7

Here is the situation :

I use a malloc to allocate memory for a struct. The struct contains various items such as pointers, string variables and vectors.

The fact is, when we use malloc, no constructors are called. Using a code similar to the following one, I've experienced some situation where some variables worked while others didn't.

Note : The following code doesn't compile. It's purpose is only to illustrate the situation.

struct MyStruct 
{
    MyClass*    mFirstClass;
    bool        mBool;
    std::string mString;
    std::vector<MyClass> mVector;
};


int main()  
{  

    MyStruct* wMyStructure;  
    wMyStructure = (MyStruct*) malloc (sizeof(MyStruct));  

    MyClass wMyClassObject;

    wMyStructure->mFirstClass = new MyClass();  
    wMyStructure->mFirstClass->func();  
    wMyStructure->mBool = false;  
    wMyStructure->mString = "aString";  
    wMyStructure->mVector.push_back(wMyClassObject);

    return 0;
}

By using pointers instead of those variables (std::string* mString), followed by a call to the object constructor (mString = new std::string;) Exception are not thrown.

However, I've experienced a situation where the mString was used without problem without the constructor being called, but when it came to the vector, the application exit automatically.

This left me with many questions:

  1. When will an object throw an exception if no constructor were used?

  2. In the situation I experienced, only the vector caused problem. Could mString be left as it is or should I call it's constructor?

  3. What would be the safest way, using malloc, to do the whole thing?

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5
  • 1
    You may have an interesting question, but please please do rewrite this one, from scratch, and work on the presentation. Nothing in the question makes sense or is C++ (there's no void main() to start with). And please don't call your class "Class", for your respected readers' sake. Commented Aug 16, 2011 at 22:21
  • 3
    i see no call to malloc in your code. Lookup the 'placement new operator'. Commented Aug 16, 2011 at 22:24
  • 2
    @bmargulies: More aptly perhaps the "placement-new expression". The operator is fairly boring, and you don't ordinarily call the operator directly anyway. Commented Aug 16, 2011 at 22:26
  • Sorry, I was distracted when I wrote the question. I hope this time it will make more sense. Commented Aug 16, 2011 at 23:08
  • Well I can't really post the code I'm working on since it's not mine. As for the casting, I made a mistake, it should be MyStruct*. My main goal was not really to get the solution to the problem, but to understand the cause of the problem. Commented Aug 16, 2011 at 23:20

4 Answers 4

Reset to default
5

Your code causes undefined behaviour, because your wMyStructure does not point to an object, so you may not use the accessor operator -> on it.

An object only commences its life after its constructor has completed. Since you don't call any constructor, you do not have an object.

(If your struct were a POD, i.e. just consisting of primitive types and PODs, then this would be OK, because PODs have trivial constructors, which do nothing.)

The concrete problem you're facing is that the string and vector members of your struct didn't get to call their constructors, so those members don't exists, and hence the entire object doesn't.

If you want to decouple memory management from object construction, you can use placement syntax:

// get some memory
char arena[HUGE_VAL];
void * morespace = malloc(HUGE_VAL);

// construct some objects
MyClass   * px = new (arena + 2000) MyClass;  // default constructor
YourClass * py = new (morespace + 5000) YourClass(1, -.5, 'x');  // non-default constructor

(You have to destroy those objects manually, px->~MyClass(); etc., when you're done with them.)

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6 Comments

Ok, This I can understand. What I don't get is why the string variable was working well even though no constructor was called. Shouldn't it have caused an exception when I tried to assign a value to the variable ?
@Sim: Well, it's undefined behaviour, so anything could happen, including (most dangerously) exactly what you'd expect. Assignment overwrites, so it's possible that you might have got away with that; accessing (or even just calling size() may get you into trouble sooner. Anyway, just don't, it's not allowed and undefined, and there's nothing meaningful you can reason about.
@ Kerrek : Ok, this answer my question fairly well. I'll just call the constructors/destructors on every object contained in the struct.
@Sim: You can't call the constructors of member objects directly. You can only explicitly construct objects with new expressions. But why would you need to do this? This almost guaranteedly makes no sense. Post what your real situation is and we can surely find a proper solution.
@Sim: If your memory comes pre-allocated, then that sounds like a good use case for the placement-new. That way you don't need to turn any members into pointers.
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5

Using object without constructing it must be an undefined behaviour. Anything may happen at any moment. If you do this, you must not rely on any part of your code to run smoothly, because the language doesn't guarantee anything in this case.

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1 Comment

Agreed. Even if it looks like it's working, bad things may happen down the line.
2

1 ) When will an object throw an exception if no constructor were used ?

If you don't call the constructor, there is no object. You have just allocated some space.

2 ) In the situation I experienced, only the vector caused problem. Could mString be left as it is or should I call it's constructor ?

This is all undefined behavior, just about anything could happen. There are no rules.

3 ) What would be the safest way, using malloc, to do the whole thing ?

The safest way would be not to use malloc, but allocate using new that will call constructors. It is as simple as this

MyStruct* wMyStructure = new MyStruct;
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Comments

1

It is undefined behaviour to use a non-initialized object. Exception may be thrown at any time- or not at all.

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1 Comment

Pedantically, there's no such thing as a "non-initialized object". An object begins its life when the constructor has completed. Otherwise there's no object.

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