What is the most efficient way to append one std::vector to the end of another?

Question

Let v1 be the target vector, v2 needs to be appended to the back of it.

I'm now doing:

v1.reserve(v1.size() + v2.size()); 
copy(v2.begin(), v2.end(), back_inserter(v1));

Is this the most efficient way? Or can it maybe be done just via copying a chunk of memory? Thanks!

Answer 1

After a lot of arguing (and a reasonable comment from Matthieu M. and villintehaspam), I'll change my suggestion to

v1.insert( v1.end(), v2.begin(), v2.end() );

I'll keep the former suggestion here:

v1.reserve( v1.size() + v2.size() ); 
v1.insert( v1.end(), v2.begin(), v2.end() );

There are some reasons to do it the latter way, although none of them enough strong:

• there is no guarantee on to what size will the vector be reallocated -- e.g. if the sum size is 1025, it may get reallocated to 2048 -- dependant on implementation. There is no such guarantee for reserve either, but for a specific implementation it might be true. If hunting for a bottleneck it might be rasonable to check that.
• reserve states our intentions clear -- optimization may be more efficient in this case (reserve could prepare the cache in some top-notch implementation).
• also, with reserve we have a C++ Standard guarantee that there will be only a single reallocation, while insert might be implemented inefficiently and do several reallocations (also something to test with a particular implementation).

Answer 2

I simply did a quick performance measurement with the following code and

v1.insert( v1.end(), v2.begin(), v2.end() );

seems to be the right choice (as already stated above). Nevertheless, you find the reported performance below.

Test code:

#include <vector>
#include <string>

#include <boost/timer/timer.hpp>

//==============================================================================
// 
//==============================================================================

/// Returns a vector containing the sequence [ 0, ... , n-1 ].
inline std::vector<int> _range(const int n)
{
    std::vector<int> tmp(n);
    for(int i=0; i<n; i++)
        tmp[i] = i;
    return tmp;
}

void test_perf_vector_append()
{
    const vector<int> testdata1 = _range(100000000);
    const vector<int> testdata2 = _range(100000000);

    vector<int> testdata;

    printf("--------------------------------------------------------------\n");
    printf(" METHOD:  push_back()\n");
    printf("--------------------------------------------------------------\n");
    testdata.clear();
    { vector<int>().swap(testdata); }
    testdata = testdata1;
    {
        boost::timer::auto_cpu_timer t;
        for(size_t i=0; i<testdata2.size(); i++)
        {
            testdata.push_back(testdata2[i]);
        }
    }

    printf("--------------------------------------------------------------\n");
    printf(" METHOD:  reserve() + push_back()\n");
    printf("--------------------------------------------------------------\n");
    testdata.clear();
    { vector<int>().swap(testdata); }
    testdata = testdata1;
    {
        boost::timer::auto_cpu_timer t;
        testdata.reserve(testdata.size() + testdata2.size());
        for(size_t i=0; i<testdata2.size(); i++)
        {
            testdata.push_back(testdata2[i]);
        }
    }

    printf("--------------------------------------------------------------\n");
    printf(" METHOD:  insert()\n");
    printf("--------------------------------------------------------------\n");
    testdata.clear();
    { vector<int>().swap(testdata); }
    testdata = testdata1;
    {
        boost::timer::auto_cpu_timer t;

        testdata.insert( testdata.end(), testdata2.begin(), testdata2.end() );
    }

    printf("--------------------------------------------------------------\n");
    printf(" METHOD:  reserve() + insert()\n");
    printf("--------------------------------------------------------------\n");
    testdata.clear();
    { vector<int>().swap(testdata); }
    testdata = testdata1;
    {
        boost::timer::auto_cpu_timer t;

        testdata.reserve( testdata.size() + testdata.size() ); 
        testdata.insert( testdata.end(), testdata2.begin(), testdata2.end() );
    }

    printf("--------------------------------------------------------------\n");
    printf(" METHOD:  copy() + back_inserter()\n");
    printf("--------------------------------------------------------------\n");
    testdata.clear();
    { vector<int>().swap(testdata); }
    testdata = testdata1;
    {
        boost::timer::auto_cpu_timer t;

        testdata.reserve(testdata.size() + testdata2.size()); 
        copy(testdata2.begin(), testdata2.end(), back_inserter(testdata));
    }

    printf("--------------------------------------------------------------\n");
    printf(" METHOD:  reserve() + copy() + back_inserter()\n");
    printf("--------------------------------------------------------------\n");
    testdata.clear();
    { vector<int>().swap(testdata); }
    testdata = testdata1;
    {
        boost::timer::auto_cpu_timer t;

        testdata.reserve(testdata.size() + testdata2.size()); 
        copy(testdata2.begin(), testdata2.end(), back_inserter(testdata));
    }

}

With Visual Studio 2008 SP1, x64, Release mode, /O2 /LTCG the output is as follows:

--------------------------------------------------------------
 METHOD:  push_back()
--------------------------------------------------------------
 0.933077s wall, 0.577204s user + 0.343202s system = 0.920406s CPU (98.6%)

--------------------------------------------------------------
 METHOD:  reserve() + push_back()
--------------------------------------------------------------
 0.612753s wall, 0.452403s user + 0.171601s system = 0.624004s CPU (101.8%)

--------------------------------------------------------------
 METHOD:  insert()
--------------------------------------------------------------
 0.424065s wall, 0.280802s user + 0.140401s system = 0.421203s CPU (99.3%)

--------------------------------------------------------------
 METHOD:  reserve() + insert()
--------------------------------------------------------------
 0.637081s wall, 0.421203s user + 0.218401s system = 0.639604s CPU (100.4%)

--------------------------------------------------------------
 METHOD:  copy() + back_inserter()
--------------------------------------------------------------
 0.743658s wall, 0.639604s user + 0.109201s system = 0.748805s CPU (100.7%)

--------------------------------------------------------------
 METHOD:  reserve() + copy() + back_inserter()
--------------------------------------------------------------
 0.748560s wall, 0.624004s user + 0.124801s system = 0.748805s CPU (100.0%)

Answer 3

Probably better and simpler to use a dedicated method: vector.insert

v1.insert(v1.end(), v2.begin(), v2.end());

As Michael mentions, unless the iterators are input iterators, the vector will figure out the required size and copy appended data at one go with linear complexity.

Answer 4

If you happen to use Boost you can download the development version of the RangeEx library from the Boost Vault. This lib. was accepted into Boost a while ago but so far it hasn't been integrated with the main distribution. In it you'll find a new range-based algorithm which does exactly what you want:

boost::push_back(v1, v2);

Internally it works like the answer given by UncleBens, but the code is more concise and readable.

Answer 5

If you have a vector of pod-types, and you really need the performance, you could use memcpy, which ought to be faster than vector<>.insert(...):

v2.resize(v1.size() + v2.size());
memcpy((void*)&v1.front(), (void*)&v2[v1.size()], sizeof(v1.front())*v1.size());

Update: Although I would only use this if performance is really, really, needed, the code is safe for pod types.