tobi303 tobi303 - 3 months ago 17
C++ Question

How to prevent implicit conversion from int to unsigned int?

Suppose you have this:

struct Foo {
Foo(unsigned int x) : x(x) {}
unsigned int x;
};

int main() {
Foo f = Foo(-1); // how to get a compiler error here?
std::cout << f.x << std::endl;
}


Is it possible to prevent the implicit conversion?

The only way I could think of is to explicilty provide a constructor that takes an
int
and generates some kind of runtime error if the
int
is negative, but it would be nicer if I could get a compiler error for this.

I am almost sure, that there is a duplicate, but the closest I could find is this question which rather asks why the implicit conversion is allowed.

I am interested in both, C++11 and pre C++11 solutions, preferably one that would work in both.

Answer

Uniform initialization prevents narrowing.

It follows a (not working, as requested) example:

struct Foo {
    explicit Foo(unsigned int x) : x(x) {}
    unsigned int x;
};

int main() {
    Foo f = Foo{-1};
    std::cout << f.x << std::endl;
}

Simply get used to using the uniform initialization (Foo{-1} instead of Foo(-1)) wherever possible.

EDIT

As an alternative, as requested by the OP in the comments, a solution that works also with C++98 is to declare as private the constructors getting an int (long int, and so on).
No need actually to define them.
Please, note that = delete would be also a good solution, as suggested in another answer, but that one too is since C++11.

EDIT 2

I'd like to add one more solution, event though it's valid since C++11.
The idea is based on the suggestion of Voo (see the comments of Brian's response for further details), and uses SFINAE on constructor's arguments.
It follows a minimal, working example:

#include<type_traits>

struct S {
    template<class T, typename = typename std::enable_if<std::is_unsigned<T>::value>::type>
    S(T t) { }
};

int main() {
    S s1{42u};
    // S s2{42}; // this doesn't work
    // S s3{-1}; // this doesn't work
}