Does the boost::enable_if_c<> conditional argument use short circuiting?

Question

See the is >> i toward the bottom of my code? I want g++ (C++03; I have reasons) to use the first operator>>() template--the one that prints "non-container type" because the right-hand expression is an int and not, for example, a vector. Instead, it considers the last one--the one that prints "fixed-length container type." I can see from the error messages that it is evaluating the last template's enable_if_c<> conditional argument, which leads to all sorts of problems because has_resize<T>::value does stuff that won't work on a container.

I would have thought that since the first subexpression in enable_if_c's conditional parameter, is_container<C>::value, presumably evaluates to false, the second subexpression, has_resize<C>::value, would not be evaluated. Either the && operator separating the two does not do short-circuiting or the first subexpression inexplicably evaluates to true for an int. Any idea which one it is and what I can do about it? (Debugging TMP is really difficult. I'd like to step through compilation as the compiler considers each template.)

Oh, and if you change #if 1 to #if 0, an alternate has_resize<T> template is used, which works as expected. However, that template doesn't do as good a job determining whether the type is resizable, which is what I'm trying to do. The one I'm trying to get to work doesn't do a perfect job, either, but it's better.

If you'd like to play around with the code, it's also available on Wandbox. (C++ shell, too. I'm playing around with online compilers. I made a list of them.)

#include <iostream>
#include <boost/spirit/home/support/container.hpp>

#if 1
// has_resize<T>::value is whether the (presumably) container class contains resize.
template<class T>
class has_resize
{
    struct Fallback { int resize; };
    struct Derived : T, Fallback { };

    template<class C, C>
    class check;

    typedef uint8_t no;
    typedef uint16_t yes;

    template<typename C> static no test(check<int Fallback::*, &C::resize> *);
    template<typename C> static yes test(...);

public:
    static const bool value = sizeof test<Derived>(0) == sizeof(yes);
};
#else
// has_resize<T>::value is whether the (presumably) container class contains allocator_type.
template <class T>
class has_resize
{
    typedef uint8_t yes;
    typedef uint16_t no;

    template <typename C> static yes test(class C::allocator_type *);
    template <typename C> static no test(...);

public:
    static const bool value = sizeof test<T>(0) == sizeof(yes);
};
#endif

class xstream { }; // For this example, the class doesn't need to do anything.

template <typename T>
typename boost::enable_if_c<
    !boost::spirit::traits::is_container<T>::value,
    xstream &>::type
    operator>>(xstream &ibs, T &b)
{
    std::cout << "non-container type" << std::endl;
    return ibs;
}

template <typename C>
typename boost::enable_if_c<
    boost::spirit::traits::is_container<C>::value && has_resize<C>::value,
    xstream &
>::type
operator>>(xstream &ibs, C &c)
{
    std::cout << "variable-length container type" << std::endl;
    ibs >> *c.begin();
    return ibs;
}

template <typename C>
typename boost::enable_if_c<
    boost::spirit::traits::is_container<C>::value && !has_resize<C>::value,
    xstream &
>::type
operator>>(xstream &ibs, C &c)
{
    std::cout << "fixed-length container type" << std::endl;
    ibs >> *c.begin();
    return ibs;
}

int main()
{
    int i;
    xstream is;
    is >> i;
}

UPDATE: Here is the code with the fix that @Jarod42 suggested:

#include <iostream>
#include <vector>
#include <set>
#if __cplusplus > 199711L
#include <array>
#endif
#include <boost/spirit/home/support/container.hpp>

#define DEFINE_HAS_SIGNATURE(traitsName, funcName, signature)               \
    template <typename U>                                                   \
    class traitsName                                                        \
    {                                                                       \
    private:                                                                \
        template<typename T, T> struct helper;                              \
        template<typename T>                                                \
        static char check(helper<signature, &funcName>*);                   \
        template<typename T> static int check(...);                         \
    public:                                                                 \
        static                                                              \
        const bool value = sizeof(check<U>(0)) == sizeof(char);             \
    }

#if __cplusplus > 199711L
DEFINE_HAS_SIGNATURE(has_resize, T::resize, void (T::*)(typename T::size_type));
#else
DEFINE_HAS_SIGNATURE(has_resize, T::resize, void (T::*)(typename T::size_type, typename T::value_type));
#endif

class xstream { }; // For this example, the class doesn't need to do anything.

template <typename T>
typename boost::enable_if_c<
    !boost::spirit::traits::is_container<T>::value,
    xstream &>::type
    operator>>(xstream &ibs, T &b)
{
    std::cout << "non-container type" << std::endl;
    return ibs;
}

template <typename C>
typename boost::enable_if_c<
    boost::spirit::traits::is_container<C>::value && has_resize<C>::value,
    xstream &
>::type
operator>>(xstream &ibs, C &c)
{
    std::cout << "variable-length container type" << std::endl;
    ibs >> *c.begin();
    return ibs;
}

template <typename C>
typename boost::enable_if_c<
    boost::spirit::traits::is_container<C>::value && !has_resize<C>::value,
    xstream &
>::type
operator>>(xstream &ibs, C &c)
{
    std::cout << "fixed-length container type" << std::endl;
    ibs >> *c.begin();
    return ibs;
}

int main()
{
    int i;
    std::vector<int> vi;
    std::set<int> si;
#if __cplusplus > 199711L
    std::array<int, 1> ai;
#endif
    xstream xs;
    xs >> i >> vi >> si;
#if __cplusplus > 199711L
    xs >> ai;
#endif
}

Answer 1

In your case, you have a hard error in struct Derived : T, Fallback { }; with T = int The ::value force the instantiation of the class.

I use the following:

#define DEFINE_HAS_SIGNATURE(traitsName, funcName, signature)               \
    template <typename U>                                                   \
    class traitsName                                                        \
    {                                                                       \
    private:                                                                \
        template<typename T, T> struct helper;                              \
        template<typename T>                                                \
        static std::uint8_t check(helper<signature, &funcName>*);           \
        template<typename T> static std::uint16_t check(...);               \
    public:                                                                 \
        static                                                              \
        constexpr bool value = sizeof(check<U>(0)) == sizeof(std::uint8_t); \
    }

DEFINE_HAS_SIGNATURE(has_resize, T::foo, int (T::*));

Live Demo