I think you can accomplish what you wish with the following class, which is very simplified to illustrate the main concept.
template <typename T>
struct Container
{
void push_back(bool isconst, T const& item)
{
data.push_back(std::make_pair(isconst, item));
}
T& at(size_t index)
{
// Check whether the object at the index is const.
if ( data[index].first )
{
throw std::runtime_error("Trying to access a const-member");
}
return data[index].second;
}
T const& at(size_t index) const
{
return data[index].second;
}
T const& at(size_t index, int dummy) // Without dummy, can't differentiate
// between the two functions.
{
return data[index].second;
}
T const& at(size_t index, int dummy) const // Without dummy, can't differentiate
// between the two functions.
{
return data[index].second;
}
std::vector<std::pair<bool, T> > data;
};
Here's a test program and its output.
#include <stdio.h>
#include <iostream>
#include <utility>
#include <stdexcept>
#include <vector>
//--------------------------------
// Put the class definition here.
//--------------------------------
int main()
{
Container<int> c;
c.push_back(true, 10);
c.push_back(false, 20);
try
{
int value = c.at(0); // Show throw exception.
}
catch (...)
{
std::cout << "Expected to see this.\n";
}
int value = c.at(0, 1); // Should work.
std::cout << "Got c[0]: " << value << "\n";
value = c.at(1); // Should work.
std::cout << "Got c[1]: " << value << "\n";
value = c.at(1, 1); // Should work.
std::cout << "Got c[1]: " << value << "\n";
// Accessing the data through a const object.
// All functions should work since they are returning
// const&.
Container<int> const& cref = c;
value = cref.at(0); // Should work.
std::cout << "Got c[0]: " << value << "\n";
value = cref.at(0, 1); // Should work.
std::cout << "Got c[0]: " << value << "\n";
value = cref.at(1); // Should work.
std::cout << "Got c[1]: " << value << "\n";
value = cref.at(1, 1); // Should work.
std::cout << "Got c[1]: " << value << "\n";
// Changing values ... should only work for '1'
try
{
c.at(0) = 100; // Show throw exception.
}
catch (...)
{
std::cout << "Expected to see this.\n";
}
c.at(1) = 200; // Should work.
std::cout << "Got c[1]: " << c.at(1) << "\n";
}
Output from running the program:
Expected to see this. Got c[0]: 10 Got c[1]: 20 Got c[1]: 20 Got c[0]: 10 Got c[0]: 10 Got c[1]: 20 Got c[1]: 20 Expected to see this. Got c[1]: 200