C++ Segmentation fault while dereferencing a void pointer to a vector - c++

Question

#include <iostream>
#include <vector>
#include <mutex>
struct STRU_Msg
{
std::string name;
void *vpData;
};
class CMSG
{
public:
template <typename T>
int miRegister(std::string name)
{
STRU_Msg msg;
msg.name = name;
msg.vpData = malloc(sizeof(T));
msgtable.push_back(msg);
std::cout << "registeratio ok\n";
return 0;
}
template <typename T>
int miPublish(std::string name, T tData)
{
for (int i = 0; i < msgtable.size(); i++)
{
if (!name.compare(msgtable[i].name))
{
(*(T *)msgtable[i].vpData) = tData;
std::cout << "SUccess!\n";
return 0;
}
else
{
std::cout << "cannot find\n";
return 0;
}
}
}
private:
std::vector<STRU_Msg> msgtable;
};
int main()
{
CMSG message;
std::string fancyname = "xxx";
std::vector<float> v;
// message.miRegister< std::vector<float> >(fancyname);
// for (int i = 0; i < 1000; i++)
// {
// v.push_back(i);
// }
// std::cout << "v[0]: " << v[0] << ", v[-1]: " << v[v.size()-1] << '\n';
// message.miPublish< std::vector<float> >(fancyname, v);
for (int i = 0; i < 1000; i++)
{
v.push_back(i);
}
std::cout << "v[0]: " << v[0] << ", v[-1]: " << v[v.size()-1] << '\n';
message.miRegister< std::vector<float> >(fancyname);
message.miPublish< std::vector<float> >(fancyname, v);
return 0;
}
What I want to achieve is to write a simple publish/subscribe (like ROS) system, I use void pointer so that it works for all data type. This is the simplified code.
If I publish an int, it works fine, but what really confuse me are:
If I pass a long vector (like this code), it gave me the
"segmentation fault (core dump)" error.
If I define the vector between "register" and "publish" (i.e. like
the commented part), this error goes away.
If I use a shorter vector, like size of 10, no matter where I define
it, my code run smoothly.
I use g++ in Linux.
Please help me fix my code and explain why above behaviors will happen, thanks in ahead!

Answer 1

You cannot copy std::vector or any other non-trivial type like that. Before you do anything (even assignment-to) with such an object, you need to construct it using a constructor and placement new.
A way to do this would be
new(msgtable[i].vpData) T;
Do this in the register function.
Then you can assign a value as you do.
Still better, do not use malloc at all, allocate your object with (normal, non-placement) new.
I however strongly suggest ditching void* and moving to a template based implementation of STRU_Msg. If you don't feel like reinventing the wheel, just use std::any.