unordered_set::remove_if(): C3892: cannot assign to a variable that is const

Question

I have code like: ``` unordered_set<AttrValue> output; ... auto requiredType = variables.at(arg.value); auto end = remove_if(output.begin(), output.end(), [&](AttrValue x) { return !matchingOutputType(requiredType, ast->getNodeType(ast->getNodeKeyAttribute(x))); }); // queryevaluator_getcandidatelist.cpp(179) output.erase(end); ``` Error is on line 4 of the code. So I think its because of `remove_if`. But whats wrong? output is not defined constant? ``` Error 90 error C3892: '_Next' : you cannot assign to a variable that is const c:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm 1840 Error 109 error C3892: '_Next' : you cannot assign to a variable that is const c:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm 1840 ``` Output window: ``` 3>c:\program files (x86)\microsoft visual studio 10.0\vc\include\algorithm(1840): error C3892: '_Next' : you cannot assign to a variable that is const 3> c:\program files (x86)\microsoft visual studio 10.0…

Accepted Answer

According to the standard § 23.2.4.6

For associative containers where the value type is the same as the key type, both iterator and const_iterator are constant iterators.

So, you can't even do

```
std::unordered_set<int> s{1, 2, 3, 4};
*s.begin() = 42;
```

And, of course, you can't use `std::remove_if(ForwardIt first, ForwardIt last, ...)` function for removing elements from `std::set` and `std::unordered_set`:

The type of dereferenced ForwardIt must meet the requirements of MoveAssignable.

unordered_set::remove_if(): C3892: cannot assign to a variable that is const

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