I want to have boost::variant
with empty state. So I define a boost::variant
with boost::blank
as the first alternative. But then I want to pass this as function parameter:
void f(Variant v);
...
void g()
{
f(boost::blank{});
}
It does not look nice due to braces. Seem to be better if it accepted boost::none
:
void g()
{
f(boost::none);
}
But I don't think I have seen boost::variant<boost::none_t, ...>
anywhere. boost::none_t
is a satellite of boost::optional
. Is it fine to use with boost::variant
?
You can just default construct, which will initialize the first element type.
Alternatively you can define your own constant of type boost::blank
:
#include <boost/variant.hpp>
#include <iostream>
using Variant = boost::variant<
boost::blank,
int,
std::string>;
void foo(Variant v = {}) {
std::cout << "foo: " << v.which() << "\n";
}
int main()
{
foo();
foo({});
foo(Variant{});
foo(boost::blank{});
static constexpr boost::blank blank;
foo(blank);
foo(42);
foo("LtUaE");
}
Prints
foo: 0
foo: 0
foo: 0
foo: 0
foo: 0
foo: 1
foo: 2