Forbid integer conversion with precision loss

Question

How to prevent such code from compiling?

#include 
#include 
#include 
#include 

int main() {
  std::         


        

Answer 1

You could start writing a wrapper for your integral types to match the exact type (or some conditions).

#include 
#include 
#include 
#include 
#include 

template  struct conjunction : std::true_type {};
template  struct conjunction : B1 {};
template 
struct conjunction
    : std::conditional_t, B1> {};

template  struct int_wrapper {
  explicit int_wrapper() : _val{T{}} {}

  explicit int_wrapper(const int_wrapper &other) : _val{other._val} {}

  template  explicit int_wrapper(U val) : _val{val} {
    static_assert(sizeof(T) >= sizeof(U), "Size mismatch.");
    static_assert(conjunction, std::is_signed>::value,
                  "sign mismatch");
  }

  explicit operator T() { return _val; }
  explicit operator T() const { return _val; }

  T _val;
};

std::ostream &operator<<(std::ostream &stream, const int_wrapper &v) {
  stream << v._val;
  return stream;
}

int main() {
  std::vector> v;
  v.emplace_back(std::numeric_limits::max());
  std::cout << v.back() << std::endl;

  return 0;
}

clang on macOS gives you error like:

so.cpp:18:60: error: non-constant-expression cannot be narrowed from type 'unsigned long long' to 'short' in initializer list
      [-Wc++11-narrowing]
  template  explicit int_wrapper(U val) : _val{val} {
                                                           ^~~
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:1752:31: note: in
      instantiation of function template specialization 'int_wrapper::int_wrapper' requested here
            ::new((void*)__p) _Up(_VSTD::forward<_Args>(__args)...);
                              ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:1668:18: note: in
      instantiation of function template specialization 'std::__1::allocator >::construct,
      unsigned long long>' requested here
            {__a.construct(__p, _VSTD::forward<_Args>(__args)...);}
                 ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/memory:1514:14: note: in
      instantiation of function template specialization 'std::__1::allocator_traits >
      >::__construct, unsigned long long>' requested here
            {__construct(__has_construct(),
             ^
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/../include/c++/v1/vector:1643:25: note: in
      instantiation of function template specialization 'std::__1::allocator_traits >
      >::construct, unsigned long long>' requested here
        __alloc_traits::construct(this->__alloc(),
                        ^
so.cpp:37:5: note: in instantiation of function template specialization 'std::__1::vector,
      std::__1::allocator > >::emplace_back' requested here
  v.emplace_back(std::numeric_limits::max());
    ^
so.cpp:18:60: note: insert an explicit cast to silence this issue
  template  explicit int_wrapper(U val) : _val{val} {
                                                           ^~~
                                                           static_cast( )
so.cpp:19:5: error: static_assert failed "Not the same size."
    static_assert(sizeof(T) >= sizeof(U), "Not the same size.");
    ^             ~~~~~~~~~~~~~~~~~~~~~~
so.cpp:20:5: error: static_assert failed "sign mismatch"
    static_assert(conjunction, std::is_signed>::value,
    ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
3 errors generated.