std::sort does not always call std::swap

Question

Consider the following code:

#include 
#include 
#include 

namespace my_space
{

struct A
{
    double  a;
           


        

Answer 1

I modified the code to be more verbose. The sorting for 20 elements uses many swaps, uses assignment end copy. Sorting for 4 elements uses only assignment and copy. Don't know about the specs, but it might be something to go on.

#include 
#include 
#include 

namespace my_space
{

struct A
{
    double  a;
    double* b;
    A()
        : a(0)
        , b(NULL)
    { }
    A(const A &rhs)
        : a(rhs.a)
        , b(rhs.b)
    {
        std::cerr << "copy" << std::endl;
    }
    A& operator=(A const &rhs)
    {
        if(this==&rhs)
                return *this;
        a = rhs.a;
        b = rhs.b;
        std::cerr << "=" << std::endl;
        return *this;
    }
    bool operator<(const A& rhs) const
    {
        return this->a < rhs.a;
    }
};

void swap(A& lhs, A& rhs)
{
    std::cerr << "My swap.\n";
    std::swap(lhs.a, rhs.a);
    std::swap(lhs.b, rhs.b);
}

} // namespace my_space

int main()
{
    const int n = 20;

        std::cerr << "=== TEST CASE: n = " << n << std::endl;
    std::cerr << "=== FILL ===" << std::endl;
    std::vector vec(n);
    for (int i = 0; i < n; ++i) {
        vec[i].a = -i;
    }

    std::cerr << "=== PRINT ===" << std::endl;
    for (int i = 0; i < n; ++i) {
        std::cerr << vec[i].a << " ";
    }
    std::cerr << "\n";

    std::cerr << "=== SORT ===" << std::endl;
    std::sort(vec.begin(), vec.end());

    std::cerr << "=== PRINT ===" << std::endl;
    for (int i = 0; i < n; ++i) {
        std::cerr << vec[i].a << " ";
    }
    std::cerr << "\n";
}

Outputs

=== TEST CASE: n = 4
=== FILL ===
copy
copy
copy
copy
=== PRINT ===
0 -1 -2 -3
=== SORT ===
copy
=
=
copy
=
=
=
copy
=
=
=
=
=== PRINT ===
-3 -2 -1 0

And

=== TEST CASE: n = 20
=== FILL ===
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
copy
=== PRINT ===
0 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10 -11 -12 -13 -14 -15 -16 -17 -18 -19
=== SORT ===
copy
My swap.
My swap.
My swap.
My swap.
My swap.
My swap.
My swap.
My swap.
My swap.
My swap.
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
copy
=
copy
=
copy
=
copy
=
copy
=
=== PRINT ===
-19 -18 -17 -16 -15 -14 -13 -12 -11 -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0