Compile error using map iterators

Question

In my header file I have included the std::map and use the appropriate namespace.
One of my members is:

map<unsigned int, double> pT_Spam;

And in my .cpp file I attempt to do something that I have been doing frequently for some time now:

for(map<unsigned int, double>::iterator it=pT_Spam.begin() ; it!=pT_Spam.end() ; it++) {/*code*/}

The above is even mentioned in one the examples of using std::map at cplusplus.com . Even though I have done pretty much the same in other parts of the code which cause no compile errors, on this particular line I get the following error from Cygwin:

error: conversion from `std::_Rb_tree_const_iterator<std::pair<const unsigned int, double> >' to non-scalar type `std::_Rb_tree_iterator<std::pair<const unsigned int, double> >' requested

Which seems rather strange. Any idea what might be wrong? (my header is, of course, included in my .cpp)

Solution 1

It would seem that at the scope this loop exists, the map is const. For example, is the loop in a class method declared const, like so?

void method() const // const method
{
    // Do stuff.
}

or passed as a const argument, like this?

void function(const map<unsigned int, double>& pT_Spam)
{
    // Do stuff.
}

If it is, you must use const iterators:

for(map<unsigned int, double>::const_iterator it=pT_Spam.begin() ; it!=pT_Spam.end() ; it++)
{
    /*code*/
}

Or, if you're using C++11, then you should use the auto keyword:

for(auto it=pT_Spam.begin() ; it!=pT_Spam.end(); it++)
{
    /*code*/
}

Since in the case you've shown you must use const iterators, you can't use them to modify the map or the data within it. That's const correctness, and it's a good thing :).

Solution 2

Well, the error says you're trying to cast a const_iterator to an iterator. You say that pT_Spam is a member. Is it a member of a const object? If it is, begin() and end() will return const_iterators.

Solution 3

You need to use const_iterators for maps, so it should be:

for(map<unsigned int, double>::const_iterator it = \\and so on

Edit: As pointed out the above is right, but for completely wrong reasons (maps have non-const iterators. What was I thinking exactly? I don't know). Most likely your map is defined as const (as pointed out in another answer).

Solution 4

for(map::iterator it=pT_Spam.begin() ; it!=pT_Spam.end() ; it++)

chage it to

for(map::const_iterator it=pT_Spam.begin() ; it!=pT_Spam.end() ; it++)

As it is pointing to constant value, so the iterator must be also of const type.

View more solutions

Comments

[jathanasiou]

Won't that cause issues when altering their value(it++)? Also, I have been using regular iterators for similar loops so far and never had problems.

[Peter Milley]

Um, what? map has non-const iterators.

[Kerrek SB]

@JohnAthanasiou: A mutable const-iterator is not the same as a const non-const iterator! Think T const * vs T * const.

[Peter Milley]

John: const_iterators aren't themselves const, they just refer to a const object.

[jathanasiou]

Not sure to be honest, it's a declared private member of my 'nbclassifier' class, while the method that performs the loop is a member of that class as well.

[Aaron McDaid]

An alternative is to mark the member as mutable. But don't tell anyone I told you that :-)

[Captain Obvlious]

-.25 for using auto in a way that kills kittens. ;)

[Aaron McDaid]

An alternative is to mark the member as mutable. But don't tell anyone I told you that :-)

[jathanasiou]

The method running the loop is indeed const, and that makes me understand why the map is const in it's scope as well. Considering I don't need to edit it in this method, const iterators seem to the answer.

[Peter Milley]

Const is coming in somewhere. If the member variable isn't declared const, and if the method isn't declared const, then maybe the method is being called on a const object (which would make the member variable implicitly const). Does the compiler error come with a call stack? EDIT: wait, it would have to be a const method if it's being called on a const object.

[Liam M]

@Chet Simpson Herb Sutter says my kittens are safe :).

[Erik Aronesty]

Where in the error does it say that? it says : conversion from GARBAGE to non-scalar type LINE_NOISE requested.

[vappolinario]

What would be the problem with the auto in this case?