Note that STL containers allow you to bounds check "[]" indexing even in release...

Note that STL containers allow you to bounds check "[]" indexing even in release mode if you set the right preprocessor directives. The flag differs between compilers and stdlib implementations, though. For libstdc++ (default on linux), use _GLIBCXX_ASSERTIONS. For example, the following code aborts on release builds too:

  // g++ -O3 -D_GLIBCXX_ASSERTIONS=1 file.cpp
  #include <iostream>
  #include <vector>

  int main() {
    std::vector<int> vec{1, 2, 3, 4};
    std::cout << vec[4];  
  }

MSVC provides similar functionality using the _ITERATOR_DEBUG_LEVEL directive. The point is that there is no inherent limitation that one has to use the .at() method for bounds checking STL container access.