c ++：通过地图实例分配从地图的向量填充地图的地图 [英] c++ : filling map of maps via map instance allocation from a vector of maps

问题描述

第2部分：我的 https://stackoverflow.com/questions/ 21780627 / c-map-of-maps-typedef-doubtts-queries

然后，我将继续创建 InnerMap ， MiddlMap 并执行以下操作：

I then move ahead to create vectors (resizeable) of InnerMap, MiddlMap and do the following:

InnerMap inmap;
vector<InnerMap> vec_inmap;
vec_inmap.resize (8);
int vec_inmap_sz = 8;
vec_inmap.insert (vec_inmap.end (), 8, inmap);
vector<InnerMap>::iterator vec_inmap_it = vec_inmap.begin ();
InnerMap::iterator inmap_it;

MiddlMap mdmap, curr_mdmap;
vector<MiddlMap> vec_mdmap;
vec_mdmap.resize (8);
int vec_mdmap_sz = 8;
vec_mdmap.insert (vec_mdmap.end (), 8, mdmap);
vector<MiddlMap>::iterator vec_mdmap_it = vec_mdmap.begin ();
MiddlMap::iterator mdmap_it;

OuterMap otmap;
OuterMap::iterator otmap_it;

ie。我存储（空）副本的 inmap 和 mdmap （是，这些副本是通过引用吗？各向量，然后通过相应的向量迭代器从向量中提取这些向量，然后相应地填充映射。以下是如何：

i.e. I store (empty) copies of inmap and mdmap (Q. is it that these copies are by reference ?) in the respective vectors, and then pick up these later from the vectors through the respective vector iterators, and then fill the maps accordingly. Here is how:

for (i = 0; i != trainSize; i++) {
...
if (curr_key_otmap != int_key) {
    otmap[int_key] = *vec_mdmap_it;
    vec_mdmap_it++;
    mdmap_count++;
    if (mdmap_count == vec_mdmap_sz) {
        vec_mdmap_sz += 8;
        vec_mdmap.resize (vec_mdmap_sz);
        vec_mdmap.insert (vec_mdmap.end(), 8, mdmap);
    }
    curr_key_otmap = int_key;
    curr_mdmap = otmap[curr_key_otmap];
}

mdmap_it = curr_mdmap.find (int_val);
if (mdmap_it == curr_mdmap.end ()) {
    curr_mdmap[int_val] = *vec_inmap_it;       <--
    curr_mdmap[int_val][char_str] = 1;

    vec_inmap_it++;
    inmap_count++;

    if (inmap_count == vec_inmap_sz) {
        vec_inmap_sz += 8;
        vec_inmap.resize (vec_inmap_sz);
        vec_inmap.insert (vec_inmap.end(), 8, inmap);
    }
} else {
    inmap_it = (*mdmap_it).second.find (char_str);
    if (inmap_it == (*mdmap_it).second.end ()) {
        (*mdmap_it).second[char_str] = 1;
    } else {
        (*mdmap_it).second[char_str] += 1;
    }
}
...
} //for ends

在运行时间..我得到以下错误在< - ed行：有人可以详细说明吗？

over the course of run time .. i get the following error at the <--ed line: could someone elaborate?

Program received signal SIGSEGV, Segmentation fault.
0x0804f9d4 in __gnu_cxx::new_allocator<std::pair<char* const, int> >::construct (this=0xbffff06f, __p=0x8057428, __val=...)
at /usr/include/c++/4.6/ext/new_allocator.h:108
108       { ::new((void *)__p) _Tp(__val); }


#0    0x0804fa2a in __gnu_cxx::new_allocator<std::pair<char* const, int> >::construct (this=0xbffff1bf, __p=0x8057428, __val=...)
at /usr/include/c++/4.6/ext/new_allocator.h:108
#1  0x0804f3e4 in std::_Rb_tree<char*, std::pair<char* const, int>, std::_Select1st<std::pair<char* const, int> >, std::less<char*>, std::allocator<std::pair<char* const, int> > >::_M_create_node (this=0x8098d1c, __x=...) at /usr/include/c++/4.6/bits/stl_tree.h:381
#2  0x0804e25f in std::_Rb_tree<char*, std::pair<char* const, int>, std::_Select1st<std::pair<char* const, int> >, std::less<char*>, std::allocator<std::pair<char* const, int> > >::_M_clone_node (this=0x8098d1c, __x=0xfffffffd) at /usr/include/c++/4.6/bits/stl_tree.h:427
#3  0x0804c645 in std::_Rb_tree<char*, std::pair<char* const, int>, std::_Select1st<std::pair<char* const, int> >, std::less<char*>, std::allocator<std::pair<char* const, int> > >::_M_copy (this=0x8098d1c, __x=0xfffffffd, __p=0x8098d20) at /usr/include/c++/4.6/bits/stl_tree.h:1036
#4  0x0804bda5 in std::_Rb_tree<char*, std::pair<char* const, int>, std::_Select1st<std::pair<char* const, int> >, std::less<char*>, std::allocator<std::pair<char* const, int> > >::operator= (this=0x8098d1c, __x=...) at /usr/include/c++/4.6/bits/stl_tree.h:945
#5  0x0804a714 in std::map<char*, int, std::less<char*>, std::allocator<std::pair<char* const, int> > >::operator= (this=0x8098d1c, __x=...)
at /usr/include/c++/4.6/bits/stl_map.h:255
#6  0x080493ff in MyProg::classify (trainData=..., testsData=...) at my_prog.cpp:83 <-- this is the line I marked
#7  0x08049c72 in main () at my_prog.cpp:200

如果有任何东西不可破译，请向我要求编辑/澄清。

Please ask me for edits/clarifications if anything is undecipherable.

推荐答案

当您调整向量大小时，该向量的所有迭代器将变为无效，除非该空间已分配给 vector :: reserve 之前创建迭代器。这是因为如果在当前分配的块之后没有可用的内存，数据将被复制到别处。如果重定位发生，在下一次迭代时，vec_inmap_it将指向可能已被覆盖的未分配内存。因此未定义的行为。

When you resize a vector, all iterators to that vector become invalid unless that space was already allocated with vector::reserve before the iterators were created. This is because if there is no available memory after currently allocated block, the data will be copied elsewhere. If the relocation happens, on next iteration, vec_inmap_it will then be pointing to unallocated memory that may have been overwritten. Thus undefined behaviour.

// inside for loop
if (mdmap_it == curr_mdmap.end ()) {
    curr_mdmap[int_val] = *vec_inmap_it; // here you dereference vec_inmap_it
    curr_mdmap[int_val][char_str] = 1;

    vec_inmap_it++;
    inmap_count++;

    if (inmap_count == vec_inmap_sz) {
        vec_inmap_sz += 8;
        vec_inmap.resize (vec_inmap_sz); // here you resize the vector, vec_inmap_it is now invalid
        vec_inmap.insert (vec_inmap.end(), 8, inmap);
    }
...

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