Destructors in STL?

sawerr

Hi I see that all stl components have destructors. string s; s.~string(); But i know that we don't need to delete stl componenets.I read all stl components automatically delete themself.Is this wrong? If true what is the function of destructors in STL? Thanks...

Christian Graus

How do you expect STL components to magically delete themselves ? Cleaning up after themselves is what C++ classes have destructors for.

Christian Graus - Microsoft MVP - C++ Metal Musings - Rex and my new metal blog

sunit5

say u have vector in such case the vector will delete the pointers but the memory referred by the pointers.So it leads to memory leak

never say die

Zac Howland

sawerr wrote:

But i know that we don't need to delete stl componenets

I don't think you quite understand C++ fundamentals. STL classes (string, vector, list, map, etc) all allocate memory on the heap to store whatever data you are throwing at them. string, for example, allocates an array of characters. As you fill up that array, it evaluates if it needs to allocate a bigger buffer, etc. All that is handled for you. The destructor cleans up that heap memory. When you write:

void f()
{
	string s;
}

s is allocated on the stack. At the end of the scope (in this case, when the function returns), s's destructor is called, which cleans up anything s had allocated. If you were to write this:

void f()
{
	string* s = new string;
}

you would have a memory leak (not good!).

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week Zac

Anonymuos

Christian Graus wrote:

How do you expect STL components to magically delete themselves ?

He probably read somwhere that he need not learn the basics but jump right into STL programming. :suss:

George L Jackson

If you have a vector of pointers to dynamically created objects, you must explicitly delete those objects before the vector goes out of scope: using std::wcout; using std::endl; using std::vector; class Foo { public: Foo(int index) : m_index(index) { wcout << L"Creating Foo #" << m_index << endl; } ~Foo() { wcout << L"Destroying Foo #" << m_index << endl; } private: int m_index; }; int _tmain(int argc, _TCHAR* argv[]) { vector pFooList; for (int i = 0; i < 10; ++i) { pFooList.push_back(new Foo(i)); } for (int i = 0; i < 10; ++i) { delete pFooList[i]; } return 0; } -- modified at 0:01 Saturday 12th August, 2006

dfields326

Use auto_ptr<> will delete them for you #include using std::wcout; using std::endl; using std::vector; class Foo { public: Foo(int index) : m_index(index) { wcout << L"Creating Foo #" << m_index << endl; } ~Foo() { wcout << L"Destroying Foo #" << m_index << endl; } private: int m_index; }; int _tmain(int argc, _TCHAR* argv[]) { vector< std::auto_ptr< Foo* > > pFooList; for (int i = 0; i < 10; ++i) { pFooList.push_back( std::auto_ptr< Foo* >(new Foo(i))); } /* no need to delete, std::auto_ptr takes care if it for you. //see #include for (int i = 0; i < 10; ++i) { delete pFooList[i]; } */ return 0; } :cool:

George L Jackson

:omg: Unfortunately, that should not compile! According to the book, "The C++ Standard Library: A Tutorial and Reference" by Nicolai M. Josuttis, I quote: auto_ptrs don't meet the requirements for container elements. An auto_ptr does not meet one of the most fundamental requirements for elements of standard containers. That is, after a copy or an assignment of an auto_ptr, source and sink are not equivalent. In fact, when an auto_ptr is assigned or copied, the source auto_ptr gets modified because it transfers its value rather than copying it. So, you should not use an auto_ptr as an element of a standard container. ... However, boost::shared_ptr can be used with containers! See: http://www.boost.org/libs/smart_ptr/shared_ptr.htm[^] George -- modified at 17:52 Thursday 17th August, 2006

George L Jackson

#include #include #include using std::wcout; using std::endl; using std::vector; using boost::shared_ptr; class Foo { public: Foo(int index) : m_index(index) { wcout << L"Creating Foo #" << m_index << endl; } ~Foo() { wcout << L"Destroying Foo #" << m_index << endl; } private: int m_index; }; int _tmain(int argc, _TCHAR* argv[]) { vector< shared_ptr > pFooList2; for (int i = 0; i < 10; ++i) { shared_ptr sptr(new Foo(i)); pFooList2.push_back(sptr); } return 0; }

Zac Howland

dfields326 wrote:

Use auto_ptr<> will delete them for you

Containers of auto_ptr's is a BIG No-No! It will compile on some older compilers, but it will not do anything close to what you expect it to. It won't even compile on newer compilers.

If you decide to become a software engineer, you are signing up to have a 1/2" piece of silicon tell you exactly how stupid you really are for 8 hours a day, 5 days a week Zac