C++ Cookbook

9.2. Making a Constructor Exception-Safe

Problem

Your constructor needs to uphold basic and strong exception-safety guarantees. See the discussion that follows for the definitions of “basic” and “strong” guarantees.

Solution

Use try and catch in the constructor to clean up properly if an exception is thrown during construction. Example 9-2 presents examples of the simple Device and Broker classes. Broker constructs two Device objects on the heap, but needs to be able to properly clean them up if an exception is thrown during construction.

Example 9-2. An exception-safe constructor

#include <iostream>
#include <stdexcept>

using namespace std;

class Device {
public:
   Device(int devno) {
      if (devno == 2)
         throw runtime_error("Big problem");
   }
  ~Device() {}
};

class Broker {
  
public:
   Broker (int devno1, int devno2) :
      dev1_(NULL), dev2_(NULL) {
      try {                            
         dev1_ = new Device(devno1);  // Enclose the creation of heap
         dev2_ = new Device(devno2);  // objects in a try block...
      }
      catch (...) {
         delete dev1_;                // ...clean up and rethrow if
         throw;                       // something goes wrong.
      }
   }
  ~Broker() {
      delete dev1_;
      delete dev2_;
   }

private:
   Broker();
   Device* dev1_;
   Device* dev2_;
};

int main() {

   try {
      Broker b(1, 2);
   }
   catch(exception& e) {
      cerr << "Exception: " << e.what() << endl;
   }
}

Discussion

To say that a constructor, member function, destructor, or anything else is “exception-safe” is to guarantee that it won’t leak resources and possibly that it won’t leave its object in an inconsistent state. In C++, these two kinds ...