How to make a safer C++ variant visitor, similar to switch statements?

Question

The pattern that a lot of people use with C++17 / boost variants looks very similar to switch statements. For example: (snippet from cppreference.com)

std::varia         


        

Answer 1

You may add an extra layer to add those extra check, for example something like:

template  struct IVisitorHelper;

template  struct IVisitorHelper {};

template 
struct IVisitorHelper
{
    virtual ~IVisitorHelper() = default;
    virtual Ret operator()(T) const = 0;
};

template 
struct IVisitorHelper : IVisitorHelper
{
    using IVisitorHelper::operator();
    virtual Ret operator()(T) const = 0;
};

template  struct IVarianVisitor;

template 
struct IVarianVisitor> : IVisitorHelper
{
};

template 
Ret my_visit(const IVarianVisitor>& v, V&& var)
{
    return std::visit(v, var);
}

With usage:

struct Visitor : IVarianVisitor>
{
    void operator() (double) const override { std::cout << "double\n"; }
    void operator() (std::string) const override { std::cout << "string\n"; }
};


std::variant v = //...;
my_visit(Visitor{}, v);