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emplace系列函数

Implementation

在学习perfect forwarding的时候，阅读了thegreenplace Perfect forwarding and universal references in C++ ，其中以 emplace_back 为例来进行说明进行说明，所以我又查阅了 cppreference std::vector<T,Allocator>::emplace_back，其中有这样的说明:

Appends a new element to the end of the container. The element is constructed through std::allocator_traits::construct, which typically uses placement-new to construct the element in-place at the location provided by the container.

emplace的实现是依赖于:

1、placement new的

2、perfect forwarding

3、Variadic Template

Optimization: avoid copy

emplace 通过avoid copy来实现Optimization。消除了temporary。

stackoverflow push_back vs emplace_back

A

Instead of taking a value_type it takes a variadic list of arguments, so that means that you can now perfectly forward the arguments and construct directly an object into a container without a temporary at all.

That's useful because no matter how much cleverness RVO and move semantic bring to the table there is still complicated cases where a push_back is likely to make unnecessary copies (or move). For example, with the traditional insert() function of a std::map, you have to create a temporary, which will then be copied into a std::pair<Key, Value>, which will then be copied into the map :

std::map<int, Complicated> m;
int anInt = 4;
double aDouble = 5.0;
std::string aString = "C++";

// cross your finger so that the optimizer is really good
m.insert(std::make_pair(4, Complicated(anInt, aDouble, aString))); 

// should be easier for the optimizer
m.emplace(4, anInt, aDouble, aString);

stackoverflow C++ std::vector emplace vs insert [duplicate]

A

Emplace takes the arguments necessary to construct an object in place, whereas insert takes (a reference to) an object.

struct Foo
{
  Foo(int n, double x);
};

std::vector<Foo> v;
v.emplace(someIterator, 42, 3.1416);
v.insert(someIterator, Foo(42, 3.1416));

zpjiang Copy constructor and emplace_back() in C++

abseil Tip of the Week #112: emplace vs. push_back

这篇介绍地非常好

emplace_back如何选择constructor？

20200404

今天碰到了这样的一个问题：

class CIStockInfoWrapper{
    CIStockInfoWrapper(int MktType, std::string Code)
            : m_MktType(MktType), m_Code(Code)
    {
        m_HQInfo = new HQInfo(m_Code, m_MktType);
    }
};

......

std::list<CIStockInfoWrapper> m_HQInfoList;
m_HQInfoList.emplace_back(MktType, std::to_string(i));

如果将CIStockInfoWrapper(int MktType, std::string Code)的说明修改为CIStockInfoWrapper(int MktType, std::string& Code)，则程序就编译报错：

hq_client_wrapper/mock_hq_accessor.h:73:63:   required from here
/usr/include/c++/4.8.2/bits/stl_list.h:114:71: 错误：对‘CIStockInfoWrapper::CIStockInfoWrapper(int&, std::basic_string<char>)’的调用没有匹配的函数
  : __detail::_List_node_base(), _M_data(std::forward<_Args>(__args)...)

./hq_client_wrapper/hqsvr_stock_info_wrapper.h:22:7: 附注： 备选需要 1 实参，但提供了 2 个

STL container emplace系列函数

基本上STL container都提供了emplace系列函数。
std::vector emplace (C++11)
emplace_back (C++11)