CODEWITHSUNDEEP
c++stdvector
nazim sami
nazim sami

Reputation: 75

Limits of C++ Vector

i've got a issue with std::vector class, i created a struct

struct Triplet{
    int first;
    int second;
    int third;
};

And i created a vector<Triplet> T. My problem is that it won't contain as many elements as i need, even though T.max_size() = 357913941 i got only T.size() = 60540697 or T.size() = 40360465 using that function

vector<Triplet> T;
while(true)
{
    Triplet t;
    t.first = 1; t.second = 1 ; t.third = 1;
    try {
        T.push_back(t);
    } catch (...) {
        break;
    }
}

qDebug() << T.size();

can anyone explain why is it doing that please ? i'm running on Windows 10 and 16Go of RAM, with Qt and VSC++ 2017 x86 (due to Lemon library which i couldn't compile for x64),

Upvotes: 1

Views: 582

Answers (1)

Botje
Botje

Reputation: 30832

An std::vector needs a contiguous (=no holes) chunk of memory to exist in. Furthermore, when pushing elements to a vector you can overshoot the internal capacity, meaning that it has to allocate memory for another std::vector (usually double the size) and copy the elements over.

Keep in mind that in 32-bit Windows programs, you only have 2 GB of available memory space to play with in a single process, regardless of how much memory your system has. Your vector with size 60540697 * 12 is taking up 700+ MB of that. There is simply no place to allocate the next size (1.4GB) because the memory space is too small.

The simplest solution is to compile in 64-bit mode, which has plenty of virtual memory. As a stop-gap solution you could try pre-allocating space in the std::vector with T.reserve(80000000) or so. This will avoid an intermediate copy but probably will not be enough. It might even fail if your memory space is fragmented in a bad way!

Upvotes: 8

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