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()         


        

Answer 1

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 
#include 

#include 

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

/// Returns a vector containing the sequence [ 0, ... , n-1 ].
inline std::vector _range(const int n)
{
    std::vector tmp(n);
    for(int i=0; i testdata1 = _range(100000000);
    const vector testdata2 = _range(100000000);

    vector testdata;

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