CODEWITHSUNDEEP
c++dynamic-memory-allocation
PYPL
PYPL

Reputation: 1849

Freeing dynamic memory

I'm just playing around c++ to understand how the memory leak occurs.

% g++ --version
Apple clang version 11.0.0 (clang-1100.0.33.17)
Target: x86_64-apple-darwin19.3.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Consider the code below

int main() {
const int c=10000;
int* ptrs[c];
for (int n=0; n<c; n++) {
    int s=1000;
    int *x = new int[s];
    for (int i=0; i<s; i++){
        x[i] = i*10;
    }
    ptrs[n] = x;
    //delete[] x; // <-- (1)
}

//Print pointers
for (int n=0; n<c; n++) {
    std::cout << ptrs[n] << std::endl;
    //delete[] ptrs[n]; // <-- (2)
}
std::getchar();
return 0;

}

When I check the program memory usage through the task manager at the point where program reaches std::getchar() I see that it uses ~360KB memory when I free up space with delete[] x; and ~7.6MB when doing it by delete[] ptrs[n];. My question is why does the second method consume more memory than the first one? They are technically doing the same thing aren't they?

Upvotes: 0

Views: 95

Answers (1)

Caleth
Caleth

Reputation: 62694

In the first case, you have implementation defined behaviour, because you inspect a pointer after having called delete[] on it. Notwithstanding that, you only have 1000 ints in dynamic storage at a time.

In the second case, you have 10000000 ints in dynamic storage at the point where you start printing values.

It is likely that your implementation is re-using the same memory over and over.

Upvotes: 1

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