What one doesn't understand, one doesn't possess.
– Johann Wolfgang von Goethe
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Page 9 of: C++
auto and decltype Explained,
by Thomas Becker
about me
Miscellaneous Properties of decltype
An important property of decltype is that its operand
never gets evaluated. For example, you can use an out-of-bounds
element access to a vector as the operand of decltype
with impunity:
std::vector<int> vect; assert(vect.empty()); typedef decltype(vect[0]) integer;Another property of decltype that is worth pointing out
is that when decltype(expr) is the name of a
plain user defined type (not a reference or pointer, not a basic or function type),
then decltype(expr) is also a class name. This means that you
can access nested types directly:
template<typename R>
class SomeFunctor {
public:
typedef R result_type;
result_type operator()() {
return R();
}
SomeFunctor(){}
};
SomeFunctor<int> func;
typedef decltype(func)::result_type integer; // access nested type
You can even use decltype(expr) to specify a base class:
auto foo = [](){return 42;};
class DerivedFunctor : public decltype(foo)
{
public:
MyFunctor(): decltype(foo)(foo) {}
// ...
};
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