为什么 C++ 数组类比 C 风格的数组需要更多的时间来操作？

Question

I have written a simple code to compare the time taken to operate on the elements of two arrays (both of same size), one defined by C++ array class and the other by plain C-style array. The code I have used is

    #include <iostream>
    #include <array>
    #include <chrono>   

    using namespace std;

    const int size = 1E8;
    const int limit = 1E2;

    array<float, size> A;
    float B[size];

    int main () {

       using namespace std::chrono;

    //-------------------------------------------------------------------------------//
       auto start = steady_clock::now();
           
       for (int i = 0; i < limit; i++)
       for (int j = 0; j < size; j++)
                A.at(j) *= 1.;     

       auto end = steady_clock::now();
       auto span = duration_cast<seconds> (end - start).count();
       
       cout << "Time taken for array A is: " << span << " sec" << endl;
    //-------------------------------------------------------------------------------//
       start = steady_clock::now();
       
       for (int i = 0; i < limit; i++)
       for (int j = 0; j < size; j++) 
               B[j] *= 1.;
    
       end = steady_clock::now();
       span = duration_cast<seconds> (end - start).count();
       
       cout << "Time taken for array B is: " << span << " sec" << endl;
    //-------------------------------------------------------------------------------//

       return 0;
    }

which I have compiled and run with

g++ array.cxx
./a.out

The output I get is the following

Time taken for array A is: 52 sec
Time taken for array B is: 22 sec

Why does the C++ array class takes much longer to operate on?

Answer 1

The std::array::at member function does bounds-checking so, of course, there is some extra overhead. If you want a fairer comparison use std::array::operator[] , just like the plain array.