } {
        x = new T[n];
        for (int i = 0; i < n; i++)
            x[i] = v.x[i];
    }
    ~Vector() = default;

    int get_size() const { return size; }
    T& at(int i) const { return x[i]; }
    T& operator[](const int i) { return x[i]; }

    friend void output(Vector& v) {
        cout << v.x[0];
        for (int i = 1; i < v.n; i++)
            cout << ", " << v.x[i];
        cout << endl;
    }

private:
    int n;
    T* x;
};

task4.cpp

#include <iostream>
#include "Vector.hpp"

void test() {
    using namespace std;

    int n;
    cin >> n;

    Vector<double> x1(n);
    for (auto i = 0; i < n; ++i)
        x1.at(i) = i * 0.7;

    output(x1);

    Vector<int> x2(n, 42);
    Vector<int> x3(x2);

    output(x2);
    output(x3);

    x2.at(0) = 77;
    output(x2);

    x3[0] = 999;
    output(x3);
}

int main() {
    test();
}

task5.cpp

#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

void output(ostream& out) {
    out << "   ";
    int a[200];
    for (int i = 97; i < 123; i++)
        a[i] = i;
    for (int i = 97; i < 123; i++)
        out << char(a[i]) << " ";
    out << endl;
    for (int i = 1; i <= 26; i++) {
        out << setw(2) << right << i;
        for (int j = 97; j < 123; j++) {
            int t = a[j] + i - 32;
            if (a[j] + i > 122)
                t -= 26;
            out << setw(2) << right << char(t);
        }
        out << endl;
    }
}

int main() {
    output(cout);
    ofstream out;
    out.open("cipher_key.txt");
    if (!out.is_open()) {
        cout << "fail to open cipher_key.txt!\n";
        return 0;
    }
    output(out);
    out.close();
}