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Getting a substr from a std::variant<std::string, std::vector<std::string>>> in C++

August 2, 2022

So I’ve got a map with a string key and either a string or vector value. I want to cycle through all the string values (whether they’re found in a vector or they can be directly checked) in the map and check if they match a given string (it’s irrelevant what the string is). If that string matches completely or partly, I will put the map that the string was found in inside a vector.

The issue I’m having is that even though I’m separating strings from vectors using an if conditional the substr function thinks I’m dealing with a std::variant<std::string, std::vector<std::string>>> and therefore gives me an error.

The begin functions in the else functions also tell me that variant doesn’t have a member called "begin" because of this reason. I’m totally lost, any help would be great.

for (std::unordered_map<std::string,
                        std::variant<std::string,
                        std::vector<std::string>>>::iterator h = MyMap.begin();
    h != MyMap.end();
    h++) {
    if (typeid(h->second) == typeid(std::string)) {
        std::string MapValueString = h->second.substr(0, TextCtrlValue.length());
        if (MapValueString == TextCtrlValue) {
            RightSearchResultsVector.insert(RightSearchResultsVector.end(), Maps.begin(), Maps.end());
        }
    }
    else {
        for (std::vector<std::string>::iterator f = h->second.begin(); f != h->second.end(); f++) {
            // Go through vector and find matching/semi matching strings
        }
    }
}

>Solution :

Here’s how to tell what type a variant holds

std::string* pstr = std::get_if<std::string>(&h->second);
if (pstr != nullptr)
{
    // do stuff with string
}
else
{
    std::vector<std::string>& vec = std::get<std::vector<std::string>>(h->second);
    // do stuff with vector
}

BTW you can simplify this monstrosity

for (std::unordered_map<std::string,
                    std::variant<std::string,
                    std::vector<std::string>>>::iterator h = MyMap.begin();

by writing