a null pointer Error

[mohammedmergani]

when i run my code i found a null pointer in the line following :
TFile * file = new TFile("ResultData18JUL.root", "READ");
TIter nextkey(gDirectory->GetListOfKeys());
TKey* key;
while (key == (TKey*)nextkey()){
if( !strcmp( key->GetName(), "OutputCent0_10")){
TList * V0Xi = (TList*)key->ReadObj();
could someone give a suggestion?
Thanks

[Peter87]

Use a debugger see where the "null pointer error" is coming from.

[mohammedmergani]

I didn't have idea about the debugger How can i used

[MikeyBoy]

The debugger will be part of your IDE. The best way to learn to use it is to consult the documentation for your specific IDE.

[coder777]

One of the problems is certrainly this:

while (key == (TKey*)nextkey()){

Note that == compares the key. In this case you want it assigned (Note: single =).

[jonnin]

it could be almost anything. you didnt check that your file opened correctly.
nextkey uses something we don't know what is.
key ==, key had no value (not initialized) and probably bug (agree with earlier post)
assumes key is valid and dereferences it in strcmp