A surprising autoconversion

Autoconversion of types is a handy thing, and easy to miss when you don't have it. This is especially so in languages like C++ that have many static type distinctions, but don't do many of the conversions automatically. C++ has a built-in autoconversion mechanism, but it's too dangerous to use much, and the alternative of manually overloading every operation with type-converting wrappers is too tedious for anything but a few basic types, so in practice C++ is a non-autoconverting language.

That's the standard excuse, anyway. C++ actually does provide quite a lot of autoconversions, but some of them are bizarrely unhelpful. Take the most obvious autoconversion, for example: converting integers to strings. I ran into this one today:

string s;
int n = 40;
s = n;

A naïve (or even not-so-naïve) user might expect this to set s to "40", but it's actually "(". Assigning a single integer to a string is treated as assignment of a single character, not the decimal representation of the integer. But this is only for assignment; the constructor doesn't accept integers at all. I can think of some excuses for this (characters are sometimes (e.g. by getc) represented as ints; string should be consistent with other collections; base 10 is arbitrary; conversions between semantically different types shouldn't happen automatically; STL is supposed to provide a solid foundation, not be easy to use), but user expectation is so strong here that I'm still surprised.

I'm also surprised I've never run into this before. I habitually use printf-like functions for generating strings, but not always, so you'd think I'd encounter this more than once a decade, especially since += has the same surprising overloading. Do I really never generate strings from single integers? Is this most obvious of autoconversions really so rare in practice?

1 comment:

  1. This isn't supper surprising to me. In general, C never performs operates on chars, just ints. This is because most machines have integer operations only for one or two word sizes.

    In C, the expression 'a'+'b' actually implicitly up-converts the chars to ints before doing the addition.

    C++ inherits a lot of this from C, so there you have it.


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