c++ - emptying an std::queue through a function that returns the front() and pops() -

i'm writing template class supposed provide queue protected against concurrent access, able write classes can treat list of items variable number of concurrent worker threads.

the template class has method:

bool getfront(t &value) { critsectex::scope scope(listlock);   bool ret = false;     if( !itemlist.empty() ){         value = itemlist.front();         itemlist.pop();         ret = true;     }     return ret; } 

which locks queue, , if it's not empty fetches 1st element, pops , returns true, each worker can do

    while( getfront(entry) ){         // entry         ntreated += 1;     }     // exit worker 

the question is: returning here through value reference? when test code, double free error because type t i'm using contains pointer (let's call b) that's deleted in dtor (when b not marked being pointer global structure). dtor has been called during pop() in getfront(), seems logical. took care type t's ctors allocate copy of b when creating new t existing instance , not subclass provide '=' operator, i'm not sure how i'd end 2 instances of t each contain same value of b , "we own b" member.

i don't sort of thing enough, i'm overlooking here, what? suggestions how troubleshoot this?

additional observation: appears it's last element popped queue shows behaviour.

to make less vague, here's use typename t:

struct folder_info {     // plenty of members snipped #ifdef __cplusplus public:     folder_info()     {         memset( this, 0, sizeof(struct folder_info) );     }     folder_info(const struct folder_info *src)     {         init(src);     }     folder_info(const struct folder_info &src)     {         init(&src);     } private:     void init(const struct folder_info *src)     {         memcpy( this, src, sizeof(struct folder_info) );         // don't duplicate filetypeslist!         filetypeslist = null;         filetypeslistlen = filetypeslistsize = 0;     } #endif };  typedef struct fileentry { public:     std::string filename;     struct stat fileinfo;     folderinfo *folderinfo;     bool freefolderinfo;     fileentry();     fileentry( const char *name, const struct stat *finfo, folderinfo *dinfo, const bool owninfo=false );     fileentry( const char *name, const struct stat *finfo, folderinfo &dinfo );     fileentry(const fileentry &ref);     ~fileentry(); } fileentry;  fileentry::fileentry() {     folderinfo = null;     freefolderinfo = false; }  fileentry::fileentry( const char *name, const struct stat *finfo, folderinfo *dinfo, const bool owninfo ) {     filename = name;     fileinfo = *finfo;     folderinfo = (owninfo)? new folderinfo(dinfo) : dinfo;     freefolderinfo = owninfo; }  fileentry::fileentry( const char *name, const struct stat *finfo, folderinfo &dinfo ) {     // creating fileentry folderinfo reference makes copy     // of folderinfo structure     fileentry( name, finfo, new folderinfo(dinfo), true ); }  fileentry::fileentry(const fileentry &ref) {     filename = ref.filename;     fileinfo = ref.fileinfo;     if( ref.freefolderinfo ){         folderinfo = new folderinfo(ref.folderinfo);     }     else{         folderinfo = ref.folderinfo;     }     freefolderinfo = ref.freefolderinfo; }  fileentry::~fileentry() {     if( freefolderinfo && folderinfo ){         delete folderinfo;         folderinfo = null;     } } 

in case right, problem lack of user-defined assignment operator fileentry.

your copy constructor takes care of fact each fileentry has new folderinfo if copied fileentry has freefolderinfo == true, whereas assignment operator not.

if assign 1 fileentry 1 , if both have freefolderinfo == true, you'll delete same pointer twice, last of 2 goes out of scope / deleted.

what rule of three?

if need explicitly declare either destructor, copy constructor or copy assignment operator yourself, need explicitly declare 3 of them.