#Constness of a member variable based on a bool template param

6 messages · Page 1 of 1 (latest)

versed forge
#

I want to do a custom iterator for a game scene that contains characters in a std::vector. I can't use the iterator of a vector itself since, due to pooling reasons, not only it stores extra chunking data, but also not all values within vector are "valid". Of course, I could declare the type itself as a template argument (which I will do if nothing else works), but this iterator is very specific, and using it with other types will likely break things. However, I do need a reference to a scene itself, and that reference must be either const or non-const depending on the constness of an iterator itself. My first and only thought was to use enable_if, but it doesn't work for member variables, as I hoped it would

This is the mockup code I have so far so I don't have to compile the entire project just for this:

#include <iostream>
#include <type_traits>

struct IntContainer
{
    int Value = 5;
}

template<bool Const = true>
class TestMember
{
public:
    typename std::enable_if<(!Const), IntContainer&>::type CurContainer;
    typename std::enable_if<Const, const IntContainer&>::type CurContainer;
    
    template<std::enable_if_t<Const, bool> = true>
    TestMember(const IntContainer& cur_container) 
        : CurContainer(cur_container)
    {}
    template<std::enable_if_t<(!Const), bool> = true>
    TestMember(IntContainer& cur_container)
        : CurContainer(cur_container)
    {}
}

int main()
{
    IntContainer cont;
    TestMember<true>(cont);
    
    std::cout << cont.CurContainer.Value;
    
    TestMember<false>(cont);
    
    std::cout << cont.CurContainer.Value;
}```

The error is in the attachment. Compiled with rextester's "C++ (gcc)"

How do?
📎 error_log.txt
vapid capeBOT
#

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merry quartz
#

templates are not my specialty but could you make a template specialization for true and false? a bit copy-pastey tbf but it works and lets both const and non-const members have the same name

fervent wharf
#

I don't fully understand the context, but just looking through the compile errors you have a couple of them that aren't related to TestMember and const/non-const stuff
Skipping those as well, my typical recommendation would be to introduce a type alias, and use that alias for everything
If you have access to c++20 I would also recommend looking at concepts/constraints, most uses of enable_if are better served with concepts instead

# 
template<bool Const = true>
class TestMember
{
public:
    using ref2cont = std::conditional_t<Const, const IntContainer&, IntContainer&>;
    ref2cont CurContainer;
    
    TestMember(ref2cont cur_container) 
        : CurContainer(cur_container)
    {}
};
versed forge