STL map insertion efficiency: [] vs. insert

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走了就别回头了 2021-02-12 22:23

There are two ways of map insertion:

m[key] = val;

m.insert(make_pair(key, val));

My question is, which op

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故里飘歌 (楼主)

2021-02-12 22:54

On a lightly loaded system with plenty of memory, this code:

#include 
#include 
#include 
#include 

using namespace std;

typedef map  MapType;
const unsigned int NINSERTS = 1000000;

int main() {
    MapType m1;
    string s = "foobar";
    clock_t t = clock();
    for ( unsigned int i = 0; i < NINSERTS; i++ ) {
        m1[i] = s;
    }
    cout << clock() - t << endl;
    MapType m2;
    t = clock();
    for ( unsigned int i = 0; i < NINSERTS; i++ ) {
        m2.insert( make_pair( i, s ) );
    }
    cout << clock() - t << endl;
}

produces:

1547
1453

or similar values on repeated runs. So insert is (in this case) marginally faster.

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