static_assert if expressions is constexpr

问题

I want to create a class template

template <class T>
class X {
  // here I'll use T::value (among other things)
};

T::value will often be a constexpr static variable, but not always. T::value has to be positive value, so I want to let people know it during compilation, when possible.

If T::value was always constexpr, I'd add static_assert like

static_assert(T::value > 0, "need positive number");

Is it possible to add this static_assert only for cases when T::value is constexpr?

回答1:

We can write an is_valid template function (come up with a better name) with two overloads:

template <typename T, int N = T::value>
constexpr bool is_valid(int) {
    return N > 0;
}

template <typename T>
constexpr bool is_valid(...) {
    return true;
}

The first overload will only be valid if T::value is a constant expression, otherwise it will be SFINAEd out. The second overload is valid no matter what, so we disambiguate the overload with a dummy int parameter.

Now we test it like so:

static_assert(is_valid<T>(0), "need positive number");

Live Demo

回答2:

This works for me on clang++:

#include <type_traits>

// The default case, returns true if the value is not constant.
template <typename T, typename = void>
struct IsNonConstantOrPositive {
    static const bool value = true;
};

// The `constexpr` case. We check if we can evaluate `T::value == 0` which can only
// be evaluated at compile-time if `T::value` is constant. The whole `enable_if` thing
// is to provide a substitution to ensure SFINAE.
template <typename T>
struct IsNonConstantOrPositive<T, typename std::enable_if<T::value==0||true>::type> {
    static const bool value = T::value > 0;
};

template <typename T>
struct X {
    static_assert(IsNonConstantOrPositive<T>::value, "T::value should be positive");
};

Example:

struct A {  // const > 0, should succeed
    static const int value = 123;
};

struct B {  // const <= 0, should fail
    static const int value = -1234;
};

struct C {   // non-const, should succeed
    static int value;
};

int main() {
    X<A> a;     // ok
    //X<B> b;   // error
    X<C> c;     // ok
}

回答3:

I think it would be nice to have a test for const(expr)-ness of a variable, to be used like:

struct T {
    ...
    static_assert(!IS_CONSTANT_VAR(value) || value > 0, "trouble is afoot");
};

The implementation below uses a similar strategy to kennytm's solution to fail out on non-constant references. It works in Clang and GCC.

#include <type_traits> // enable_if

template<typename T, T& var, typename = void> struct is_constant_var_impl {
    static constexpr bool value = false;
};
template<typename T, T& var>
struct is_constant_var_impl <T, var, typename std::enable_if<(double)var == (double)var>::type> {
    // (double) cast above to thwart GCC's agressive constant folding;
    // perhaps could be removed with a bit more finesse
    static constexpr bool value = true;
};
#define IS_CONSTANT_VAR(...) (is_constant_var_impl<decltype(__VA_ARGS__), (__VA_ARGS__)>::value)