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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>

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

    const std::map<int, char> B = A;

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

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

t01.cpp:14: error: passing ‘const std::map<int, char, std::less, std::allocator<std::pair<const int, char> > >’ 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<std::pair<const int, char> >]’ discards qualifiers

The compiler used is g++ 4.2.1

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5 Answers 5

Reset to default
177

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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11 Comments

Is is possible to have "at" return a default value instead of throwing an exception?
I'm using 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... ??
I just need to comment that it doesn't make sense to omit the const operator[], which could also throw an exception for an unmapped element instead of changing the map.
@Spencer It would be surprising if the const and non-const overloads of operator[] had different effects. Normally, we expect that if some non-const objects or references in a program are made const, the program will continue to behave in the same way, as long as it compiles. Allowing only the non-const overload to throw exceptions can result in bugs that are not caught until runtime.
@Brian Trading a silent, possibly corrupting error for a noisy one. That's a good thing.
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41

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).

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2 Comments

Does the C++ standard forbid the implementation from defining additional non-standard member functions in library classes?
@Tim I believe the interface is fixed, yes.
4

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.

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3

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().

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2 Comments

Could be a good idea to implement in a custom map.
The article link is dead.
1

std::map::operator[] does not have a const-qualifier, which means that you cannot call it with a const std::map. See also What is the meaning of 'const' at the end of a member function declaration? if you're unfamiliar with this.

operator[] returns a reference to the element, just like at, but there is a key difference:

Since at() doesn't need to modify the map, it has a const overload:

T& at( const Key& key );
const T& at( const Key& key ) const; // <-- this is what operator[] is lacking

Non-mutating access in a std::map

Besides these two functions, there are three more ways to access elements in a const std::map:

const std::map<K, V> map = { /* ... */ };

auto a = map.at(key);   // OK, C++11, throws exception if key not found
auto b = map.find(key); // OK, returns iterator (possibly equal to .end() if key not found)

auto c = map.count(key);    // OK, returns size_type (0 or 1)
auto d = map.contains(key); // OK, C++20, returns bool
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