NonCopyable

By default, C++ objects are copyable. However, some types of objects, such as resources or polymorphic types, are not meant to be copied because they have unique identities.

To prevent copying from happening, a common practice has been to declare the copy constructor and copy assignment operator of noncopyable types as private. This pattern has the disadvantage of not being semantically explicit. Therefore, it can be difficult to find out whether a type is copyable.

The NonCopyable class solves these issue. Use it by inheriting privately from it:

class Resource : NonCopyable<Resource> { /* resource code */ };

Resource r1, r2;

// Copy construction generates a compile time error.
Resource r3 = r1;

// Copy assignment generates a compile time error.
r1 = r2;

The noncopyable properties also transfer to classes that derive from a noncopyable class, as well as classes that own a noncopyable instance:

class DerivedResouce : public Resource { /* DerivedResource code */ };

DerivedResouce r1, r2;

// Copy construction generates a compile time error.
DerivedResouce r3 = r1;

// Copy assignment generates a compile time error.
r1 = r2;

class ResourceOwner {
    /* code */
    Resource r;
};

ResourceOwner o1, o2;

// Copy construction generates a compile time error.
ResourceOwner o3 = o1;

// Copy assignment generates a compile time error.
r1 = r2;

We recommend inheriting from the NonCopyable class whenever a class is a resource or owns a resource (lock/hardware/file) that should or could not be copied.

NonCopyable class reference

NonCopyable example

Copying objects of classes used for locking, network encapsulation, hardware bus and so on should not occur. Mbed OS has noncopyable classes, such as Mutex, EventFlags, BusOut, InterruptIn and I2C.

Related content

• Mutex API reference.
• EventFlags API reference.
• BusOut API reference.
• InterruptIn API reference.
• I2C API reference.