How can i estimate memory usage of std::map?
The estimate would be closer to
(sizeof(A) + sizeof(B) + ELEMENT_OVERHEAD) * N + CONTAINER_OVERHEAD
There is an overhead for each element you add, and there is also a fixed overhead for maintaining the data structure used for the data structure storing the map. This is typically a binary tree, such as a Red-Black Tree. For instance, in the GCC C++ STL implementation ELEMENT_OVERHEAD would be sizeof(_Rb_tree_node_base) and CONTAINER_OVERHEAD would be sizeof(_Rb_tree). To the above figure you should also add the overhead of memory management structures used for storing the map's elements.
It's probably easier to arrive at an estimate by measuring your code's memory consumption for various large collections.
You could use MemTrack, by Curtis Bartley. It's a memory allocator that replaces the default one and can track memory usage down to the type of allocation.
An example of output:
-----------------------
Memory Usage Statistics
-----------------------
allocated type blocks bytes
-------------- ------ -----
struct FHRDocPath::IndexedRec 11031 13.7% 2756600 45.8%
class FHRDocPath 10734 13.3% 772848 12.8%
class FHRDocElemPropLst 13132 16.3% 420224 7.0%
struct FHRDocVDict::IndexedRec 3595 4.5% 370336 6.2%
struct FHRDocMDict::IndexedRec 13368 16.6% 208200 3.5%
class FHRDocObject * 36 0.0% 172836 2.9%
struct FHRDocData::IndexedRec 890 1.1% 159880 2.7%
struct FHRDocLineTable::IndexedRec 408 0.5% 152824 2.5%
struct FHRDocMList::IndexedRec 2656 3.3% 119168 2.0%
class FHRDocMList 1964 2.4% 62848 1.0%
class FHRDocVMpObj 2096 2.6% 58688 1.0%
class FHRDocProcessColor 1259 1.6% 50360 0.8%
struct FHRDocTextBlok::IndexedRec 680 0.8% 48756 0.8%
class FHRDocUString 1800 2.2% 43200 0.7%
class FHRDocGroup 684 0.8% 41040 0.7%
class FHRDocObject * (__cdecl*)(void) 36 0.0% 39928 0.7%
class FHRDocXform 516 0.6% 35088 0.6%
class FHRDocTextColumn 403 0.5% 33852 0.6%
class FHRDocTString 407 0.5% 29304 0.5%
struct FHRDocUString::IndexedRec 1800 2.2% 27904 0.5%
If you really want to know the runtime memory footprint, use a custom allocator and pass it in when creating the map. See Josuttis' book and this page of his (for a custom allocator).
Maybe it's easier to ask for upper bound?
The upper bound will depend on the exact implementation (e.g. the particular variant of balanced tree used). Maybe, you can tell us why you need this information so we can help better?