c++11--Improving Container performance

using Emplace_back in-place construction

Emplace_back can be in- situ through the parameters of the construction of objects, do not need to copy or move memory, compared to push_back can better avoid the memory copy and move, so that the performance of the container insert element is further improved. in most cases, emplace_back should be preferred instead of push_back.
All of the standard storage (except for the array, since it is immutable and cannot be inserted) adds a similar approach: Emplace, Emplace_hint, Emplace_front, Emplace_after, Emplace_back.
If you need to use EMPLACE_XX to construct in-place, you need to provide a constructor for the class or struct.

struct a{    int A;    int b;    A (int x, int y): A (x), B (y) {};}; int main () {    vector<a> v;    (v.emplace_back);            In-place construction requires a struct or class to provide constructors    cout << v.size () << Endl;    return 0;}

using unordered container unordered containers

    c++11 added unordered container unordered_map/unordered_multimap/unordered_set/ Unordered_multiset, the elements in these containers are not sorted, are found and stored using hashes, and are therefore more efficient.  
     because a hash table is used internally for unordered containers, the key of the unordered container needs to provide the Hash_value function , other usages and map/set are the same, and for a custom key, you need to provide a hash function and a comparison function .

struct key{    std::string first;    std::string second;}; struct keyhash{    std::size_t operator () (const key& k) const{        return std::hash<std::string> () ( K.first) ^ (std::hash<std::string> () (K.second) << 1);};    }; struct keyequal{    bool operator () (const key& K1, const key& K2) {        return K1.first = = K2.first && K1. Second = = K2.second;    }}; int main () {    std::unordered_map<std::string, int> M1;    Decltype (m1) m2 = {{"Hello", 1}, {"World", 2}};        Std::unordered_map<key, std::string, Keyhash, keyequal> M6 = {{    "John", "Doe"}, "Example"},    {"Mary," Sue "}," another "}; For a custom type key, you need to provide a hash function and a comparison function    return 0;};

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c++11--Improving Container performance