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C++ const map element access

Posted by: admin November 30, 2017 Leave a comment

Questions:

I tried to use the operator[] access the element in a const C++ map, but this method failed. I also tried to use “at()” to do the same thing. It worked this time. However, I could not find any reference about using “at()” to access element in a const C++ map. Is “at()” a newly added function in C++ map? Where can I find more info about this? Thank you very much!

An example could be the following:

#include <iostream>
#include <map>

using namespace std;

int main()
{
        map<int, char> A;
        A[1] = 'b';
        A[3] = 'c';

        const map<int, char> B = A;

        cout << B.at(3) << endl; // it works
        cout << B[3] << endl;  // it does not work

}

For using “B[3]”, it returned the following errors during compiling:

t01.cpp:14: error: passing ‘const
std::map,
std::allocator > >’ as ‘this’ argument of ‘_Tp&
std::map<_Key, _Tp, _Compare,
_Alloc>::operator[](const _Key&) [with _Key = int, _Tp = char, _Compare = std::less, _Alloc =
std::allocator >]’ discards qualifiers

The compiler used is g++ 4.2.1

Answers:

at() is a new method for std::map in C++11.

Rather than insert a new default constructed element as operator[] does if an element with the given key does not exist, it throws a std::out_of_range exception. (This is similar to the behaviour of at() for deque and vector.)

Because of this behaviour it makes sense for there to be a const overload of at(), unlike operator[] which always has the potential to change the map.

Questions:
Answers:

If an element doesn’t exist in a map, the operator [] will add it – which obviously cannot work in a const map so C++ does not define a const version of the operator. This is a nice example of the compiler’s type checker preventing a potential runtime error.

In your case, you need to use find instead which will only return an (iterator to the) element if it exists, it will never modify the map. If an item doesn’t exist, it returns an iterator to the map’s end().

at doesn’t exist and shouldn’t even compile. Perhaps this is a “compiler extension” (= a bug new in C++0x).

Questions:
Answers:

The []-operator will create a new entry in the map if the given key does not exists. It may thus change the map.

See this link.

Questions:
Answers:

This comes as quite a surprise to me, but the STL map doesn’t have a const index operator. That is, B[3] cannot be read-only. From the manual:

Since operator[] might insert a new element into the map, it can’t possibly be a const member function.

I have no idea about at().