Odd Template behaviour when T == bool

Question

I don't really know how to describe this problem but I have a template class that stores an array of value and casts it out to various data types. Generally I'm not going to cast a boolean to a float but it has to be implemented because of the way the templates work.

This causes a weird issue where when T[] = bool[] I get an error on this code:

virtual bool getBool(uint i) {
 bool b;
 b = *reinterpret_cast<bool*>(&values[i]);
 return b;
}

Compliler error:

error: taking address of temporary

But this compiles and works fine:

 virtual bool getBool(uint i) {
  bool b;
  T c = values[i];
  b = *reinterpret_cast<bool*>(&c);
  return b;
 }

These two pieces of code work exactly the same but the first will not compile when T is bool. int, float, and std::string work as intended with both versions of this code. (gcc c++11)

Why does this happen?

Is the 'array' a std::vector? (if so, std::vector is special) — , May 21 '15 at 17:13
http://stackoverflow.com/q/30376032/560648 just a few hours ago! — Lightness Races in Orbit, May 21 '15 at 17:17
[This is](http://en.cppreference.com/w/cpp/container/vector_bool). Don't say array if you mean `std::vector`. — Useless, May 21 '15 at 17:17
Next time present a minimal, _complete_ testcase please. It avoids issues like this, where you leave out crucial details not realising that they're crucial. — Lightness Races in Orbit, May 21 '15 at 17:18
You have a class template that stores an array of any arbitrary type `T`, and you have a member function `getBool` that'll `reinterpret_cast` whatever `T` is into a `bool`?! Seems like an awesome idea. — Praetorian, May 21 '15 at 17:18
@Praetorian There are catches for invalid casts and special cases for bool to string being changed to "true" and "false". I just wanted learn about this weird case. If there are better ways to do stuff like this I want to learn. Being condescending/sarcastic is not constructive at all. — new Objekt, May 21 '15 at 17:29

score 1 · Accepted Answer · answered May 21 '15 at 17:16

1

Unlike the generic implementation of std::vector<T>, std::vector<bool> is many times a specialization that is implemented such that std::vector<boo>::operator[](size_t) returns a bool, not a bool&.

Hence,

std::vector<bool> v{0, 1};
v[0]; // Returns an object
bool* ptr = &v[0]; // Not allowed since v[0] returns an object
                   // by value.

However,

bool b = v[0];
bool* ptr = &b; // OK since you are taking address of b.

More informaton can be found at http://en.cppreference.com/w/cpp/container/vector_bool.

answered May 21 '15 at 17:16

R Sahu

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Interesting. Are there any other weird type cases with vector? – new Objekt May 21 '15 at 17:19
@newObjekt, `vector` is the only one that I know of. – R Sahu May 21 '15 at 17:20

score 1 · Answer 2 · edited May 23 '17 at 11:58

values is a std::vector<bool>, which is a special version of vector that packs the bools into bits.

The result of values[i] is a special proxy class (or simply a copy of the bool, if the vector is const) that acts like a reference to a bool, but is unfortunately not a real reference. The "temporary" referred to by the error message is this proxy class.

See also: Why vector<bool>::reference doesn't return reference to bool? and this article.

Odd Template behaviour when T == bool

2 Answers2