C++ Initialization lists - I don't get it

c++ initialization-list
helpermethod picture helpermethod · Mar 1, 2011 · Viewed 33.2k times · Source

In Effective C++, it is said that data elements in the initialization list need to be listed in the order of their declaration. It is further said that the reasoning for this is that destructors for data elements get called in the reverse order of their constructors.

But I just don't see how this could be a problem...

Answer

sharptooth picture sharptooth · Mar 1, 2011

Well consider the following:

class Class {
    Class( int var ) : var1( var ), var2(var1 ) {} // allright
    //Class( int var ) : var2( var ), var1(var2 ) {} // var1 will be left uninitialized

    int var1;
    int var2;
};

The second (commented out) constructor looks allright, but in fact only var2 will be initialized - var1 will be initialized first and it will be initialized with var2 that is not yet initialized at that point.

If you list initializers in the same order as member variables are listed in the class declaration risk of such errors becomes much lower.

Related questions

How do I initialize a stl vector of objects who themselves have non-trivial constructors?

suppose I have the following class: class MyInteger { private: int n_; public: MyInteger(int n) : n_(n) {}; // MORE STUFF }; And suppose this class don't have a default trivial constructor MyInteger(). I must always supply an int to initialize it for …

Read More
What does a colon following a C++ constructor name do?

What does the colon operator (":") do in this constructor? Is it equivalent to MyClass(m_classID = -1, m_userdata = 0);? class MyClass { public: MyClass() : m_classID(-1), m_userdata(0) { } int m_classID; void *m_userdata; };

Read More
Initialize parent's protected members with initialization list (C++)

Is it possible to use the initialization list of a child class' constructor to initialize data members declared as protected in the parent class? I can't get it to work. I can work around it, but it would be nice …

Read More