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0 pointsfusiongyro13y ago0 comments

One thing you cannot do is infer `const`ness. This isn't a problem in Hindley-Milner systems because variables are not assignable--they're all `const`, all the time. Literals are inherently immutable but when you assign a C++ variable a literal value, sometimes you want a const variable and sometimes you don't.

I don't think C++ can figure out the map<..> part simply because lots of things could have list initializers that accept lists of 2 item lists. C++'s overloading and implicit conversion conflicts with perfect type inferencing. This is an example of the kind of thing the FQA talks about, where several of C++'s issues collide to produce counter-intuitive behavior.

I do wonder if you could get away with this:

    const map<const auto, const auto> TagDataMap { ...

I don't have access to a C++11 compiler from where I am to find out though. I am particularly unclear on the interaction between `auto` and other aspects of a type declaration--I don't know if you can nest `auto` like this deep inside some other type declaration. I don't see why you couldn't, but wouldn't be shocked either.

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3 comments · 2 top-level

bcoates13y ago· 1 in thread

You can't do it with that syntax. 'auto' never infers constructor calls, just takes the static type of an LHS to capture a temporary into. You'd have to use a non-constructor template function that inspects the type of its argument to guess what kind of map you want.

  const auto TagDataMap = map_initializer( { "foo", { 1, 20, func }, ... } );

where map_initializer would be a template function that inspects the typedefs of its std::initializer_list<T> argument and generates an appropriate map.

In real C++ code, this would almost never be what you actually want, though, as the types in an initializer list in C++ do not imply the types being initialized, they are parameters to a constructor to be determined elsewhere.

fusiongyroOP13y ago

Thanks for clearing that up. I'm just starting to play with C++11 and have some trouble getting to a supporting compiler from work.

Evbn13y ago

It would help a lot if the keyword 'const' were replaced by a punctuation character or shorter simpler keyword like 'val'

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