Using arrays or std::vectors in C++, what's the performance gap?

Question

In our C++ course they suggest not to use C++ arrays on new projects anymore. As far as I know Stroustroup himself suggests not to use arrays. But are there significant perf

Answer 1

Sometimes arrays are indeed better than vectors. If you are always manipulating a fixed length set of objects, arrays are better. Consider the following code snippets:

int main() {
int v[3];
v[0]=1; v[1]=2;v[2]=3;
int sum;
int starttime=time(NULL);
cout << starttime << endl;
for (int i=0;i<50000;i++)
for (int j=0;j<10000;j++) {
X x(v);
sum+=x.first();
}
int endtime=time(NULL);
cout << endtime << endl;
cout << endtime - starttime << endl;

}

where the vector version of X is

class X {
vector vec;
public:
X(const vector& v) {vec = v;}
int first() { return vec[0];}
};

and the array version of X is:

class X {
int f[3];

public:
X(int a[]) {f[0]=a[0]; f[1]=a[1];f[2]=a[2];}
int first() { return f[0];}
};

The array version will of main() will be faster because we are avoiding the overhead of "new" everytime in the inner loop.

(This code was posted to comp.lang.c++ by me).