Static_cast和reinterpret_cast的int()转换操作符失败

Question

我有一个简单的类需要转换:

#include <iostream>
using namespace std;

template<int p>
class Mod {
    int n;

    friend Mod operator+(const Mod &x, const Mod &y) {
        const int sum = x.n + y.n;

        if (sum < p) {
            Mod z(sum);
            return z;
        } else if (sum >= p) {
            Mod z2(sum - p);
            return z2;
        }
    }

    operator int() {
        if (p == 0) return n;
        if (p == 1) return 0;

        for (int r = 0; r <= abs(n); r++) {
            if (((r - n) % p) == 0) {
                return r;
            }
        }
    }

    friend Mod operator-(const Mod &x) {
        if (x.n == 0) return 0;

        Mod z(-static_cast<int>(x) + p);
        return z;
    }

    friend std::ostream &operator<<(std::ostream &out, const Mod &x) {
        out << reinterpret_cast<int>(x); // invalid cast from type ‘const Mod<5>’ to type ‘int’ out << reinterpret_cast<int>(x);
        return out;
    }

public:
    Mod(int x) {
        n = x;
    }
};

int main() {
    Mod<5> z(3), x(2);

    cout << z + x << endl; // error: invalid cast from type ‘const Mod<5>’ to type ‘int’ out << reinterpret_cast<int>(x);
    cout << z << endl; //error: invalid cast from type ‘const Mod<5>’ to type ‘int’ out << reinterpret_cast<int>(x); // invalid cast from type ‘const Mod<5>’ to type ‘int’ out << reinterpret_cast<int>(x);
    cout << -x << endl; //error: invalid static_cast from type ‘const Mod<5>’ to type ‘int’ Mod z(-static_cast<int>(x) + p);

    return 0;
}

当我尝试编译和运行它们时，我看到以下信息：

[ 33%] Building CXX object CMakeFiles/TestCpp.dir/main.cpp.o
/home/user/ClionProjects/TestCpp/main.cpp: In instantiation of ‘std::ostream& operator<<(std::ostream&, const Mod<5>&)’:
/home/user/ClionProjects/TestCpp/main.cpp:75:17:   required from here
/home/user/ClionProjects/TestCpp/main.cpp:56:13: error: invalid cast from type ‘const Mod<5>’ to type ‘int’
out << reinterpret_cast<int>(x); // invalid cast from type ‘const Mod<5>’ to type ‘int’ out << reinterpret_cast<int>(x);
^
/home/user/ClionProjects/TestCpp/main.cpp: In instantiation of ‘Mod<5> operator-(const Mod<5>&)’:
/home/user/ClionProjects/TestCpp/main.cpp:77:14:   required from here
/home/user/ClionProjects/TestCpp/main.cpp:50:36: error: invalid static_cast from type ‘const Mod<5>’ to type ‘int’
Mod z(-static_cast<int>(x) + p);

- user902691

Answer 1

对于static_cast，它使用隐式和用户定义的转换来在类型之间进行转换。

将考虑用户定义的转换，但是您正在对const对象进行int 强制转换，应该将operator int()设置为常量成员函数。

operator int() const {
               ~~~~~
    ...
}

对于 reinterpret_cast，

它通过重新解释底层位模式来在类型之间进行转换。

与 static_cast 不同，但类似于 const_cast，reinterpret_cast 表达式不会编译为任何 CPU 指令。它完全是一条编译器指令，指示编译器将表达式的比特序列（对象表示）视为具有新类型。

因此，reinterpret_cast<int>(x) 无论如何都行不通，并且不会考虑用户定义的转换。

- songyuanyao