pointer to a stl map

Hi,
I have some pointer to a map, because I want to be able to dynamically allocate this map at a certain time, however, I can't seem to assign any values or keys to the map. Here's some mockup code (which gives me the same error)

Code:

#include<map>
#include<stdio.h>

using namespace std;

int main()
{
  map<int, char*> * the_map;
  the_map = new map<int, char*>();
  the_map[0] = "zero";
  char * val = the_map[0];
  printf("val: %s.\n", val);
  return 0;
}

The error is:

Code:

stupid.cpp: In function ‘int main()’:
stupid.cpp:10: error: no match for ‘operator=’ in ‘* the_map = "zero"’
/usr/include/c++/4.2.1/bits/stl_map.h:226: note: candidates are: std::map<_Key, _Tp, _Compare, _Alloc>& std::map<_Key, _Tp, _Compare, _Alloc>::operator=(const std::map<_Key, _Tp, _Compare, _Alloc>&) [with _Key = int, _Tp = char*, _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, char*> >]
stupid.cpp:11: error: cannot convert ‘std::map<int, char*, std::less<int>, std::allocator<std::pair<const int, char*> > >’ to ‘char*’ in initialization

What have I missed?

Thank you so much!

That's the right syntax for assigning keys to a map, except what you have is a map pointer. Dereference the pointer, then index into the map.

Also, if you're putting string literals in, the type should be const char *.

Why do you need to dynamically allocate the map?

Originally Posted by laserlight

Why do you need to dynamically allocate the map?

The map is contained inside some struct, which is allocated via malloc.

That worked! thanks!

Originally Posted by dayalsoap

The map is contained inside some struct, which is allocated via malloc.

Why are you allocating things with malloc? Why not use new, which will automatically construct all the bits inside the struct for you so you wouldn't have to go through the insanity of using a pointer to a map?

Originally Posted by tabstop

Why are you allocating things with malloc? Why not use new, which will automatically construct all the bits inside the struct for you so you wouldn't have to go through the insanity of using a pointer to a map?

Much neater, thank you!!

Is it OK to put this struct in a header file then, with a constructor that does some conditional initialization?

Or would the code for the constructor have to go in my .cpp file?

Generally speaking, definitions of structs are exactly what header files are built for. Just like with a class, you can inline the constructor inside the struct definition if it's nice'n'short, or you can define it separately in a .cpp file.

Basically if you have a pointer to a map then your program is wrong.
To my knowledge. nobody who has asked a question on here has ever dynamically allocated a map, where that was the right thing to do.
A std::map is dynamically allocated internally already, you don't ever need to dynamically allocate it.
You also should not use malloc in a C++ program.