CODEWITHSUNDEEP
c++vector
ClockChen7414
ClockChen7414

Reputation: 11

vector of pairs initialization in C++

In competitive programming, I see people initializing vector of pairs like below:

#include <bits/stdc++.h>
using namespace std;

typedef pair<int, int> ii;
typedef vector<ii> vii;

vector<vii> AL;
int main(){
  int V, E; scanf("%d %d", &V, &E);
  AL.assign(V, vii());
}

I want to ask about AL.assign(V, vii()); I know that it is going to have V pairs in AL, but what's the meaning of vii() here?

Upvotes: 1

Views: 276

Answers (2)

user4581301
user4581301

Reputation: 33982

In

AL.assign(V, vii());

vii() executes the std::vector default constructor to provide a temporary object to be used as a template (English term, not a C++ template) by std::vector::insert. This temporary will effectively be copied V times, filling AL up with V vectors of pairs, and then it will go out of scope.

Upvotes: 0

gsamaras
gsamaras

Reputation: 73444

From the ref:

void assign (size_type n, const value_type& val);

The new contents are n elements, each initialized to a copy of val.

val

Value to fill the container with. Each of the n elements in the container will be initialized to a copy of this value. Member type value_type is the type of the elements in the container, defined in vector as an alias of its first template parameter (T).

As @user4581301 commented: "vii() constructs a temporary variable that will be used in the assign method and then go out of scope. Fortunately a copy is made of the temporary (assuming the compiler doesn't do some time-saving trickery) so AL isn't left referring to dead space".

Upvotes: 1

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