how to use emplace_back to insert a new element into a vector of vector?-CodePudding

I'm confused by the syntax of some C concepts.

Let's say I have a vector of vector:

vector<vector<int>> data;

I can use push_back() to insert a new element:

data.push_back({1, 1});

In this way, I list initialized a new element, then a copy of this element is pushed to data?

I can also do it in this way:

vector<int> tmp{1, 1};
data.emplace_back(tmp);

But if I directly call emplace(), like in this way:

data.emplace_back(1, 1);

It does not give me the expected result.

Did I misunderstand something here?

---update: sorry, what I mean is emplace_back. My question is how to initialize directly using emplace_back instead of initializing a tmp vector first, vector<int> tmp{1, 1};.

CodePudding user response：

I can use push_back to insert a new element: data.push_back({1, 1}); In this way, I list initialized a new element, then a copy of this element is pushed to data?

exactly.

data.emplace(1, 1);

vector<Type>::emplace_back forwards its arguments to the constructor of Type. Now, std::vector<int> has a constructor that takes two integer arguments! The one specifying length (1) and fill element (1).

You can, however, inform the compiler you actually mean list initialization!

#include "fmt/format.h"
#include "fmt/ranges.h"
#include <initializer_list>
#include <vector>
int main() {
  std::vector<std::vector<int>> a;
  a.emplace_back(std::initializer_list<int>{1, 1});
  a.emplace_back(std::initializer_list<int>{2, 2});
  fmt::print("{}\n", fmt::join(a.begin(), a.end(), ";"));
}

yields the expected

 {1, 1};{2, 2}

To show it really does the in-place construction: Follow this link to gcc.godbolt.org, and observe how push_back({3,3,3,3}); actually calls the vector<int> constructor and then the insert of the vector<vector>, while emplace_back really just calls the initializer-list constructor.

CodePudding user response：

There is an emplace function, but this takes an iterator (see docs).

emplace_back will add a new element to the end of the vector.

emplace emplaces the element at a specified a position indicated by an iterator. v.emplace_back(1) and v.emplace(v.end(), 1) are the same, because the indicated position is the end of v.

see this playground