C++学习——format

在学习C++的时候总是习惯于写一些简单的demo来测试这些新方法的使用。今天我就基于之前写的关于运算符重载的demo——复数类的实现进行重写。新的demo中我实现了使用std::format输出自定义类型。

我这里参考的是文章C++20 使用std::format输出自定义类型 - 知乎 (zhihu.com)

#include <iostream>
#include <format>
#include <string>
#include <sstream>

using std::cin;
using std::cout;
using std::format;
using std::istream;
using std::ostream;

class complex
{
public:
    complex(double real = 0.0, double imag = 0.0) : m_real(real), m_imag(imag){};

public:
    friend complex operator+(const complex &A, const complex &B);
    friend complex operator-(const complex &A, const complex &B);
    friend complex operator*(const complex &A, const complex &B);
    friend complex operator/(const complex &A, const complex &B);
    friend istream &operator>>(istream &in, complex &A);
    friend ostream &operator<<(ostream &out, complex &A);
    friend class std::formatter<complex, char>;

private:
    double m_real; //实部
    double m_imag; //虚部
};

template <>
class std::formatter<complex, char>
    : public std::_Formatter_base<complex, char, std::_Basic_format_arg_type::_Custom_type>
// _Basic_format_arg_type是个枚举类，_Custom_type用于说明是自定义类型
{
public:
    // format用于输出格式化的内容，比较简单输出格式和cout输出格式类似
    template <class _FormatContext>
    auto format(const complex &val, _FormatContext &format_ctx) noexcept
    {
        std::ostringstream out_sstream;
        out_sstream << val.m_real << "+" << val.m_imag << "i";
        return formatter<string, char>().format(out_sstream.str(), format_ctx);
    }
};

//重载加法运算符
complex operator+(const complex &A, const complex &B)
{
    return complex(A.m_real + B.m_real, A.m_imag + B.m_imag);
}

//重载减法运算符
complex operator-(const complex &A, const complex &B)
{
    return complex(A.m_real - B.m_real, A.m_imag - B.m_imag);
}

//重载乘法运算符
complex operator*(const complex &A, const complex &B)
{
    return complex(A.m_real * B.m_real - A.m_imag * B.m_imag,
                   A.m_imag * B.m_real + A.m_real * B.m_imag);
}

//重载除法运算符
complex operator/(const complex &A, const complex &B)
{
    double square = A.m_real * A.m_real + A.m_imag * A.m_imag;
    return complex((A.m_real * B.m_real + A.m_imag * B.m_imag) / square,
                   (A.m_imag * B.m_real - A.m_real * B.m_imag) / square);
}

//重载输入运算符
istream &operator>>(istream &in, complex &A)
{
    in >> A.m_real >> A.m_imag;
    return in;
}

//重载输出运算符
ostream &operator<<(ostream &out, complex &A)
{
    out << A.m_real << " + " << A.m_imag << " i ";
    return out;
}

int main()
{
    complex c1, c2;
    cin >> c1 >> c2;
    cout << format("{} + {} = {}\n", c1, c2, c1 + c2);

    cout << format("{} - {} = {}\n", c1, c2, c1 - c2);

    cout << format("{} * {} = {}\n", c1, c2, c1 * c2);

    cout << format("{} / {} = {}\n", c1, c2, c1 / c2);

    return 0;
}