How to check result from boost::fusion:at_key?

Question

Example from boost tutorial:

using namespace fields;
std::string person_name = at_key<name>(a_person);
int person_age = at_key<age>(a_person);

How to check wheather there is no such key in this map?

Answer 1

To check if a given sequence contains an element associated with a given Key, one can (and should) use has_key<Key> (seq), as in the below example.

if (has_key<fields::age> (some_seq)) {
  int person_age = at_key<fields::age> (some_seq);
}

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Documentation:

• boost.org - has_key - 1.55.0