no == defined for boost::tuples

问题:

I have this code:

...
#include "boost/tuple/tuple_comparison.hpp"
...
template <typename ReturnType, typename... Args>
function<ReturnType(Args...)> memoize(const Args && ... args)
{
    using noRef = boost::tuple<typename std::remove_reference<Args>::type...>;
    static map<noRef, ReturnType, less<>> cache;
    auto key = std::tie(noRef{ boost::make_tuple(args ...) });
    auto it = cache.lower_bound(key);
    ReturnType result;
    if (it->first == key) { ...

But when I try to compile it I receive this error:

error C2678: binary '==': no operator found which takes a left-hand operand of type 'const noRef' (or there is no acceptable conversion)

Why this happens, since noRef is an alias for boost::tuple and tuple_comparison should manage this case?

ERROR FOUND, DON'T KNOW HOW TO SOLVE IT:

It seems that the error was in the std::tie operation. So rewriting it as:

    auto key = noRef{ boost::make_tuple(args ...) };

Works fine. The problem is that this solution is inefficient, since key is a potentially expensive copy of the whole tuple, while using tie is a tuple of references (much smaller). So, how can I take a reference to it->first tuple? Should I use the same tie trick?

回答1:

The only reason this line compiles at all is MSVC's Evil Extension^TM which allows non-const lvalue references to bind to temporaries:

auto key = std::tie(noRef{ boost::make_tuple(args ...) });

This should simply be

auto key = boost::tie(args...);

which creates a boost::tuple of references to be used for lookup later.

Also, as noted in the comments, the if check should verify it != cache.end() first before attempting to dereference it (thanks!).

Finally, const Args && ... doesn't make much sense, as one is unlikely to want to accept const rvalues. It probably should be either const Args&... or Args&&....