Checking if all elements of a vector are equal in C++

you can use std::equal

version 1:

//assuming v has at least 1 element
if ( std::equal(v.begin() + 1, v.end(), v.begin()) )
{
    //all equal
}

This will compare each element with the previous one.

version 2:

//assuming v has at least 1 element
int e = v[0]; //preferably "const auto& e" instead
bool all_equal = true;
for(std::size_t i = 1,s = v.size();i<s && all_equal;i++)
    all_equal = e == v[i];

Edit:

Regarding performance, after testing with 100m elements i found out that in Visual Studio 2015 version 1 is about twice as fast as version 2. This is because the latest compiler for vs2015 uses sse instructions in c++ std implementations when you use ints, float , etc..

if you use _mm_testc_si128 you will get a similar performance to std::equal

You need not to use std::sort. It can be done in a simpler way:

if ( std::adjacent_find( myvector.begin(), myvector.end(), std::not_equal_to<>() ) == myvector.end() )
{
    std::cout << "All elements are equal each other" << std::endl;
}

Given no constraints on the vector, you have to iterate through the vector at least once, no matter the approach. So just pick the first element and check that all others are equal to it.