C++ why does new keyword works and malloc does not?

Question

I'm learning about pointers and structures and I ran into this hard to understand the problem for me. I have created this simple program for testing purpose:

#include <iostream>

struct testStructure
{
    int a = 0;
    int b = 0;
    int c = 400;
};

int main()
{
    struct testStructure* testStruct;
    testStruct = new testSctructure;

    std::cout << testStruct->c;

    delete testStruct;
    return 0;
}

the above program works just fine, it prints the value 400. But when i try to do it with malloc:

#include <iostream>

struct testStructure
{
    int a = 0;
    int b = 0;
    int c = 400;
};

int main()
{
    struct testStructure* testStruct;
    testStruct = (testStructure*)malloc(sizeof testStructure);

    std::cout << testStruct->c;

    free(testStruct);
    return 0;
}

it gives me this value: -842150451

Why? The above examples were written and build in Visual Studio 2019.

I know that in C++ you should almost always want to use new keyword but I wanted to experiment a bit.

In the second case, you have allocated memory, but it has not been initialized yet. Undefined behavior. — Eljay, Aug 28 '20 at 18:14
`-842150451` is a magic debugging code. `0xcdcdcdcd` see here what that means: [https://stackoverflow.com/questions/127386/in-visual-studio-c-what-are-the-memory-allocation-representations](https://stackoverflow.com/questions/127386/in-visual-studio-c-what-are-the-memory-allocation-representations) — drescherjm, Aug 28 '20 at 18:23

score 3 · Accepted Answer · answered Aug 28 '20 at 18:17

3

new initializes the allocated memory with the class' constructor (which is implicit in this this case).

malloc performs no initialization, it just allocates a portion of memory. Reading uninitiazlied memory will have undefined behavior.

answered Aug 28 '20 at 18:17

Mureinik

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score 2 · Answer 2 · answered Aug 28 '20 at 18:20

Here's how your second example can be made to work. In general, this is not a recommended way of working with C++, but there are some valid use cases.

#include <cstdlib>
#include <iostream>

struct testStructure {
    int a = 0;
    int b = 0;
    int c = 400;
};

int main() {
    auto testStruct_memory = std::malloc(sizeof(testStructure));

    // In-place constructor.
    auto testStruct = new(testStruct_memory) testStructure();

    std::cout << testStruct->c << "\n";

    // In-place destructor.
    testStruct->~testStructure();

    std::free(testStruct_memory);
}

Jeffrey · Answer 3 · 2020-08-28T18:39:36.073

In the case of malloc, you allocated a block of memory, but did not create an object. The testStructure constructor was not called.

You could call the constructor on the memory area with a placement new:

char* ptr = malloc(sizeof testStructure);
testStruct = new(ptr) testStructure;

but this is hard to read, confusing, hard to maintain, and fraught with risk. For example, you need

to free() not delete ptr
you need to similarly explicitly call the destructor.

So, not recommended.

C++ why does new keyword works and malloc does not?

3 Answers3