Question

I am observing strange behaviour of std::map::clear(). This method is supposed to call element’s destructor when called, however memory is still accessible after call to clear().

For example:

struct A {   ~A() { x = 0; }   int x; };  int main( void ) {   std::map< int, A * > my_map;   A *a = new A();   a->x = 5;   my_map.insert( std::make_pair< int, *A >( 0, a ) );    // addresses will be the same, will print 5   std::cout << a << ' ' << my_map[0] << ' ' << my_map[0]->x << std::endl;    my_map.clear();    // will be 0   std::cout << a->x << std::endl;    return 0; } 

The question is, why is variable a still accessible after its destructor was called by map::clear()? Do I need to write delete a; after calling my_map.clear() or is it safe to overwrite the contents of a?

Thanks in advance for your help, sneg

Answer 1

std::map does not manage the memory pointed to by the pointer values – it’s up to you to do it yourself. If you don’t want to use smart pointers, you can write a general purpose free & clear function like this:

template <typename M> void FreeClear( M & amap )      for ( typename M::iterator it = amap.begin(); it != amap.end(); ++it ) {         delete it->second;     }     amap.clear(); } 

And use it:

std::map< int, A * > my_map; // populate FreeClear( my_map ) 

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