Question 135516 in Visual Studio Code Question posted in
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Visual Studio Code – error C2784 c++ std::map with 2 user-defined types

could someone please explain why this code produces. i was able to narrow the error to this segment regardless of what I set the map value to.

C:Program FilesMicrosoft Visual Studio2022CommunityVCToolsMSVC14.33.31629includexstddef(117,1): error C2784: ‘bool std::operator <(std::nullptr_t,const std::shared_ptr<_Ty> &) noexcept’: could not deduce template argument for ‘const std::shared_ptr<_Ty> &’ from ‘const _Ty’

struct Vector2i
{
    int x;
    int y;
};
std::map<Vector2i, Chunk*> map{};
map.insert({ Vector2i{0,0}, nullptr });

thanks 🙂

I tried commenting out all other instances of the Vector2i struct and this segment seemed to be the only place that causes this error.

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2

Answers


  1. 0

    The std::map is an ordered container, and therefore requires keys to provide a "less-than" operator for comparison. This can be provided in the map’s template arguments, or it can be implicit if you have defined operator< for your type.

    struct Vector2i
{
    int x;
    int y;

    bool operator<(const Vector2i& other) const {
        return x < other.x || x == other.x && y < other.y;
    }
};
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  2. 0

    You should provide comparison operator to your map. Here is some ways.

    Define operator< inside struct.

    struct Vector2i
{
    ...   
    bool operator<(const Vector2i& rhs) const {
        return x < rhs.x;
    }
};

    Define outside the struct.

    bool operator< (const Vector2i& lhs, const Vector2i& rhs) {
    return lhs.x < rhs.x;
}

    Define struct comparator and pass to constructor of map.

    struct cmpByStringLength {
    bool operator()(const Vector2i& lhs, const Vector2i& rhs) const {
       return lhs.x < rhs.x;  
    }
};

std::map<Vector2i, Chunk*, cmpByStringLength> mp{};

    Using lambda function.

    auto comp = [](const Vector2i& a, const Vector2i& b) { return a.x < b.x; };
std::map<Vector2i, Chunk*, decltype(comp)> mp{};
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