C++ Cookbook

8.1. Initializing Class Member Variables

Problem

You need to initialize member variables that are native types, pointers, or references.

Solution

Use an initializer list to set the initial values for member variables. Example 8-1 shows how you can do this for native types, pointers, and references.

Example 8-1. Initializing class members

#include <string>

using namespace std;

class Foo {
public:
   Foo() : counter_(0), str_(NULL) {}
   Foo(int c, string* p) :
       counter_(c), str_(p) {}
private:
   int counter_;
   string* str_;
};

int main() {

   string s = "bar";
   Foo(2, &s);
}

Discussion

You should always initialize native variables, especially if they are class member variables. Class variables, on the other hand, should have a constructor defined that will initialize its state properly, so you do not always have to initialize them. Leaving a native variable in an uninitialized state, where it contains garbage, is asking for trouble. But there are a few different ways to do this in C++, which is what this recipe discusses.

The simplest things to initialize are native types. ints, chars, pointers, and so on are easy to deal with. Consider a simple class and its default constructor:

class Foo {
public:
   Foo() : counter_(0), str_(NULL) {}
   Foo(int c, string* p) :
       counter_(c), str_(p) {}
private:
   int counter_;
   string* str_;
};

Use an initializer list in the constructor to initialize member variables, and avoid doing so in the body of the constructor. This leaves the body of the constructor for any logic ...