stl - C++ const map element access

Sunday, 19 June 2016

stl - C++ const map element access

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 
#include 


using namespace std;

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

        const map 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

Answer

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.

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Sunday, 19 June 2016

stl - C++ const map element access

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