I have following simple class with conversion:
#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; } When I try compile and run them I see following message:
[ 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); 
