What does 'modern C++' actually mean in practice?_

Resource Acquisition Is Initialization means every resource — memory, file handles, locks, sockets — is owned by an object whose destructor releases it. If you follow RAII rigorously you essentially cannot leak resources. Smart pointers are just RAII for heap memory.

C++17 gave us std::optional<T> (nullable without null) and std::variant<A,B,C> (type-safe union). These replace null pointer hacks and void* casts respectively. Combined with std::visit and pattern-like matching they enable much safer code.

Modern C++ (C++11 and beyond) means: (1) smart pointers instead of raw new/delete, (2) range-based for loops, (3) auto for type deduction, (4) move semantics to avoid copies. Adopt these four and you have already eliminated the majority of classic C++ bugs.

The C++ Core Guidelines (github.com/isocpp/CppCoreGuidelines) are the official answer to 'what is modern C++'. Maintained by Bjarne Stroustrup and Herb Sutter. Read the first 30 rules and you will have a better handle on the language than most developers.