CODEWITHSUNDEEP
c++malloc
vladCovaliov
vladCovaliov

Reputation: 4403

C++ dynamic allocation program crashes

I have a very simple piece of code with 2 structures and one dynamic allocation. The program crashes on the "nume" initialization. 

typedef struct{
    int key;
    string name;
} TElement;

typedef struct nod {
   int cheie;
   string nume;
   struct nod *stg, *dr;
} NOD;

when i try to do this 

void ABC::inserare_element(TElement e){
    NOD *p, *q;
    int n;
    /* construction nod p*/
    n=sizeof (NOD);
    p=(NOD*)malloc(n);
    p->cheie = e.key;
    p->nume = e.name; // on this line the program crashes

Thanks

Upvotes: 2

Views: 256

Answers (3)

hmjd
hmjd

Reputation: 122001

malloc() will not invoke the constructor of NOD, meaning the constructor of nume will not have been invoked resulting in an attempt to use std::string::operator= on an unconstructed/uninitialized std::string: use new.

Upvotes: 7

user405725
user405725

Reputation:

You have a hot mix here of high-level C++ objects, such as std::string and C-style memory allocations such as malloc. The thing is that C++'s new operator not only allocates memory, but also calls constructor of high-level objects. The problem you are facing is that nume object of type std::string is not initialized properly, thus you run into undefined behavior that leads to a crash. That is because you are lucky. It could have been a lot worse if program was actually working, but producing strange, unexpected results.

To make it work like you want, you can simply use new instead of malloc. For example:

p = new NOD;

If it so happens that you really need to use malloc or other memory management API that does not care about C++ objects, then you have to call a constructor of nume manually, using a placement new. For example:

p = (NOD*)malloc(n);
new ((void *)&p->nume) std::string();

In case you go that way - don't forget to call destructor as well or you will end up with a memory leak.

Upvotes: 4

Loupi
Loupi

Reputation: 701

You should use new instead of malloc. malloc is a C function and it only allocates a chunk of memory. Using new will call the default constructor of your class, and the string constructor will also be called at this moment.

Upvotes: 3

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