Question

EDIT: thank you for the answers! I declared the tellSomething method with a std::string return type while it should have been void!

I was tripping myself up and blamed the poor guiltless delete operator :)!

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Let’s consider a pointer to a dynamically allocated vector which contains pointers to dynamically allocated objects:

// Create the vector of pointers
std::vector<A *>* v = new std::vector<A *>;

// Create two objects
A *a1 = new A;
A *a2 = new A;

// Populate the vector
v->push_back(a1);
v->push_back(a2);

// Delete the vector
delete v;

// Try accessing one of the objects
a1->tellSomething();    --> // Segmentation fault

As expected, if I delete the vector, the delete on the contained objects is not called (I also verified that A::~A() is never called in the above code), however, the last instruction gives a segmentation fault.

What I expect from the delete v is two things:

• The destructor for every contained object is called
• The container is deallocated

But in this case the contained objects are pointers, so no destructor is called.

Also, a1 is not NULL at the end of the listing.

So, why the segmentation fault?

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Complete example here: http://ideone.com/r8YC0.

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Note: I don’t usually use raw pointers with STL containers, please, consider this code as a purely theoretic example to help me understanding the logic of the delete v instruction.

Answer 1

The crash you get is completely unrelated; you declared tellSomething as returning a std::string, but you never return anything, so you go into undefined behavior-land; the fact that the program crashed after deallocating the vector is sheer luck, it could have crashed even at the first call to tellSomething.

Fixing that problem makes your program run fine (although you are leaking a1 and a2).

By the way, this teaches you to turn up all the warnings: with -Wall that code would have given you an explicit warning about this potential problem:

matteo@teolapmint ~/cpp $ g++ -Wall testwarns.cpp 
testwarns.cpp: In member function ‘std::string A::tellSomething()’:
testwarns.cpp:12:5: warning: no return statement in function returning non-void [-Wreturn-type]

(just for the record: personally I recommend to compile with -Wall -Wextra -ansi -pedantic, often one warning can save you a lot of debug time).