I am having issues storing values into a void* and successfully retrieving what I stored in initially. The below is my pseudo code/train of thought:
Inside method 1 on a client
StatusCode doSomething() {
string filename = "somefile.txt";
void* server_checksum;
//Stat signature (string &filename, void* file_status)
StatusCode fileStatus = Stat(filename, &server_checksum); //Passing the address of the pointer
//We received the fileStatus from Stat, I expect the value of server_checksum to match what the server sent
//However, this prints a completely different number, and I do not know how to ensure it holds the right value
cout << *((uint32_t *)serverCrc) << endl;
return StatusCode::OK;
}
Inside the Stat method on the client, there's a protobuf via grpc that has the checksum for the file on the server:
StatusCode Stat(string &filename, void* file_status) {
//call the grpc method on the server (abstracted)
.
.
.
//Contains the checksum of the file on the server - this works fine
uint32_t s_crc = response.server_crc();
// I print it in both the server and the client to confirm it is the same value - this works fine
cout << s_crc << endl;
//In my understanding, here I am assigning the value of s_crc to the void * file status, which I passed the address for inside of method 1 - this works fine
file_status = (uint32_t *) &s_crc;
// I print file_status to make sure it still matches the value the server sent - this works fine
cout<<"file_status " << *((uint32_t *)file_status) << endl;
return StatusCode::OK; -> Continues inside method 1 above
}
There's no reason to use a void*
here at all. C++ has a type system; you should use it.
Instead of declaring your out parameter as a void*
, declare it to be either a pointer or reference to the type you want to write. In this case that appears to be uint32_t
:
StatusCode Stat(const std::string& filename, uint32_t& file_status) {
//call the grpc method on the server (abstracted)
// ...
//Contains the checksum of the file on the server - this works fine
file_status = response.server_crc();
return StatusCode::OK;
}
And then you can call it without doing any special gymnastics:
StatusCode doSomething() {
std::string filename = "somefile.txt";
uint32_t server_checksum;
StatusCode fileStatus = Stat(filename, server_checksum);
std::cout << server_checksum << std::endl;
return StatusCode::OK;
}
If there's some reason you must use a void*
and thus explicitly give up the protections offered by the type system then the pointer still has to point to something . In the end the code will look very similar, just with an extra cast and significantly more opportunity to mess up and wander into the realm of undefined behavior:
StatusCode Stat(const std::string& filename, void* file_status) {
//call the grpc method on the server (abstracted)
// ...
// cast to the appropriate pointer type
uint32_t* status_ptr = static_cast<uint32_t*>(file_status);
// now write the status to the object pointed to by the pointer passed to us
*status_ptr = response.server_crc();
return StatusCode::OK;
}
Not much extra is needed when calling the function, since any pointer-to-object type can be implicitly converted to void*
:
StatusCode doSomething() {
std::string filename = "somefile.txt";
uint32_t server_checksum;
StatusCode fileStatus = Stat(filename, &server_checksum);
std::cout << server_checksum << std::endl;
return StatusCode::OK;
}
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