Question

I have a std::map of Poco::Any which I’m trying to iterate and output to a stream but i’m getting a compiler error. My code is below:

map<string, Poco::Any>::const_iterator it;
map<string, Poco::Any>::const_iterator end = _map.end();
map<string, Poco::Any>::const_iterator begin = _map.begin();
for(it = begin; it != end; ++it) {
    const std::type_info &type = it->second.type();

    // compile error here:
    os << "  " << it->first << " : " << Poco::RefAnyCast<type>(it->second) << endl;
}

2 errors on that line:

'type' cannot appear in a constant-expression
no matching function for call to 'RefAnyCast(Poco::Any&)'

UPDATE:

I understand that templates are compile time whereas type() is runtime so won’t work. Thanks for underlining that. Also DynamicAny won’t work because it only accepts types which have DynamicAnyHolder implementations, not ideal. The only rule i’d like to impose on the types is that they have << overloaded.

Below is what i’m currently doing, works to a degree, but only dumps known types, which is not what I’m after.

string toJson() const {
    ostringstream os;
    os << endl << "{" << endl;
    map<string, Poco::Any>::const_iterator end = _map.end();
    map<string, Poco::Any>::const_iterator begin = _map.begin();
    for(map<string, Poco::Any>::const_iterator it = begin; it != end; ++it) {
        const std::type_info &type = it->second.type();
        os << "  " << it->first << " : ";

        // ugly, is there a better way?
        if(type == typeid(int)) os << Poco::RefAnyCast<int>(it->second);
        else if(type == typeid(float)) os << Poco::RefAnyCast<float>(it->second);
        else if(type == typeid(char)) os << Poco::RefAnyCast<char>(it->second);
        else if(type == typeid(string)) os << Poco::RefAnyCast<string>(it->second);
        else if(type == typeid(ofPoint)) os << Poco::RefAnyCast<ofPoint>(it->second);
        else if(type == typeid(ofVec2f)) os << Poco::RefAnyCast<ofVec2f>(it->second);
        else if(type == typeid(ofVec3f)) os << Poco::RefAnyCast<ofVec3f>(it->second);
        //else if(type == typeid(ofDictionary)) os << Poco::RefAnyCast<ofDictionary>(it->second);
        else os << "unknown type";

        os << endl;
    }
    os<< "}" << endl;
    return os.str();
}

Answer 1

Runtime type information cannot be used to instantiate templates. An instance of type_info whose value will only be known when you run the program, doesn’t magically turn into a type like int, std::string or struct FooBar when the compiler is compiling this code.

I don’t know Poco library, but perhaps you could use their other Any type, DynamicAny (see documentation) which would hopefully allow you to convert the stored value to std::string for outputting:

os << "  " << it->first << " : " << it->second.convert<std::string>() << endl;