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Item 15: Provide access to raw resources in resource-managing classes

Concept​

RAII classes exist to prevent resource leaks, but many APIs require access to raw resources. You need to provide a way to get at the raw resource. This can be done via explicit conversion (a get() member function) or implicit conversion (an operator conversion function). Explicit conversion is safer; implicit conversion is more convenient for clients but increases the chance of errors. Generally, explicit conversion via get() is preferred.

Code Example​

#include <memory>

class Investment {
public:
    virtual ~Investment() = default;
    bool isTaxFree() const { return false; }
};

// Existing C API that expects raw pointer
int daysHeld(const Investment* pi) { return 100; }
bool isTaxFree(const Investment* pi) { return pi->isTaxFree(); }

void example() {
    std::shared_ptr<Investment> pInv(new Investment());

    // Explicit access via get()
    int days = daysHeld(pInv.get());

    // Implicit via operator-> and operator*
    bool taxFree = pInv->isTaxFree();
    bool taxFree2 = (*pInv).isTaxFree();
}

// Custom RAII class with explicit conversion
class Font {
public:
    explicit Font(int fontHandle) : f(fontHandle) {}
    ~Font() { releaseFont(f); }

    int get() const { return f; }  // explicit conversion

private:
    int f;
    static void releaseFont(int) { /* release */ }
};

Full source code

Things to Remember​

  • APIs often require access to raw resources, so each RAII class should offer a way to get at the resource it manages.
  • Access may be via explicit conversion (e.g., get()) or implicit conversion. In general, explicit conversion is safer, though implicit conversion is more convenient.
  • Item 13 — RAII and smart pointers
  • Item 14 — Copying behavior in resource-managing classes
  • Item 18 — Making interfaces easy to use correctly