Performance penalty using 'auto' keyword in Visual Studio 2010

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 = ... 

Answer 2

As Bo said, you have to use auto& instead of auto (Note, that there is also auto* for other cases). Here is an updated version of your code:

#include <functional> #include <iostream> #include <map> #include <vector> #include <deque> #include <time.h>  using namespace std;  typedef map<int, vector<deque<float>>> FooType; // this should have a meaningful name  void func1(FooType& m) {     vector<deque<float>>& v = m[1]; }  void func2(FooType& m) {     auto v = m[1]; }  void func3(FooType& m) {     auto& v = m[1]; }  void measure_time(std::function<void(FooType&)> func, FooType& m) {     clock_t begin=clock();     for(int i = 0; i < 100000; ++i) func(m);     cout << "100000 x func: " << (((double)(clock() - begin))/CLOCKS_PER_SEC) << " sec." << endl; }  void main() {     FooType m;     m[1].push_back(deque<float>(1000,1));      measure_time(func1, m);     measure_time(func2, m);     measure_time(func3, m); } 

On my computer, it gives the following output:

100000 x func: 0 sec. 100000 x func: 3.136 sec. 100000 x func: 0 sec.