An experiment to understand ways of copying C arrays to vector

fcarney

I found out today that most (a lot of?) std containers can accept raw pointers.

Here is the code I tested with:

{
        using namespace std;

        auto loop = [](const char* name, vector<int>& v){
            qInfo() << name;
            qInfo() << v.size() << v.capacity();
            qInfo() << v;
        };

        int arr[] = {0,1,2,3,4,5,6,7,8,9};
        size_t arrsize = sizeof(arr)/sizeof(*arr);

        // at initialization
        vector<int> vec(arr, arr+arrsize);
        loop("vec", vec);

        // resizes vector and assigns values
        vector<int> vec2;
        vec2.assign(arr, arr+arrsize);
        loop("vec2", vec2);

        // using copy
        vector<int> vec3;
        copy(arr, arr+arrsize, back_inserter(vec3));
        loop("vec3", vec3);

        // using copy with reserve
        vector<int> vec4;
        vec4.reserve(arrsize);
        copy(arr, arr+arrsize, back_inserter(vec4));
        loop("vec4", vec4);
    }

The results:

vec
10 10
std::vector(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
vec2
10 10
std::vector(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
vec3
10 16
std::vector(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
vec4
10 10
std::vector(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

I am particularly fascinated by what happened to vec3. Some kind of resize to multiple of 8 on insert? Kinda neat. That would mean that the vector resized twice if that is true.

JKSH

@fcarney said in An experiment to understand ways of copying C arrays to vector:

I found out today that most (a lot of?) std containers can accept raw pointers.

These are called range constructors. They actually accept iterators like vec.begin(), vec.end(). It just so happens that vector iterators are implemented as raw pointers.

In contrast, a linked list's iterators are not raw pointers.

I am particularly fascinated by what happened to vec3. Some kind of resize to multiple of 8 on insert? Kinda neat. That would mean that the vector resized twice if that is true.

No resizing was done. The system just "prepared" more memory than necessary during initialization. This allows you to append a few more elements without reallocating memory: http://www.cplusplus.com/reference/vector/vector/capacity/ ( EDIT: Actually, I can't tell if multiple allocations were done or not )

fcarney

@JKSH said in An experiment to understand ways of copying C arrays to vector:

No resizing was done.

        // using copy
        vector<int> vec3;
        qInfo() << vec3.capacity();
        copy(arr, arr+arrsize, back_inserter(vec3));
        loop("vec3", vec3);

Output:

0
vec3
10 16
std::vector(0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

Maybe I said the wrong term. A reserve was done.