Re: Call virtual function in constructor

Kira Yamato <>
Sat, 16 Feb 2008 19:05:20 -0500
On 2008-02-16 18:02:00 -0500, Pavel
<dot_com_yahoo@paultolk_reverse.yourself> said:

Kira Yamato wrote:

On 2008-02-16 14:25:25 -0500, said:

Is there any problem if call virtual function in constructor? For

class A

           data = getnumber();

    print() { cout << data << endl;

    virtual int getnumber() {return 100;}
    int data;

class B : public A
         B() { }

         virtual int getnumber() {return 22222;}


int main()
     A test1;
     test1.print() ;

     B test2;
     test2.print() ;


Technically, it's perfectly fine to call virtual functions in the
constructor as long as you understand that it will not invoke any
possible derived-class's overriding version of the same method. Also,
your compiler will complain if you're calling a pure virtual function.

However, semantically speaking, it seems weird why you would want to
invoke a method that your object wishes to override. I can't pinpoint
it yet, but it seems like a bad design. Sorry for being so vague.
Perhaps an expert can help point out the problem in a clear way.

It is sometimes useful to initialize different classes of a hierarchy
with the derived class-dependent code and then return back to the base
class constructor to execute some of its code again to avoid
duplicating that latter common code.

For example (this code will not work in C++ but the analogous code will
work in other programming languages (e.g. Java) and I do not see any
fundamental design flaws in this code).:

typedef std::map<std::string> ConnectionParameters;
class FooConnection {
    virtual void init(const ConnectionParameters &pars) = 0;
    FooConnection(const ConnectionParameters &pars) {
    void validateConnection()
        throw(FooConnectionException /*defined elsewhere*/)
        /* perform some uniform validation here, for example
            some select from "FOO_MAIN_TABLE" */
class OracleFooConnection : public FooConnection {
    void init(const ConnectionParameters &pars) {
        // .. do Oracle-specific initialization
class MySqlFooConnection : public FooConnection {
    void init(const ConnectionParameters & pars) {
        // .. do MySql-specific initialization

This way, you can perform all validation in the constructor, that is,
according to the best practices, and without duplicating the common
validation code in the derived class.

Interesting example. However, I would like to propose that in any case
where the construction of an object is complicated enough, the factory
pattern should be considered.

In your example above, a FooConnectionFactory can be used to invoke the
derived-class-specific initiation code as well as
derived-class-specific validation code. This way avoids the need to
invoke derived-class overridden methods.



// kira

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