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Thinking in C++
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Hidden activities in constructors & destructors

Constructors and destructors are two places where you can be fooled into thinking that an inline is more efficient than it actually is. Constructors and destructors may have hidden activities, because the class can contain subobjects whose constructors and destructors must be called. These subobjects may be member objects, or they may exist because of inheritance (covered in Chapter 14). As an example of a class with member objects:

//: C09:Hidden.cpp
// Hidden activities in inlines
#include <iostream>
using namespace std;

class Member {
  int i, j, k;
  Member(int x = 0) : i(x), j(x), k(x) {}
  ~Member() { cout << "~Member" << endl; }

class WithMembers {
  Member q, r, s; // Have constructors
  int i;
  WithMembers(int ii) : i(ii) {} // Trivial?
  ~WithMembers() {
    cout << "~WithMembers" << endl;

int main() {
  WithMembers wm(1);
} ///:~

The constructor for Member is simple enough to inline, since there’s nothing special going on – no inheritance or member objects are causing extra hidden activities. But in class WithMembers there’s more going on than meets the eye. The constructors and destructors for the member objects q, r, and s are being called automatically, and those constructors and destructors are also inline, so the difference is significant from normal member functions. This doesn’t necessarily mean that you should always make constructor and destructor definitions non-inline; there are cases in which it makes sense. Also, when you’re making an initial “sketch” of a program by quickly writing code, it’s often more convenient to use inlines. But if you’re concerned about efficiency, it’s a place to look.

Thinking in C++
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire