Question

Using the new auto keyword has degraded my code execution times. I narrowed the problem to the following simple code snippet:

#include <iostream>
#include <map>
#include <vector>
#include <deque>
#include <time.h>

using namespace std;

void func1(map<int, vector<deque<float>>>& m)
{
    vector<deque<float>>& v = m[1];
}

void func2(map<int, vector<deque<float>>>& m)
{
    auto v = m[1];
}

void main () {

    map<int, vector<deque<float>>> m;
    m[1].push_back(deque<float>(1000,1));

    clock_t begin=clock();
    for(int i = 0; i < 100000; ++i) func1(m);
    cout << "100000 x func1: " << (((double)(clock() - begin))/CLOCKS_PER_SEC) << " sec." << endl;

    begin=clock();
    for(int i = 0; i < 100000; ++i) func2(m);
    cout << "100000 x func2: " << (((double)(clock() - begin))/CLOCKS_PER_SEC) << " sec." << endl;

}

The output I get on my i7 / Win7 machine (Release mode; VS2010) is:

100000 x func1: 0.001 sec.
100000 x func2: 3.484 sec.

Can anyone explain why using auto results in such a different execution times?

Obviously, there is a simple workaround, i.e., stop using auto altogether, but I hope there is a better way to overcome this issue.

Answer 1

You are copying the vector to v.

Try this instead to create a reference

auto& v = ...