C++ foreach-loop picks mutable begin() even

Question

When writing

for(const auto& val: my_container)
  sum += val

Visual Studio picks the mutable version of begin(), is this by design or a bug?

As I using a copy on write container, this is quite a performance issue in my code.

Answer 1

This is by design. The "foreach" loop doesn't look at the qualifiers or the reference qualifiers of the iteration variable when determining whether to treat my_container as const. Workaround is to add the const explicitly

const auto& my_container_const = my_container;
for(const auto& val: my_container_const)
   sum += val

Answer 2

This is the expected behaviour a const_iterator isn't a const iterator and in any case begin returns const depending on the constness of the container not the variable that will store the return value.