why bad_alloc when new array, resize vector and push_back to vector?

Question

I need to create a very big array in my project. I tried 3 methods, but all of them turned out to be bad_alloc . I couldn't understand, as my PC's RAM is 10GB.

Here are my implementations under MSVC2015 x86 mode.

CODE1

#include<fstream>
#include<iostream>
#include<string>
#include<vector>
using namespace std;
const long long MEM_SIZE = 1LL * 1024LL * 1024LL * 1024LL; // available memory 1GB
typedef struct MyClass {
    int a;
    unsigned char b,c,d;
    size_t e,f;
    double g, h; 
};
int main() {
    MyClass *mc = new MyClass[MEM_SIZE / sizeof(MyClass)];
    cout << "done!" << endl;
    return 0;
}

---

CODE2

#include<fstream>
#include<iostream>
#include<string>
#include<vector>
using namespace std;

const long long MEM_SIZE = 1LL * 1024LL * 1024LL * 1024LL; // available memory 1GB
typedef struct MyClass {
    int a;
    unsigned char b,c,d;
    size_t e,f;
    double g, h; 
};
int main() {
    vector<MyClass> myv;
    myv.resize(MEM_SIZE / sizeof(MyClass));
    cout << "done!" << endl;
    return 0;
}

---

CODE3

#include<fstream>
#include<iostream>
#include<string>
#include<vector>
using namespace std;
const long long MEM_SIZE = 1LL * 1024LL * 1024LL * 1024LL; // available memory 1GB
typedef struct MyClass {
    int a;
    unsigned char b,c,d;
    size_t e,f;
    double g, h; 
};
int main() {
    vector<MyClass> myv;
    MyClass tmp;
    for (int i = 0; i < 12000000; i++){
        tmp.a = i;
        myv.push_back(tmp);
    }
    cout << "done!" << endl;
    return 0;
}

The size of MyClass is 32 Bytes, I set available memory as 1GB, so the array length is 1GB/32B=33554432.

As for CODE1 and CODE2, the array size is 1GB, far less than PC's RAM, why bad_alloc ?

As for CODE3, I know when push_back , the capacity of vector will double, but it's also less than PC's RAM. In CODE3, when i==11958657 crashed.

But when I build and run in x64 mode, all are fine. To my knowledge, x86's heap is around 2GB, why my 1GB array crashed?

How do i do in x86 mode?

Answer 1

An array has to be contiguous in memory so you don't just require 1 GB of memory, you need it in one block. Even if you have enough free virtual memory (physical memory doesn't matter much), memory fragmentation may prevent that allocation.