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Item 4: Make sure that objects are initialized before they're used

Concept​

Reading uninitialized values yields undefined behavior. Rules:

  • Always initialize objects before use
  • Use member initialization lists in constructors (order must match declaration order)
  • For non-local statics in different translation units, use the Singleton/local static idiom to avoid initialization order problems

Code Example​

class PhoneNumber { /* ... */ };

class ABEntry {
public:
    // Good: use member initialization list
    ABEntry(const std::string& name, const std::string& address)
        : theName(name), theAddress(address), numTimesConsulted(0)
    {}
private:
    std::string theName;
    std::string theAddress;
    int numTimesConsulted;
};

// Replace non-local static with local static (Meyers' Singleton)
FileSystem& theFileSystem() {
    static FileSystem fs;
    return fs;
}

Full source code

Things to Remember​

  • Manually initialize objects of built-in type, because C++ only sometimes initializes them itself.
  • In a constructor, prefer use of the member initialization list to assignment inside the body. List data members in the initialization list in the same order they're declared in the class.
  • Avoid initialization order problems across translation units by replacing non-local static objects with local static objects.
  • Item 5 — Know what functions C++ silently writes and calls
  • Item 26 — Postpone variable definitions as long as possible