C++ std::move doesn't move anything: A deep dive into Value Categories

4 months ago

std::move doesn't actually move anything; it's a cast that changes the value category of an expression to an xvalue (expiring value).
Move constructors and move assignment operators must be marked noexcept to ensure standard containers use them during reallocation instead of falling back to copying.
Returning local variables by name enables NRVO (Named Return Value Optimization), which is more efficient than explicitly using std::move in return statements.
Moving from const objects silently falls back to copying because const objects cannot be modified, which is required for moving resources.
After moving from an object, it's in a 'valid but unspecified state' and should only be reassigned or destroyed.
The Rule of Five: If you implement any of the destructor, copy constructor, copy assignment, move constructor, or move assignment, you likely need to implement all five.
std::forward is used for perfect forwarding in templates, preserving the original value category (lvalue or rvalue) of the argument.
Small String Optimization (SSO) means moving small strings may still involve copying their inline buffer rather than just swapping pointers.
Inheritance requires explicit std::move of base class subobjects in derived class move constructors because named rvalue references are treated as lvalues.
C++17 made copy elision mandatory for prvalues, guaranteeing efficient returns of temporaries without any moves or copies.

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