map::emplace with muliple constructor arguments

[Florian Lindner]

Hello,

I have this piece of code:

#include <map>
#include <tuple>

class Test {
public:
Test(int a, int b) : _a(a), _b(b) {}

int _a, _b;
};


int main(int argc, char *argv[])
{
std::map<int, Test> map;

map.emplace(0, 1, 2);
return 0;
}


I try to insert a Test object that is initialized with a=1, b=2. The
std::map key should be 0.

However, this does not work like that. I also tried putting 1,2 insert
a make_tuple.

Thanks,
Florian

[Robert Jakob]

There are several examples here:
http://en.cppreference.com/w/cpp/container/map/emplace

As far as I can see, it isn't possible to do this with
one call and direct ctor argument to m.emplace only.

In your example:

map.emplace(std::piecewise_construct,
std::forward_as_tuple(0),
std::forward_as_tuple(1,2));

Best,
r.

[Ville Voutilainen]

Why not just write

map.emplace(0, Test(1, 2));

?

[codyd...@gmail.com]

Extending from this Stack Overflow post, if you write "map.emplace(0, Test(1,2))", I believe it will create a temporary object of type Test given parameters (1,2), then copy the Test object into the map, then destruct the temporary Test object. The purpose of the emplace function is to be able to construct your objects in the correct place initially, so no copying and/or destruction has to occur.

[tko...@google.com]

On Monday, 2 May 2016 13:46:16 UTC+1, Florian Lindner wrote:

Hello,

I have this piece of code:

#include <map>
#include <tuple>

class Test {
public:
  Test(int a, int b) : _a(a), _b(b) {}

  int _a, _b;
};


int main(int argc, char *argv[])
{
  std::map<int, Test> map;

  map.emplace(0, 1, 2);
  return 0;
}


I try to insert a Test object that is initialized with a=1, b=2. The
std::map key should be 0.

How about "map.try_emplace(0, 1, 2);"?

Demo: https://wandbox.org/permlink/eD95RWScaGm15wuA