Question

In an std::vector of a non POD data type, is there a difference between a vector of objects and a vector of (smart) pointers to objects? I mean a difference in the implementation of these data structures by the compiler.

E.g.:

class Test {
   std::string s;
   Test *other;
};

std::vector<Test> vt;
std::vector<Test*> vpt;

Could be there no performance difference between vt and vpt?

In other words: when I define a vector<Test>, internally will the compiler create a vector<Test*> anyway?

Answer 1

In other words: when I define a vector, internally will the compiler create a vector anyway?

No, this is not allowed by the C++ standard. The following code is legal C++:

vector<Test> vt;
Test t1; t1.s = "1"; t1.other = NULL;
Test t2; t2.s = "1"; t2.other = NULL;
vt.push_back(t1);
vt.push_back(t2);
Test* pt = &vt[0];
pt++;
Test q = *pt; // q now equal to Test(2)

In other words, a vector “decays” to an array (accessing it like a C array is legal), so the compiler effectively has to store the elements internally as an array, and may not just store pointers.

But beware that the array pointer is valid only as long as the vector is not reallocated (which normally only happens when the size grows beyond capacity).