Throw exception in constructor c++

Question

I have created a class and I want not to create the object if one member is empty. These are the code lines:

#include "verification/CVerifObj.hpp"

VerifObj::VerifObj(const fs::path& imgNameIn)
{
    m_image = cv::imread(imgNameIn.string());
    AnalyseCSV csv;
    m_plateContour = csv.getPlateRegion(imgNameIn); // search for the csv file and load the values
    if (m_plateContour.empty()) // throw exception if empty csv
    {
        throw EmptyContourException(imgNameIn.string());
    }
    m_imageName = imgNameIn.string();
}

VerifObj::~VerifObj()
{
    // are these enough for destructor?
    m_image.release();
    m_plateContour.clear();
}

Is this OK, or shall I do something more? How shall I be sure that if the exception is thrown the object is not created?

I have the following lines of code for ensuring it:

for(fs::directory_iterator iter(folderIn); iter != end; ++iter)
{
    if(!fs::is_regular_file(iter->status()))
    {
        continue;
    }
    std::string extension = iter->path().extension().string();
    if(targetExtensions.find(extension) == targetExtensions.end()) 
    { 
        continue;
    }
    try
    {
        VerifObj verifObj(iter->path());
        verifObjVecRet.push_back(verifObj);
    }
    catch (EmptyContourException& ece)
    {
        continue;
    }
    catch (CSVException &csve)
    {
        continue;
    }
}

IdeaHat · Accepted Answer

The short answer is, throwing stuff in the constructor is dangerous.

First, lets define the design problem: you have a class that can fail to initialize. If the class fails to initialize, it cannot be used (will cause additional errors if used), so the class failing is considered a "critical failure", atleast where the class is concerned.

In a nutshell, what we want to avoid is letting a user use a class that failed to initialize.

Imagine the following situation:

class A
{
  public:
  A()
  {
    stuff1 = malloc(100);//some dynamic memory allocation
    throw "This throw is crazy";
    stuff2 = malloc(100);//some dynamic memory allocation
  }
  ~A() {free(stuff1); free(stuff2);}
  private: void* stuff2;void* stuff2;
};

int main(int argc, char** argv)
{
  A a;
}

Once you throw in the constructor, what happens to stuff? well, it is an instant memory leak. The destructor is never called. Which is baaaad. If you handle the exception:

int main(int argc, char** argv)
{
  try
  {
  A a;
  }
  catch(...)
  {
     //can't do much here
  }
}

You've lost the reference to A, which is a nightmare. So some people try to get away with this instead (Up front, it is still bad)

int main(int argc, char** argv)
{
   A* a;

   try { a= new A();}
   catch(...){delete a;}
}

But that is just as bad. You may still have a reference to a (and the memory directly pointed to by a will not leak) but a is now in an unknown state...you'll still have to delete it some day, and the free for stuff2 failed.

One way to get around this is to make your constructor and destructor alot smarter. Catch any exceptions that can be thrown in the constructor, and clean up the object, returning a "zombie" object. And have the destructor easily be able to check for the zombie object. I find this method to be more complicated.

A better way to some is to use an initializer:

 class A
 {
 public:
    A() {stuff1=null; stuff2=null;}
    void init()
    {
      stuff1 = malloc(100);//some dynamic memory allocation
      throw "This throw is crazy";
      stuff2 = malloc(100);//some dynamic memory allocation
    }
    void destroy() {if (stuff1) {delete stuff1; stuff1=NULL;} if (stuff2) {delete stuff2; stuff2=NULL;}}
    ~A() {destroy();}
 };

I don't like this because you still get "zombie" objects, and have all the added upkeep of calling init and destroy. Going back to the original problem, both a smarter constructor and providing initalizers still don't solve the nutshell statement: the end user can still (pretty easily, in fact) use an instance that is in an unknown state.

I like to follow a pseudo RAII (Resource Acquisition Is Initialization) here: what you want to to ensure that, if you have a reference to an object, it is a valid reference. Else, there should be no memory leaks. The best way (IMHO) to achieve this is to use a factory method, and keep all the initializes private.

 class A
 {
 public:
    ~A() {destroy();}
     static A* MakeA()
     {
         A* ret = new A();
         try { ret->init();}
         catch(...)
         {
            delete ret;
            return NULL;
         }
         return ret;
     }
  private: void* stuff1,*stuff2;
          A() {stuff1=null; stuff2=null;}
    void init()
    {
      stuff1 = malloc(100);//some dynamic memory allocation
      throw "This throw is crazy";
      stuff2 = malloc(100);//some dynamic memory allocation
    }
    void destroy() {if (stuff1) {delete stuff1; stuff1=NULL;} if (stuff2) {delete stuff2; stuff2=NULL;}}
 };

Now the user can never make a valid reference to a failed object, which is nice.

Protips:

Exception handling is slow. If you expect this to happen often, or don't think that it will be a fatal error, use error codes in your init statements. Error codes in the factory method can give the user more info as well.
I used raw pointers in this answer cuz I'm too lazy to write all those characters in an answer. Always use smart pointers and this becomes even easier. Juggling raw pointers is only for the criminally insane. (CLARIFYING USE SMART POINTERS)

Edit: clarified what I'm lazy about

Throw exception in constructor c++

Answers (2)

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