Re: [Boost-users] Type_traits: is_convertible and std::pair problem

cp now wrote:

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The type_traits library gives me somewhat erroneous answers when dealing with pairs of pairs.

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The compiler appears to be attempting a piecewise conversion of the contents of the pair, hence the error about converting pair to 'int' rather than an error about converting pair, int> to pair.

Is there a method of converting that is_convertible has in mind when it returns true for the aforementioned conversion? I'm testing here for simple implicit conversion...

is_convertible does tell you about implicit convertibility, it answers the question: Does type "To" have a copy constructor that accepts a type "From"? And indeed std::pair has such a constructor, so is_convertible answers true. Unfortunately what no traits class can answer is the question: Does this code compile? And that's where this example fails: std::pair has a "catch all" constructor that will accept any kind of pair as an argument, so that is_convertible::value must always be true, even though the code may fail to compile inside the constructor :-( However... given that pairs (and in the future tuples) are part of the standard, it may be worth while fixing is_convertible for this special case. Interestingly, had std::pair's constructor been defined in terms of enable_if and is_convertible, it could have been defined in such a way that it's signature was only valid if the code would compile: and is_convertible would then have "just plain worked" for pairs. No doubt C++0x's concepts can achieve something similar. Hope this explanation helps, John.