Question

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Constructor initialization-list evaluation order

While writing a C++ constructor for a class, why is it that the order of initialising the member fields should be the order in which they are declared?

Consider the following C++ code. On compiling with gcc (gcc -g -Wall foo.cpp) I get the Warning

g++ -g -Wall main.cpp 
main.cpp: In constructor ‘myclass::myclass(int, int, int, int, int, int)’:
main.cpp:12: warning: ‘myclass::z’ will be initialized after
main.cpp:11: warning:   ‘int myclass::y’
main.cpp:26: warning:   when initialized here

Here is the code. In this, the member z appears in the initialization list of the
constructor class before y and throws the above warning.

#include <iostream>
#include <iomanip>

class myclass
{
public:
  int x;
  int y;
  int z;
  myclass(int num1, int num2, int num3, int num4, int num5, int num6);//constructor for the class

private:
  int a;
  int b;
  int c;
};

myclass::myclass(int num1, int num2, int num3, int num4, int num5, int num6) 
  :x(num1),z(num3),  y(num2), a(num4),b(num5),c(num6)
{}

int main(int argc, char *argv[])
{
  myclass jimmy(23,34,56,67,89,91);

  std::cout << jimmy.x << std::endl;
  std::cout << jimmy.y << std::endl;
  std::cout << jimmy.z << std::endl;

  return 0;
}

Answer 1

This may help,

Constructor initialization-list evaluation order

Please see AProgrammer’s reply in the above,

“The reason for which they are constructed in the member declaration order and not in the order in the constructor is that one may have several constructors, but there is only one destructor. And the destructor destroy the members in the reserse order of construction.” – AProgrammer Aug 7 ’09 at 6:45