If a constructor cannot construct a valid object, throw an exception

Reason

Leaving behind an invalid object is asking for trouble.

Example

class X2 {
    FILE* f;
    // ...
public:
    X2(const string& name)
        :f{fopen(name.c_str(), "r")}
    {
        if (!f) throw runtime_error{"could not open" + name};
        // ...
    }

    void read();      // read from f
    // ...
};

void f()
{
    X2 file {"Zeno"}; // throws if file isn't open
    file.read();      // fine
    // ...
}

Example, bad

class X3 {     // bad: the constructor leaves a non-valid object behind
    FILE* f;   // call is_valid() before any other function
    bool valid;
    // ...
public:
    X3(const string& name)
        :f{fopen(name.c_str(), "r")}, valid{false}
    {
        if (f) valid = true;
        // ...
    }

    bool is_valid() { return valid; }
    void read();   // read from f
    // ...
};

void f()
{
    X3 file {"Heraclides"};
    file.read();   // crash or bad read!
    // ...
    if (file.is_valid()) {
        file.read();
        // ...
    }
    else {
        // ... handle error ...
    }
    // ...
}

Note

For a variable definition (e.g., on the stack or as a member of another object) there is no explicit function call from which an error code could be returned. Leaving behind an invalid object and relying on users to consistently check an is_valid() function before use is tedious, error-prone, and inefficient.

Exception

There are domains, such as some hard-real-time systems (think airplane controls) where (without additional tool support) exception handling is not sufficiently predictable from a timing perspective. There the is_valid() technique must be used. In such cases, check is_valid() consistently and immediately to simulate RAII.

Alternative

If you feel tempted to use some "post-constructor initialization" or "two-stage initialization" idiom, try not to do that. If you really have to, look at factory functions.

Note

One reason people have used init() functions rather than doing the initialization work in a constructor has been to avoid code replication. Delegating constructors and default member initialization do that better. Another reason has been to delay initialization until an object is needed; the solution to that is often not to declare a variable until it can be properly initialized.

Enforcement

???