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Thinking in C++
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Constructor calls

Considering that

MyType* f = new MyType;

calls new to allocate a MyType-sized piece of storage, then invokes the MyType constructor on that storage, what happens if the storage allocation in new fails? The constructor is not called in that case, so although you still have an unsuccessfully created object, at least you haven’t invoked the constructor and handed it a zero this pointer. Here’s an example to prove it:

//: C13:NoMemory.cpp
// Constructor isn't called if new fails
#include <iostream>
#include <new> // bad_alloc definition
using namespace std;

class NoMemory {
  NoMemory() {
    cout << "NoMemory::NoMemory()" << endl;
  void* operator new(size_t sz) throw(bad_alloc){
    cout << "NoMemory::operator new" << endl;
    throw bad_alloc(); // "Out of memory"

int main() {
  NoMemory* nm = 0;
  try {
    nm = new NoMemory;
  } catch(bad_alloc) {
    cerr << "Out of memory exception" << endl;
  cout << "nm = " << nm << endl;
} ///:~

When the program runs, it does not print the constructor message, only the message from operator new( ) and the message in the exception handler. Because new never returns, the constructor is never called so its message is not printed.

It’s important that nm be initialized to zero because the new expression never completes, and the pointer should be zero to make sure you don’t misuse it. However, you should actually do more in the exception handler than just print out a message and continue on as if the object had been successfully created. Ideally, you will do something that will cause the program to recover from the problem, or at the least exit after logging an error.

In earlier versions of C++ it was standard practice to return zero from new if storage allocation failed. That would prevent construction from occurring. However, if you try to return zero from new with a Standard-conforming compiler, it should tell you that you ought to throw bad_alloc instead.

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