stdvariant

stdvariant is a C++17 feature that provides a type-safe union. It can hold a value of one of its specified alternative types at any given time. Unlike a traditional union, stdvariant knows which type it currently holds, preventing accidental misinterpretation of the stored data. It is a value type, meaning it owns its data.

To use stdvariant, you declare it with a list of possible types it can hold. For example, std::variant<int, float, std::string> myVariant; can hold an integer, a floating-point number, or a string. You can assign a value of any of these types to it. Attempting to assign a value of an incompatible type will result in a compile-time error.

Accessing the value stored in a stdvariant requires checking its current type. This can be done using std::get_if, which returns a pointer to the value if it is of the requested type, or nullptr otherwise. Alternatively, std::get can be used, which throws an exception if the variant does not hold the requested type. The std::visit function provides a more generic and often safer way to operate on the stored value by applying a visitor object to it.

stdvariant is useful in scenarios where a function or data structure might return or hold one of several distinct types. This includes parsing data, implementing state machines, or handling results from operations that can fail in different ways. It promotes safer code by eliminating the need for manual type tracking often associated with C-style unions.