I tried to use the operator[]
access the element in a const 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 map
. Is at()
a newly added function in 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
The compiler used is g++ 4.2.1
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.
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).
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.
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:
I have no idea about at()
.
Success story sharing
at()
with in VS2013 on a project set to use VS2010 toolkit. I thought that meant I wasn't using C++11... But yet it compiles... ??