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std::unordered_map::emplace behavior with no move/copy constructor


std::unordered_map::emplace behavior with no move/copy constructor

By : user2951243
Date : November 18 2020, 01:01 AM
like below fixes the issue You seem to have some misunderstanding about basic terminology. A default constructor is one that can be called with no arguments. FooBar(int* pFoo, int* pBar) is not a default constructor.
Moreover, to get your code to compile on gcc and clang, I had to modify your copy-constructor to FooBar(FooBar const& rhs).
code :
map.emplace(std::piecewise_construct,
            std::forward_as_tuple(40),
            std::forward_as_tuple((int*)0xDEADBEEF, (int*)0x01010101));
Foobar default constructor called


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Move constructor, non-const copy constructor, list's emplace in VS2010

Move constructor, non-const copy constructor, list's emplace in VS2010


By : Abhinav Gupta
Date : March 29 2020, 07:55 AM
This might help you I'm new to move constructor and quite confused with VS2010 behavior. I design a move constructor (of class A) which in my knowledge is like this:
code :
mylist.emplace(a);
mylist.emplace(std::move(a));
std::unordered_map::emplace issue with private/deleted copy constructor

std::unordered_map::emplace issue with private/deleted copy constructor


By : Salar Askar
Date : March 29 2020, 07:55 AM
Does that help If you look at the template error vomit more carefully you'll see this chunk of carrot in it:
code :
test.exe.cpp:8:3: error: 'constexpr test::test(const test&)' is private
Why does unordered_map's emplace with piecewise_construct argument needs default constructor?

Why does unordered_map's emplace with piecewise_construct argument needs default constructor?


By : Spamcaster
Date : March 29 2020, 07:55 AM
will be helpful for those in need std::unordered_map needs default constructor is because of operator[]. map[key] will construct new element using default constructor if key is not exist in map.
You can totally use map without default constructor. E.g. following program will compile with no error.
code :
struct A {
    int x;
    A(int x) : x(x) {}
}; 

...

std::unordered_map<int, A> b;

b.emplace(std::piecewise_construct,
    std::forward_as_tuple(1),
    std::forward_as_tuple(2));
b.at(1).x = 2;
Presence of unordered_map determines whether copy-constructor or move constructor is used

Presence of unordered_map determines whether copy-constructor or move constructor is used


By : Natan Dias Monteiro
Date : March 29 2020, 07:55 AM
I hope this helps . There is a standard defect (in my opinion) that unordered map's move ctor is not noexcept.
So the defaulted move ctor being noexcept(false) or deleted by your attempted default noexcept(true) seems plausible.
Why a vector constructor takes only last value of the parameter when using in unordered_map emplace?

Why a vector constructor takes only last value of the parameter when using in unordered_map emplace?


By : Charles Beer
Date : March 29 2020, 07:55 AM
Does that help I'm trying to implement unordered_map> with using unordered_map::emplace
code :
amap.emplace('k',(2,9));
amap.emplace('k', (9));
amap.emplace('k', vector<int>(2,9));
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