Custom Exception Types

When what() strings are not enough structure, define exception types. Derive from std::runtime_error (or logic_error) and carry the data the handler needs:

#include <stdexcept>
#include <string>

class ValidationError : public std::runtime_error {
public:
    ValidationError(std::string field, std::string message)
        : std::runtime_error{field + ": " + message},
          field_{std::move(field)}, message_{std::move(message)} {}

    const std::string& field() const noexcept { return field_; }

private:
    std::string field_;
    std::string message_;
};

Rules that keep custom exceptions safe:

• constructor must not throw (it would abort during unwinding);
• what() is satisfied for free by std::runtime_error;
• copy members exist by default — exceptions are copied as they unwind, so members must be copyable;
• catch handlers get const ValidationError& and can read .field() without parsing strings.

Design the type around the handler's decision: what would a caller do differently for this failure than for any other? If the answer is "nothing", a plain std::runtime_error string was enough.