How to pass class template argument to boost::variant? - c++

Question

I have a template method that uses boost::get of boost:variant module:
typedef boost::variant<int, std::string, bool, uint8_t> Variant;
template <class T>
void write(const Variant& t) {
size_t sizeT = boost::apply_visitor(SizeOfVisitor(), t);
memcpy(&v[offset], &boost::get<T>(t), sizeT);
}
The problem is I know the underlying type of Variant only at runtime. And AFAIK I can query for it only with which() method.
Is there any way I can pass the type T, which is the underlying type of Variant to this method?
For instance, using which() I do know, what type there is, but how to pass it?
switch (m_var.which()) { // Returns an int index of types in the order of passed template classes
case 0: // This is int
case 1: // This is std::string
...
}
...
Writer.write<???>(m_var); // How to pass the type here?
EDIT: If you know any other way to achieve the desired result - actually getting the address of boost::variant inner variable, to copy from there, please share with me your ideas
Thank you

Answer 1

I happen to have written a very similar answer here:
C++: Extract size of boost::variant element
Again, the most important would be that it's completely bogus to use memcpy with non-POD datatypes (so you cannot use it with std::string. Ever).
The way to operate on variants with the type known only at runtime is using boost::static_visitor<>.
Here's the example with main() adapted to be close to what you wanted to achieve, apparently,
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#include <boost/variant.hpp>
#include <boost/bind.hpp>
#include <boost/array.hpp> // just as a sample
#include <iostream>
namespace serialization {
namespace customization {
template<typename T, typename Out, typename R = typename boost::enable_if<boost::is_pod<T>, void>::type>
void do_serialize(T const& x, Out& out)
{
static_assert(boost::is_pod<T>(), "");
char const* rawp = reinterpret_cast<char const*>(&x);
std::copy(rawp, rawp+sizeof(T), out);
}
template<typename Out>
void do_serialize(std::string const& x, Out& out)
{
do_serialize(x.size(), out);
for(auto ch : x)
do_serialize(ch, out);
}
}
struct serialize_f : public boost::static_visitor<> {
template<typename Out, typename... T>
void operator()(boost::variant<T...> const& v, Out& out) const
{
boost::apply_visitor(boost::bind(*this, _1, boost::ref(out)), v);
}
template<typename T, typename Out>
void operator()(T const& x, Out& out) const
{
using customization::do_serialize; // ADL dispatch
do_serialize(x, out);
}
};
template <typename T, typename Out>
Out serialize(T const& v, Out out) {
const static serialize_f _vis {};
_vis(v, out);
return out;
}
}
namespace MyUserTypes {
struct A {
std::string name;
int i;
};
template<typename Out> void do_serialize(A const& v, Out& out) { // ADL will find this
serialization::serialize(v.name, out);
serialization::serialize(v.i, out);
}
}
int main() {
using namespace serialization;
std::vector<uint8_t> binary_data;
auto out_inserter = back_inserter(binary_data);
// variants and custom types
typedef boost::variant<MyUserTypes::A, boost::array<char, 42> > V;
MyUserTypes::A myA { "0123456789", 99 };
V v = boost::array<char,42>();
serialize(myA, out_inserter);
serialize(v, out_inserter);
v = myA;
serialize(v, out_inserter);
std::cout << "Bytes in binary_data vector: " << binary_data.size() << "\n";
}

Answer 2

switch (m_var.which()) { // Returns an int index of types in the order of passed template classes
case 0: // This is int
Writer.write<int>(m_var);
break;
case 1: // This is std::string
Writer.write<std::string>(m_var);
break;
...
}
:)