strange const correctness error

Question

I have a header file that contains a class. Within that class, I have a function like so:

class Definition { public: int GetID() { return Id; } //Other methods/variables private: int Id; }

When I attemped to get that ID as so:

for (std::map<Definition, std::vector<bool> >::iterator mapit = DefUseMap.begin(); mapit != DefUseMap.end(); ++mapit, defIndex++) { stream << "Definition " << (*mapit).first.GetID() << " Def Use" << endl << "\t"; }

I get the following error

CFG.cc:1145: error: passing 'const Definition' as 'this' argument of 'int Definition::GetID()' discards qualifiers

is it because I'm using definition inside a map, and I'm not allowed to call methods on that mapped definition? Is there a way to get that ID variable out?

Thanks in advance

QuantumMechanic · Accepted Answer · 2011-04-10 03:00:59Z

9

Declare the getID() method const:

int getId() const { return Id; }

Then the method can be called by a const reference, which is what operator<<() is being passed.

answered Apr 10, 2011 at 3:00

QuantumMechanic

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2 Comments

Mike DeSimone Over a year ago

The general rule is to declare a method const if it only needs const access to the object, i.e. this could be a const pointer and things will work just fine. Also note the mutable keyword, which says that a class member can be changed even in a const method. One example of a mutable member is if you wanted to record the last access time for an object. In this case, the access timestamp would be a mutable member so it could be updated from a const getter method.

QuantumMechanic Over a year ago

user127817 -- if you find my answer satisfactory, please consider accepting it. Thanks!

Bo Persson · Accepted Answer · 2011-04-10 07:50:54Z

The map<Key, Value> stores your data internally in a std::pair<const Key, Value>. This so that it will be hard to accidentally change the Key and destroy the ordering of the elements.

To be able to call GetID() for the const Key, the function will have to be declared const as well:

int GetID() const;

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