Compile Error C2662

Question

I'm trying to pass an object as a reference to a function that accepts the object as a const however the compiler is throwing:

error C2662: 'const int DataPacket::GetData(void)': cannot convert 'this' pointer from 'const DataPacket' to 'DataPacket &'

IntelliSense says:

the object has type qualifiers that are not compatible with the member function
    object type is: const DataPacket

I made a test-cast of the code to demonstrate the issue:

#include <iostream>
#include <functional>

class DataPacket
{
    int SomeVar;
public:
    DataPacket(int InitialData)
    {
        SomeVar = InitialData;
    }

    const int GetData()
    {
        return SomeVar;
    }
};

void ProcessPacket(const DataPacket& Packet)
{
    std::cout << Packet.GetData() << std::endl;
}

int main()
{
    std::function<void(const DataPacket& Packet)> f_Callback;
    f_Callback = ProcessPacket;

    while (true)
    {
        f_Callback(DataPacket(10));
    }

}

Basically I have a STD function which the user can set to use their own callback function. A lower level class creates objects of type DataPacket when new packets arrive. I then want to pass the object into the callback function as a constant reference so not to copy the entire object and restrict the user from changing the original object.

What's going on here? How can I pass DataPacket into f_Callback as a const reference?

Answer 1

The problem is that in the callback, the DataPacket object Packet is marked as const. That means you can't call non-const function in the object.

This can simply be changed by making the GetData member function const, and no it's not what you have done, you have only made the returned integer const, which makes no sense (because the value will just be copied and loose it's const-ness).

Instead declare the function as

int GetData() const;