A constructor should create a fully initialized object

Reason

A constructor establishes the invariant for a class. A user of a class should be able to assume that a constructed object is usable.

Example, bad

class X1 {
    FILE* f;   // call init() before any other function
    // ...
public:
    X1() {}
    void init();   // initialize f
    void read();   // read from f
    // ...
};

void f()
{
    X1 file;
    file.read();   // crash or bad read!
    // ...
    file.init();   // too late
    // ...
}

Compilers do not read comments.

Exception

If a valid object cannot conveniently be constructed by a constructor, use a factory function.

Enforcement

• (Simple) Every constructor should initialize every data member (either explicitly, via a delegating ctor call or via default construction).
• (Unknown) If a constructor has an Ensures contract, try to see if it holds as a postcondition.

Note

If a constructor acquires a resource (to create a valid object), that resource should be released by the destructor. The idiom of having constructors acquire resources and destructors release them is called RAII ("Resource Acquisition Is Initialization").