Invalid operands to binary expression ('const' and 'const' )

Question

I am a C++ noob, so please don’t mind if you find this question silly

I am declaring map in C++ a below:

std::map<CartesianLocation, std::list<RadioSignal<RadioDevice>>> radioMap;

Full code:

Don't know but using below code, I could able to solve my issue

class RadioMap:public std::iterator_traits<CartesianLocation>, public Cloneable<RadioMap> {
private:
    std::map<const CartesianLocation*, const std::list<RadioSignal<RadioDevice>>*> radioMap;
    std::vector<RadioDevice> radioDevices;

public:
    void add(const CartesianLocation *location, const std::list<RadioSignal<RadioDevice>> *observedSignals){
        radioMap[location] = observedSignals;
}

On this line radioMap[location] = observedSignals; I am ended with this error

“Invalid operands to binary expression ('const CartesianLocation' and 'const CartesianLocation’)” on struct _LIBCPP_TYPE_VIS_ONLY less : binary_function<_Tp, _Tp, bool> { _LIBCPP_CONSTEXPR_AFTER_CXX11 _LIBCPP_INLINE_VISIBILITY bool operator()(const _Tp& __x, const _Tp& __y) const {return __x < __y;} };

Any idea where I may be wrong?

I think because you have user defined types specifically `CatersianLocation` and whatever `RadioSignal` and `RadioDevice` is they need to define the less than operator `<` for the comparator in the `map` to order the elements correctly, see related: http://stackoverflow.com/questions/1102392/stdmaps-with-user-defined-types-as-key or you can define a comparator functor as suggested in the linked question — EdChum, May 18 '16 at 13:34

Rakete1111 · Accepted Answer · 2020-02-15T19:24:43.090

7

You are not providing a comparator for std::map, and so it uses std::less. But std::less doesn't have an overload for CartesianLocation (and CartesianLocation doesn't have an operator<), so you get an error.

You can add an operator<:

struct CartesianLocation
{
    //Other stuff
    bool operator<(const CartesianLocation& loc2) const
    {
        //Return true if this is less than loc2
    }
};

Another way is to define a custom comparator, for example:

struct comp
{
    bool operator()(const CartesianLocation& loc1, const CartesianLocation& loc2) const
    {
        //Compare the 2 locations, return true if loc1 is less than loc2
    }
};

Then you can pass it to std::map:

std::map<CartesianLocation, std::list<RadioSignal<RadioDevice>>, comp> radioMap;

edited Feb 15 '20 at 19:24

answered May 18 '16 at 13:37

Rakete1111

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For completness, two other options would be to specialize `less` for `CartesianLocation` or defined `operator<` for it. – SergeyA May 18 '16 at 13:40
@SergeyA can you really specialize `less`?? I thought you can't do that for `std` functions.. – Rakete1111 May 18 '16 at 13:41
Yes, you can. There is a special exception - while you can't define new names inside `std`, you can provide specializations. Also, your member `operator<` definition is incorrect. – SergeyA May 18 '16 at 13:45
@SergeyA oh, didn't know that. But it doesn't apply to templates, right? – Rakete1111 May 18 '16 at 13:47
@SergeyA - you can specialize templates in `std` **for user-defined types**. – Pete Becker May 18 '16 at 13:56
The two-argument `operator<` needs to be a free function. If it's a member it should take one argument and be `const`. – molbdnilo May 18 '16 at 13:56
@PeteBecker, of course, and this is something I forgot to mention in my comment. – SergeyA May 18 '16 at 14:04
Don't know but using below code, I could able to solve my issue: – Tarun May 19 '16 at 14:24

Invalid operands to binary expression ('const' and 'const' )

1 Answers1