Template function type casting at run time for json deserialization

I want to write a function which deserialise a json representation of array into a std::vector. The json library which i am using is part of Facebook's folly library. I would like to achieve somethings like the following, but unfortunately it's not working:

template<typename T>
static bool deserializeHelper(std::string fieldName, vector< T >& structField, const folly::dynamic& jsonObj) {        
    if(auto* jsonField = jsonObj.get_ptr(fieldName)){
        if(jsonField->isArray()) {
           for(auto& elem : *jsonField) {
               if(elem.isInt()) {
                   structField.push_back(elem.asInt());
               } else if(elem.isString()){
                   structField.push_back(elem.asString());
               } else if(elem.isDouble()) {
                   structField.push_back(elem.asDouble());
               } else if(elem.isBool()) {
                   structField.push_back(elem.asBool());
               } else return false;
           }
        } else return false;
    }

    return true;
}

In the above code, jsonField is a representation of array field. So the code just try to loop through the array; then for each element; it will try to push back to generic vector: vector. The problem is that the code cannot be compiled because it will complain that it cannot cast from std::string to int; when T=int;

I am not sure how to write a generic function like that without the need of implement 4 method overloading functions. static bool deserializeHelper(std::string fieldName, vector< int >& structField, const folly::dynamic& jsonObj) ....

Thanks.