C++ class with template member variable. and parameter memory out - c++

Question

struct PacketBase
{
virtual ~PacketBase() {}
template<class T>
const T& get() const
{
return static_cast<const PacketVal<T>&>(*this).val;
}
template<class T>
void set(const T& rhs)
{
return static_cast<PacketVal<T>>(*this).val = rhs;
}
};
template <typename T>
struct PacketVal : public PacketBase
{
T val;
PacketVal(const T& rhs) : val(rhs) {}
~PacketVal() {}
};
class CResponsePacket
{
private:
std::vector<std::pair<int, const PacketBase*>> m_vPacketData;
public:
void addValue(int n, const PacketBase& packetData)
{
m_vPacketData.emplace_back(std::make_pair(n, &packetData));
}
void print()
{
for ( auto& data : m_vPacketData )
{
std::printf("%d - ", data.first);
std::printf("%s\n", data.second->get<std::string>().c_str());
}
}
};
CResponsePacket has template class member-variable.
int main()
{
CResponsePacket packet;
std::string strAAA("AAA");
PacketVal<std::string> pVal(strAAA);
packet.addValue(1, pVal);
for ( int nIdx = 0; nIdx < 5; ++nIdx )
{
PacketVal<std::string> pp(strAAA);
packet.addValue(nIdx + 1, pp);
}
packet.print();
return 0;
}
The result is
1 - AAA
1 -
2 -
3 -
4 -
5 -
Because the instance is in a loop, memory is destroyed. But this is just an example, and I have to use loop actually. How can I solve this...?
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Answer 1

You can make use of std::shared_pointer or std::unique_ptr (depending of your use), and so having the vector looking like this:
std::vector<std::pair<int, std::shared_pointer<PacketBase>>> m_vPacketData;
And so then the addValue method should look like this:
void addValue(int n, const PacketBase& packetData)
{
m_vPacketData.emplace_back(
std::make_pair(
n,
std::make_shared<PacketBase>(packetData)
)
);
}
This will make a copy of packetData on the heap, and store the reference in a shared pointer. If you don't what this copy to be made, then you have to define a new constructor in PacketBase
class PacketBase{
public:
PacketBase(PacketBase&& el): attr(std::move(el.attr))... {...}
}
and then redefining the addValue that accept a rvalue reference
void addValue(int n, PacketBase&& packetData)
{
m_vPacketData.emplace_back(
std::make_pair(
n,
std::make_shared<PacketBase>(std::move(packetData))
)
);
}
And then actually calling the right addValue from main:
for ( int nIdx = 0; nIdx < 5; ++nIdx )
{
PacketVal<std::string> pp(strAAA);
packet.addValue(nIdx + 1, std::move(pp));
}
Or actually just:
for ( int nIdx = 0; nIdx < 5; ++nIdx )
packet.addValue(nIdx + 1, PacketVal<std::string>(strAAA););
This will obviously force you to rewrite the entire class, because you are changing the attribute type... the other way that you can achieve this is switch from
m_vPacketData.emplace_back(std::make_pair(n, &packetData));
to
m_vPacketData.emplace_back(std::make_pair(n, new PacketVal(packetData)));
But you have then remember to delete the objects on the heap or you will have memory leaks