Why does my result data returned as void* gets broken?

Question

I am working in a project with a huge legacy code base and have been trying to re-design parts of it to get away from old c-style code.

I've run into a problem and have prepared a short program to explain.

The legacy interface I am using needs me to pass a pointer to the result data as void* and I would like to avoid having to change this.

The unique_ptr in the example just goes to demonstrate, that in my real code base everything working on the data is using smart pointers to manage memory.

My problem is, that the result data gets broken (see last output line / last call to printPayload); everything is 0 at the end, but it doesn't seem to be a problem with converting to the void* and back as shown by the 2nd and 3rd output line.

Is this a problem related to temporaries? I don't get it...

I hope this kind of problem has relevance for some of you.

#include <iostream>
#include <memory>

struct Payload
{
    long a;
    int b;
    int c;

    Payload() : a(), b(), c() {}
    Payload(long setA, int setB, int setC) : a(setA), b(setB), c(setC) {}
};

void printPayload(const Payload& printThis)
{
    std::cout << "payload -- a: " << printThis.a << " b: " << printThis.b << " c: " << printThis.c << std::endl;
}

void doSomething(Payload* sourceData, void* targetData)
{
    if (!sourceData) return;

    std::unique_ptr<Payload> sourceDataUnique(sourceData);

    sourceDataUnique->a = 222;
    sourceDataUnique->b = 333;
    sourceDataUnique->c = 444;

    printPayload(*sourceDataUnique);

    targetData = reinterpret_cast<void*>(sourceDataUnique.release());

    printPayload(*(reinterpret_cast<Payload*>(targetData)));
}

int main(void)
{
    Payload* myPayload = new Payload(14, 8, 1982);
    Payload myResult;

    printPayload(*myPayload);

    doSomething(myPayload, &myResult);

    printPayload(myResult);
}

Output:

payload -- a: 14 b: 8 c: 1982
payload -- a: 222 b: 333 c: 444
payload -- a: 222 b: 333 c: 444
payload -- a: 0 b: 0 c: 0

Answer 1

targetData is a local variable to doSomething. After you assign an address to it, it goes out of scope.

You never actually assign to myResult.

Answer 2

In your code, the parameter targetData is local to doSomething function (i.e., any change is lost after you exit the scope of the function). However, *targetData refers to the variable myResult declared in main function.

So, the following code should work:

void doSomething(Payload* sourceData, void* targetData)
{
    if (!sourceData) return;

    sourceData->a = 222;
    sourceData->b = 333;
    sourceData->c = 444;

    printPayload(*sourceData);

    Payload* td = static_cast<Payload*>(targetData);
    *td = *sourceData;
    printPayload(*td);
}