Question

When I try to compile the following code…

struct MemPages
{
  size_t size;
  volatile sig_atomic_t acc;
}; 

typedef std::map<unsigned long, MemPages> PagesMap;
PagesMap pagesMap;
............
pagesMap.insert(pair<unsigned long, MemPages>((unsigned long)addr, memPages ));
............
// This is Line 531
MemPages& mp = pagesMap[addr]; // Error here

I get the following error…

**replication.cpp:531: error: invalid conversion from ‘void*’ to ‘long unsigned int’
replication.cpp:531: error:   initializing argument 1 of ‘_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = long unsigned int, _Tp = MemPages, _Compare = std::less<long unsigned int>, _Alloc = std::allocator<std::pair<const long unsigned int, MemPages> >]’
make: *** [all] Error 1**

Any Idea, what is going on?

Answer 1

The error says:

invalid conversion from ‘void*’ to ‘long unsigned int’

addr is apparently a void*; the key type of your map is a unsigned long. You need to pass an unsigned long (or, at least something that is convertible to an integer) to operator[].

The casting of a pointer to an integer ((unsigned long)addr) in your code is odd: there really shouldn’t be any reason to do this. If the key type of the std::map should be a pointer type, then you should make it a pointer type…