Reason

If you intend for a class to match a concept, verifying that early saves users' pain.

Example

class X {
public:
    X() = delete;
    X(const X&) = default;
    X(X&&) = default;
    X& operator=(const X&) = default;
    // ...
};

Somewhere, possibly in an implementation file, let the compiler check the desired properties of X:

static_assert(Default_constructible<X>);    // error: X has no default constructor
static_assert(Copyable<X>);                 // error: we forgot to define X's move constructor

Enforcement

Not feasible.

CPL: C-style programming

C and C++ are closely related languages. They both originate in "Classic C" from 1978 and have evolved in ISO committees since then. Many attempts have been made to keep them compatible, but neither is a subset of the other.

C rule summary:

• CPL.1: Prefer C++ to C
• CPL.2: If you must use C, use the common subset of C and C++, and compile the C code as C++
• CPL.3: If you must use C for interfaces, use C++ in the calling code using such interfaces