英文:
Using Structs as keys in maps, ignoring == operators, false positive
问题
I'm trying to use a struct as a key in a map, but despite defining < and == operators I'm getting false positives:
return models[mr]; is returning even when the new ImportSettings has reverse = true and the one in models has reverse = false.
The Visual Studio debugger also shows that the operator calls for == aren't even being called. What have I misunderstood here?
struct HE2_ImportSettings
{
bool reverseWindingOrder = false;
bool operator==(const HE2_ImportSettings& other) const
{
return reverseWindingOrder == other.reverseWindingOrder;
}
};
struct ModelReference
{
std::string filepath = "";
HE2_ImportSettings importSettings = {};
bool operator==(const ModelReference& other) const
{
return filepath == other.filepath && importSettings == other.importSettings;
}
bool operator<(const ModelReference& other) const
{
return filepath < other.filepath;
}
};
ModelReference mr = { filename, importSettings };
bool exists = models.count(mr);
if (exists)
{
// This is returning even when the new importsettings has reverse = true and the one in models has reverse = false
return models[mr];
}
英文:
I'm trying to use a struct as a key in a map, but despite defining < and == operators I'm getting false positives:
return models[mr]; is returning even when the new ImportSettings has reverse = true and the one in models has `reverse = false.
The Visual studio debugger also shows that the operator calls for == aren't even being called. What have I misunderstood here?
struct HE2_ImportSettings
{
bool reverseWindingOrder = false;
bool operator==(const HE2_ImportSettings& other)const
{
return reverseWindingOrder == other.reverseWindingOrder;
}
};
struct ModelReference
{
std::string filepath = "";
HE2_ImportSettings importSettings = {};
bool ModelReference::operator == (const ModelReference& other) const
{
return filepath == other.filepath && importSettings == other.importSettings;
}
bool ModelReference::operator< (const ModelReference& other) const
{
return filepath < other.filepath;
}
};
ModelReference mr = { filename, importSettings };
bool exists = models.count(mr);
if (exists)
{
//This is returning even when the new importsettings has reverse = true and the one in models has reverse = false
return models[mr];
}
答案1
得分: 2
std::map 使用等价性概念。当两个元素等价时，
!(a < b) && !(b < a)
换句话说，map 从不使用 == 来判断两个元素是否相同。在你的 map 中，当它们具有相同的 filepath 时，两个元素被视为等价。如果你想在 map 的排序中考虑 importSettings，你需要在 operator< 中也进行比较。
英文:
std::map uses the concept of equivalence. Two elements are equivalent when
!(a < b) && !(b < a)
In other words, a map never uses == to see if two elements are the same. In your map two elements are considered equivalent when they have the same filepath. If you want to take also importSettings into account for the ordering in the map you need to compare that also in the operator<.
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