Question

I wrote a C++ template class, but I do not have the type to instantiate the class. Types are stored in a string format. So I have to do something like this:

if ( propType == "char") {
   Property<char> pChar = ...
} else if ( propType == "int") {
   Property<int> pChar = ...
} if ( propType == "double") {
   Property<double> pChar = ...
} 

I am not liking this if-else- loop, is there any way to avoid this, or any other solution to such problem?

Answer 1

Many ways, but it’s impossible to choose a good one without knowing how you use it in your particular case.

As a demonstration:

• Let us suppose that all Property<T> classes inherit from PropertyBase
• Let us suppose that you initialize them by parsing the type from a string

Here is some code then:

using PropertyPtr = std::unique_ptr<PropertyBase>;
using Parser = std::function<PropertyPtr(std::string const&, std::string const&)>;

template <typename T>
PropertyPtr parse(std::string const& type, std::string const& value) {
    T v = boost::lexical_cast<T>(value);
    return PropertyPtr(new Property<T>(std::move(v)));
}

std::map<std::string, Parser> const parsers = {
    std::make_pair("char", parse<char>),
    std::make_pair("int", parse<int>),
    std::make_pair("double", parse<double>)
};

void dummy(std::string const& type, std::string const& value) {
    auto const it = parsers.find(type);
    assert(it == parsers.end() && "No parser");

    auto const& parser = it->second;
    PropertyPtr property = parser(type, value);

    // do something with property
}

Hope this helps.