C++ const std::map reference fails to compile

Question

Is there a reason why passing a reference to a std::map as const causes the [] operator to break? I get this compiler error (gcc 4.2) when I use const:

error: no match for ‘operator[]’ in ‘map[name]’

Here's the function prototype:

void func(const char ch, std::string &str, const std::map<std::string, std::string> &map);

And, I should mention that there is no problem when I remove the const keyword in front of std::map.

If I've been instructed correctly, the [] operator will actually insert a new pair into the map if it doesn't find the key, which would of course explain why this happens, but I can't imagine that this would ever be acceptable behavior.

If there is a better method, like using find instead of [], I'd appreciate it. I can't seem to get find to work either though... I receive const mismatched iterator errors.

Answer 1

Yes you can't use operator[]. Use find, but note it returns const_iterator instead of iterator:

std::map<std::string, std::string>::const_iterator it;
it = map.find(name);
if(it != map.end()) {
    std::string const& data = it->second;
    // ...
}

It's like with pointers. You can't assign int const* to int*. Likewise, you can't assign const_iterator to iterator.