Question

#include <iostream>
#include <cassert>
#include <vector>
#include <ctime>
#include <cstdlib>
#include <Windows.h>

using namespace std;

char randomLetter()
{
    srand(time(0));
    char rValue;

    while(1)
    if((rValue=(rand()/129)) > 31) 
        return rValue;
}


int main()
{       
    vector<char> meegaString;

    for(int i=0; i < 10000000000; i++)
    {   
        meegaString.push_back(randomLetter());

                if(!(i%10000000)) 
            cout<<"There are: " <<i+1<<" chars in the list"<<endl;

    }

    system("pause");
    return 0;
}

RAM usage before running this program was approximately 2500/8000 MB.
When it comes to 3200, the following exception is thrown:

Unhandled exception at 0x773c15de in Resources gormandizer.exe:
Microsoft C++ exception: std::bad_alloc at memory location
0x0045f864..

1) Why this program didn’t fill the whole available memory , although it was working on a 64 bit OS?

2) Why only 26 percent of processor(intel core i5) were in usage?

Answer 1

1. As noted, the elements of a vector are stored contiguously. Also, depending on the memory allocation algorithm used in your implementation of std::vector, it is probably attempting to pre-allocate memory ahead of time; allocating more memory than is being used to cut down on the number of malloc/new calls. As such, it might be getting to the point where it’s requesting more memory than a 32-bit OS can support (which would explain why a 64-bit process would work, but a 32-bit process wouldn’t, despite having enough memory available).

2. Your process is running on one core out of 4, and it’s quite busy, hence it’s taking up approximately 25% of the CPU time. Other processes would make up the rest.

See also: Being Smart About Vector Memory Allocation