Re: [boost] [Hana] Informal review request

AMDG On 03/05/2015 01:15 PM, Matt Calabrese wrote:

<snip>What this implies is that simply by writing pointer == tuple_t, the effect is that the type "Car" is implicitly instantiated at this point (if it were template instantiation), because the compiler needs to see if there are base classes in order to properly form the set of associated namespaces!

In this particular example it might not be too apparent why this is so bad, but in the general case, instantiating such types that are involved can cause a number of serious issues: 1) Perhaps the most obvious is that instantiation can potentially fail (and therefore cause a compile error even though it doesn't look like the type would be used).

I'm glad you brought this up. This problem is not academic. I've seen real bugs reported against at least one Boost library because of something like this.

2) They can cause considerable compile-time overhead for a template definition's instantiation that otherwise might not be used. 3) Worst case, but probably the least likely. It can compile but produce strange results in the program in a very subtle way, not even necessarily directly at this point in code, due to the type being instantiated "prematurely" with respect to what the user may have expected. This can can cause problems, for instance, if certain types are still incomplete at this point in the translation unit, or if important functions/overloads have not yet been declared, etc. Because of memoization, at the point that the user thinks they are first instantiating the template, they will get an instantiation that they likely did not expect.

In Christ, Steven Watanabe